Physical World/Astronomy

Planets

The International Astronomical Union (IAU) defines a planet, for the purposes of our solar system only, as a celestial body that:

(a) is in orbit around the Sun,

(b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and

(c) has cleared the neighbourhood around its orbit

Inferior planet – used in reference to Mercury and Venus, which are closer to the Sun than the Earth is

Superior planet – used in reference to Mars, Jupiter, Saturn, Uranus, Neptune, and all the known minor and dwarf planets, including Ceres and Pluto, which are farther from the Sun than the Earth is

Inner planets – those planets which lie inside the asteroid belt, i.e. Mercury, Venus, Earth and Mars

Outer planets – those that lie outside the asteroid belt, i.e. the four gas giants

Following the discovery of Neptune, there was considerable speculation that another planet might exist beyond its orbit. The search culminated at the start of the 20th century with Percival Lowell's quest for Planet X. Lowell proposed the Planet X hypothesis to explain apparent discrepancies in the orbits of the gas giants

Rogue planet – also known as orphan planet, is a planetary-mass object which has either been ejected from its system or was never gravitationally bound to any star and that therefore orbits the galaxy directly

Circumbinary planet – a planet that orbits two stars instead of one

Vulcan was a small planet proposed to exist in an orbit between Mercury and the Sun. Attempting to explain peculiarities of Mercury's orbit, the 19th-century French mathematician Urbain Le Verrier hypothesized that they were the result of another planet, which he named ‘Vulcan’

Kepler's laws of planetary motion are three scientific laws describing the motion of planets around the Sun:

First law – The orbit of every planet is an ellipse with the Sun at a focus

Second law – A line joining a planet and the Sun sweeps out equal areas during equal intervals of time

Third law – The square of the orbital period of a planet is directly proportional to the cube of the semi-major axis of its orbit (mean distance from the Sun)

Titius-Bode law (sometimes termed just Bode's law) is a hypothesis that the semi-major axes of planets in the solar system follow a simple rule. It was discredited with the discovery of Neptune

Equatorial bulge – a difference between the equatorial and polar diameters of a planet, due to the centrifugal force of its rotation. A rotating body tends to form an oblate spheroid rather than a sphere. The Earth has an equatorial bulge of 42.7 km. The Sun does not have an equatorial bulge

Earth

Moon

Moon is 240,000 miles from Earth and has a diameter of 2160 miles

14th largest body in Solar System (between Io and Europa)

Moon is 1.3 light seconds from earth

Moon daytime temperature 107^oC, nighttime temperature -153^oC

Moon orbital period is 27.32 days

The South Pole-Aitken basin is an impact crater on the Moon. Roughly 2500 kilometers in diameter and 13 kilometers deep, it is the largest known impact crater not only on the Moon, but also in the entire solar system

Terminator – the edge of the shadow between night and day on the moon

The dark areas visible on the moon, the lunar maria, are plains of flood basaltic lava flows. They were dubbed maria, Latin for ‘seas’, by early astronomers who mistook them for actual seas

Nearly the entire lunar surface is covered with regolith (a layer of loose, heterogeneous material covering solid rock)

Oceanus Procellarum, Latin for ‘Ocean of Storms’, is a vast lunar mare on the western edge of the near side of the Moon

Moon’s surface is deformed by 20 m by the gravitational effect of the Earth

Features on the moon – mare (seas), lacus (lakes), palus (marshes), sinus (bays)

Tidal forces between Earth and the Moon have slowed the moon's rotation so that the same side is always facing the Earth. The other face, which is never visible from the Earth in its entirety, is therefore called the ‘far side of the Moon’. The far side should not be confused with the ‘dark side’ (the hemisphere that is not illuminated by the Sun at a given point in time), as the two are the same only during a full moon

The Moon's rotation and orbital periods are both just under four weeks, so no matter when the Moon is observed from the Earth the same hemisphere of the Moon is always seen. Due to tidal locking

40% oxygen, 19% silicon, 14% iron in moon rocks

Mendeleev Crater – on dark side of the moon

Mare Nubium – ‘sea of clouds’ on the Moon

The giant impact hypothesis is the now-dominant scientific theory for the formation of the Moon, which is thought to have formed as a result of a collision between the young Earth and a Mars-sized body over 4.5 billion years ago that is sometimes called Theia, derived from Greek mythology, as Theia was the Titan who gave birth to the Moon goddess Selene. The hypothesis was first published in Icarus in 1975 by Hartmann and Davis

Presence of the moon prevents the earth from wobbling

At a major lunar standstill, which takes place every 18.6 years, the range of the declination of the Moon reaches a maximum. As a result, at high latitudes, the Moon's greatest altitude changes in just two weeks from high in the sky to low over the horizon

The Moon is thought to have formed nearly 4.5 billion years ago, not long after the Earth

Phases of the moon – new, waxing crescent, first quarter, waxing gibbous, full, waning gibbous, last quarter, waning crescent

Waxing – amount of lit area of the moon increases over time

Waning – amount of lit area of the moon decreases over time

Crescent moon – part way between a half moon and a new moon, or between a new moon and a half moon

Gibbous moon – between a full moon and a half moon, or between a half moon and a full moon

Blue moon – when two full moons occur in a single month, the second full moon is called a blue Moon

Harvest moon – the full moon nearest the autumnal equinox (in October)

Hunter’s moon – follows the harvest moon, in November

Selenography – study of the moon's surface

Jupiter

Moons

Moons of Jupiter are named after figures who figured in the life of Zeus

Moons

A natural satellite, or moon, is a celestial body that orbits another body (a planet, dwarf planet, or small Solar System body), which is called its primary, and that is not artificial. There are 207 known natural satellites orbiting planets in the Solar System

Shepherd moons – small moon that orbits near the outer edges of planetary rings or within gaps in the rings. The gravity of shepherd moons serves to maintain a sharply defined edge to the ring

Tidal acceleration – an effect of the tidal forces between an orbiting natural satellite (i.e. a moon), and the planet (called the primary) that it orbits. The ‘acceleration’ is usually negative, as it causes a gradual slowing and recession of a satellite, and a corresponding slowdown of the primary's rotation. The process eventually leads to tidal locking of first the smaller, and later the larger body. The Earth-Moon system is the best studied case

Tidal locking – (or captured rotation) occurs when the gravitational gradient makes one side of an astronomical body always face another, an effect known as synchronous rotation. For example, the same side of the Moon always faces the Earth

Trojan – a minor planet or natural satellite (moon) that shares an orbit with a larger planet or moon, but does not collide with it. The term originally referred to the Trojan asteroids orbiting around Jupiter's Lagrangian points. Both Tethys and Dione have two trojan moons

Late Heavy Bombardment – period of time approximately 4.1 to 3.8 billion years ago during which a large number of impact craters are believed to have formed on the Moon, and by inference on Earth, Mercury, Venus, and Mars as well

A ray system comprises radial streaks of fine ejecta thrown out during the formation of an impact crater, looking like many thin spokes coming from the hub of a wheel

Exoplanets

An exoplanet or extrasolar planet is a planet that does not orbit the Sun and instead orbits a different star, stellar remnant, or brown dwarf. More than 1800 exoplanets have been discovered (December 2014). There are also rogue planets, which do not orbit any star and which tend to be considered separately

The Kepler space telescope has also detected a few thousand candidate planets

In 1995, Michel Mayor and Didier Queloz of the University of Geneva announced the first definitive detection of an exoplanet orbiting an ordinary main-sequence star (51 Pegasi)

Hot Jupiters are a class of extrasolar planets whose characteristics are similar to Jupiter, but which have high surface temperatures because they orbit very close to their parent stars

Super-Earth – an extrasolar planet with a mass higher than Earth's, but substantially below the mass of the Solar System's smaller gas giants Uranus and Neptune

SuperWASP (Wide Angle Search for Planets) is an international academic organization performing an ultra-wide angle search for transiting extrasolar planets. WASP-1b was found in 2006

A habitable zone (HZ) is a region of space where conditions are favorable for life as it can be found on Earth. There are two regions that must be favorable, one within a solar system and the other within the galaxy. Planets and moons in these regions are the likeliest candidates to be habitable and thus capable of bearing extraterrestrial life. Life is most likely to form within the circumstellar habitable zone (CHZ) within a solar system, and the galactic habitable zone (GHZ) of the larger galaxy.The HZ may also be referred to as the ‘life zone’, ‘Green Belt’ or the ‘Goldilocks Zone’

Doppler spectroscopy (also known as the radial-velocity method, or colloquially, the wobble method) is an indirect method for finding extrasolar planets from radial velocity measurements via observation of Doppler shifts in the spectrum of the star around which the planet orbits. Otto Struve proposed in 1952 the use of powerful spectrographs to detect distant planets. He described how a very large planet would cause its parent star to wobble slightly as the two objects orbit around their centre of mass

Dwarf planets

A dwarf planet is defined, for the purposes of our solar system only, as a celestial body that:

(a) is in orbit around the Sun,

(b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape,

(c) has not cleared the neighbourhood around its orbit, and

(d) is not a satellite

The IAU currently recognizes five dwarf planets—Ceres, Pluto, Haumea, Makemake, and Eris. However, only two of these bodies, Ceres and Pluto, have been observed in enough detail to demonstrate that they fit the definition

Ceres is a dwarf planet in the asteroid belt. Its name is derived from the Roman goddess Ceres, the goddess of growing plants and of motherly love. It was the first asteroid to be discovered, in 1801 by Piazzi in Palermo

Eris orbits the Sun in the scattered disk accompanied by a moon, Dysnomia. Discovered by Mike Brown at the Mount Palomar observatory. Formerly known as 2003UB313, Xena and Lila. Named after Eris, the Greek mythological goddess personifying discord. Her Latin name is Discordia. Largest dwarf planet

Haumea is a dwarf planet in the Kuiper belt. Its mass is one-third the mass of Pluto. It was discovered in 2004 by a team headed by Mike Brown of Caltech at the Palomar Observatory. In 2008, it was accepted as a dwarf planet and named after Haumea, the Hawaiian goddess of childbirth

Haumea has two moons – Hi’iaka and Namaka

Makemake was discovered in 2005 by Mike Brown. Its diameter is roughly three-quarters that of Pluto. Makemake is named after the creator of humanity and god of fertility in the mythos of the Rapanui, the native people of Easter Island

Pluto was discovered by Clyde Tombaugh in 1930. The name Pluto was proposed by Venetia Burney, an 11-year-old schoolgirl from Oxford

Pluto was classified as a planet until 2006

Pluto and Charon are now considered to be a double dwarf planet

Charon was discovered by James Christie in 1978

Pluto has four smaller moons – Nix and Hydra, both discovered in 2005, and Kerberos and Styx, both named in 2013

Small Solar System Bodies

Small Solar System Body (SSSB) is a term defined in 2006 by the International Astronomical Union to describe objects in the Solar System that are neither planets nor dwarf planets:

This encompasses:

• all minor planets apart from the dwarf planets, i.e.
• the classical asteroids, (except for 1 Ceres, the largest);
• the Centaurs and Neptune Trojans;  the trans-Neptunian Objects (except for dwarf  planets such as Pluto, Makemake and Eris);
• all comets

Minor planet – an astronomical object in direct orbit around the Sun that is neither a planet nor originally classified as a comet. Minor planets can be dwarf planets, asteroids, trojans, centaurs, Kuiper belt objects, and other trans-Neptunian objects. The orbits of 620,000 minor planets were archived at the Minor Planet Center by 2013

Trans-Neptunian objects

A trans-Neptunian object (TNO) is any object in the solar system that orbits the Sun at a greater distance on average than Neptune. The first trans-Neptunian object to be discovered was Pluto. The largest known trans-Neptunian objects are Eris and Pluto, followed by Makemake and Haumea. The Kuiper belt, Scattered disc, and Oort cloud are names for three divisions of this volume of space

Sedna is a trans-Neptunian object which may qualify as a dwarf planet. Sedna is named after the Inuit goddess who created the sea creatures of the Arctic. Passes through the Kuiper asteroid belt

Orcus is a TNO in the Kuiper belt with a large moon, Vanth. It is a large plutino (an object in 2:3 orbital resonance with Neptune) and is likely a dwarf planet

1992 QB1, was the first trans-Neptunian object to be discovered after Pluto and Charon. It was discovered in 1992 at the Mauna Kea Observatory, Hawaii

Asteroids

Asteroids are minor planets, especially those of the inner Solar System. The larger ones have also been called planetoids

In 1975, an asteroid taxonomic system based on colour, albedo, and spectral shape was developed by Chapman, Morrison, and Zellner. These properties are thought to correspond to the composition of the asteroid's surface material. Originally, they classified only three types of asteroids:

•   C-type asteroids – carbonaceous, 75% of known asteroids
•   S-type asteroids – silicaceous, 17% of known asteroids
•   M-type asteroids – metallic, 8% of known asteroids

Kirkwood gap – a gap in the distribution of the orbital periods of the orbits of main-belt asteroids. They correspond to the locations of orbital resonances with Jupiter

Ceres was the first asteroid to be discovered, in 1801

Pallas is the second asteroid to have been discovered, and it is one of the largest asteroids in the Solar System

Vesta is the second-most-massive asteroid after the dwarf planet Ceres and it comprises an estimated 9% of the mass of the asteroid belt. The less-massive Pallas is slightly larger, making Vesta third in volume. Vesta is the only asteroid that can be seen by the naked eye

433 Eros was the first near-Earth asteroid discovered

1036 Ganymed is the largest near-Earth asteroid

1566 Icarus is an Apollo asteroid that at perihelion comes closer to the Sun than Mercury

Asteroid belt – the region of the Solar System located roughly between the orbits of Mars and Jupiter

Apollo asteroids are a group of near-Earth asteroids named after 1862 Apollo, the first asteroid of this group which was discovered by Karl Wilhelm Reinmuth. They are Earth-crosser asteroids

3753 Cruithne is an asteroid in orbit around the Sun in 1:1 orbital resonance with that of the Earth. It has been incorrectly called ‘Earth's second moon’. Named after a people of early medieval Ireland

Centaurs

Centaurs are Small Solar System Bodies with a semi-major axis between those of the outer planets. They have unstable orbits that cross or have crossed the orbits of one or more of the giant planets

The first centaur to be discovered was 944 Hidalgo in 1920. However, they were not recognized as a distinct population until the discovery of 2060 Chiron in 1977. The largest confirmed centaur is 10199 Chariklo

Three centaurs (including Chiron) have been found to display cometary comas and are therefore classified as both asteroids and comets

Neptune trojans

Neptune trojans are bodies in orbit around the Sun that orbit near one of the stable Lagrangian points of Neptune. They therefore have approximately the same orbital period as Neptune and follow roughly the same orbital path. Nine Neptune trojans are currently known

Comets

Comet is an icy Small Solar System Body that, when close enough to the Sun, displays a visible coma (a thin, fuzzy, temporary atmosphere) and sometimes also a tail. These phenomena are both due to the effects of solar radiation and the solar wind upon the nucleus of the comet. Comet nuclei are themselves loose collections of ice, dust, and small rocky particles, ranging from a few hundred metres to tens of kilometers across

Short-period comets are generally defined as having orbital periods of less than 200 years

Long-period comets have highly eccentric (elongated) orbits and periods ranging from 200 years to thousands or even millions of years

Comet tail always points directly away from the sun, pushed by the solar wind

Sungrazer – a comet that passes extremely close to the Sun at perihelion

Halley’s comet returns c. every 76 years, last seen in 1986, will next appear in 2061

Hale-Bopp was most widely observed comet of the 20th century, and was dubbed the Great Comet of 1997

Great Comet of 1860, also known as Newton's Comet, has the distinction of being the first comet discovered by telescope

Encke's Comet is a periodic comet that completes an orbit of the Sun once every three years – the shortest period of any known comet

Shoemaker-Levy 9 comet was torn into 21 fragments known as ‘string of pearls’, which crashed into Jupiter in 1994

Comets could have provided up to half the water in the earth’s oceans

Main-belt comets are bodies orbiting within the main asteroid belt which have shown cometary activity during part of their orbit

Comet Elst–Pizarro is a body that displays characteristics of both asteroids and comets, and is the prototype of main-belt comets. Its orbit keeps it within the asteroid belt, yet it displayed a dust tail like a comet while near perihelion

Maria Mitchell was an American astronomer, who in 1847, by using a telescope, discovered a comet which as a result became known as the ‘Miss Mitchell's Comet’

Outer Solar System

Kuiper belt – a great ring of debris similar to the asteroid belt, but consisting mainly of objects composed primarily of ice. It extends between 30 and 50 AU from the Sun. Though it is estimated to contain anything from dozens to thousands of dwarf planets, it is composed mainly of small Solar System bodies

Scattered disc – a distant region of the Solar System that is sparsely populated by icy minor planets, a subset of the broader family of trans-Neptunian objects. The scattered disc overlaps the Kuiper belt but extends much further outwards, and is thought to be the source of short-period comets

Oort cloud – a hypothetical spherical cloud of  icy objects that is believed to be the source for all long-period comets and to surround the Solar System at roughly 50,000 AU. It is believed to be composed of comets that were ejected from the inner Solar System by gravitational interactions with the outer planets

Heliopause – the theoretical boundary where the Sun's solar wind is stopped by the interstellar medium; where the solar wind's strength is no longer great enough to push back the stellar winds of the surrounding stars

Meteors and meteor showers

2008 TC₃ was a meteoroid 2 to 5 m in diameter that entered Earth's atmosphere in 2008. The meteoroid was notable as the first such body to be observed and tracked prior to reaching Earth. It exploded an estimated 37 km above the Nubian Desert in Sudan

Murchison meteorite is named after Murchison, Victoria, in Australia. It is one of the most studied meteorites due to its large mass (>100 kg), the fact that it was an observed fall, and it belongs to a group of meteorites rich in organic compounds. Murchison contains common amino acids such as glycine, alanine and glutamic acid

Chelyabinsk meteor entered Earth's atmosphere over Russia in 2013. The object exploded in an air burst, producing many surviving small fragmentary meteorites, as well as a large shock wave. The largest found fragments having a total mass of 654 kg were raised from the bottom of the Chebarkul lake

Neumann lines or Neumann bands are fine patterns of parallel lines seen in cross-sections of many iron meteorites

Most meteorites are stony meteorites, classed as chondrites and achondrites. Only about 6% of meteorites are iron meteorites or a blend of rock and metal, the stony-iron meteorites

Chondrites are stony meteorites that have not been modified due to melting or differentiation of the parent body. Most meteorites that are recovered on Earth are chondrites

Hexahedrites are a type of iron meteorite

Radiant or apparent radiant of a meteor shower is the point in the sky, from which (to a planetary observer) meteors appear to originate

Zenithal Hourly Rate (ZHR) of a meteor shower is the number of meteors a single observer would see in one hour under a clear, dark sky

Leonids are a prolific meteor shower associated with the comet Tempel-Tuttle. The Leonids get their name from the location of their radiant in the constellation Leo: the meteors appear to radiate from that point in the sky. Visible in November

Comet Swift-Tuttle is the parent body of the Perseid meteor shower. The shower is visible from mid-July each year, with the peak in activity being between August 9 and 14

Quadrantids are an easily visible January meteor shower

Geminids – a meteor shower caused by the object 3200 Phaethon, which is thought to be a Palladian asteroid. This would make the Geminids, together with the Quadrantids, the only major meteor showers not originating from a comet. The meteors from this shower can be seen in December

Orionid meteor shower, usually shortened to the Orionids, is the most prolific meteor shower associated with Halley's Comet. Orionids are an annual meteor shower which last approximately one week in late-October

Giovanni Schiaparelli demonstrated that the Perseids and Leonids meteor showers were associated with comets

Auroras

Auroras result from emissions of photons in the Earth's upper atmosphere, from ionized nitrogen molecules regaining an electron, and oxygen atoms and nitrogen molecules returning from an excited state to ground state. They are ionized or excited by the collision of solar wind and magnetospheric particles being funneled down and accelerated along the Earth's magnetic field lines; excitation energy is lost by the emission of a photon, or by collision with another atom or molecule. Oxygen emissions are green, nitrogen emissions are blue or red

In northern latitudes, the effect is known as the aurora borealis (or the northern lights), named after the Roman goddess of dawn, Aurora, and the Greek name for the north wind, Boreas, by Galileo. Its southern counterpart, the aurora australis (or the southern lights)

Kristian Birkeland first elucidated the nature of the Aurora borealis. In order to fund his research on the aurorae, he invented the electromagnetic cannon and the Birkeland-Eyde process of fixing nitrogen from the air

Sun

Sun takes 220 million years to travel round the galaxy

Diameter of Sun – 1.4 million km

Sun is 93 million miles from Earth (1 AU). Light takes 8 minutes to reach Earth

Core of the Sun has a temperature of close to 13.6 million kelvin (K). By contrast, the Sun's surface temperature is approximately 5800 K

Sun is 71% hydrogen, 27% helium

The Sun formed about 4.6 billion years ago from the gravitational collapse of a region within a large molecular cloud

Photosphere of an astronomical object is the region from which externally received light originates. The visible surface of the Sun

Chromosphere – the lower level of the solar atmosphere between the photosphere and the corona

Solar transition region is a region of the Sun's atmosphere, between the chromosphere and corona. It is visible from space using telescopes that can sense ultraviolet

Corona – the set of coloured rings around the Sun (or moon) created when it shines through a thin cloud. Corona is hotter than the surface of the Sun

Coronal loops form the basic structure of the lower corona and transition region of the Sun. These highly structured and elegant loops are a direct consequence of the twisted solar magnetic flux within the solar body. Coronal loops are often found with sunspots at their footpoints

Footpoint – the intersection of tubes of magnetic field lines with the surface of the photosphere

Prominence – a large bright feature extending outward from the Sun's surface, often in a loop configuration. Prominences are anchored to the Sun's surface in the photosphere, and extend outwards into the Sun's corona. Large eruptions of luminous hydrogen gas that rise thousands of kilometers above the chromosphere

Sunspot – a cooler darker spot appearing periodically on the Sun's photosphere; associated with a strong magnetic field

Solar flare – a sudden eruption of hydrogen gas on the surface of the Sun, usually associated with sunspots

Intense solar flares are known as ‘X-class’ flares

Spicule – a dynamic jet of about 500 km diameter on the Sun. It moves upwards at about 20 km/s from the photosphere

Solar maximum is a normal period of greatest solar activity in the 11 year solar cycle of the Sun. During solar maximum, large numbers of sunspots appear and the sun's irradiance output grows by about 0.1%

Sunspots were first observed telescopically in late 1610 by the English astronomer Thomas Harriot and Frisian astronomers Johannes and David Fabricius

Galileo discovered that the Sun rotated by observing sunspots

Wolf number – a quantity that measures the number of sunspots and groups of sunspots present on the surface of the Sun

Grand solar minima occur when several solar cycles exhibit lesser than average activity for decades or centuries. Solar cycles still occur during these grand solar minimum periods but the intensity of those cycles are less intense

Maunder Minimum – the period roughly spanning 1645 to 1715 when sunspots became exceedingly rare. The Maunder Minimum coincided with a period of lower-than-average global temperatures

Dalton Minimum – the period roughly spanning 1790 to 1830 when sunspots became exceedingly rare

Solar wind – a stream of charged particles ejected from the upper atmosphere of the Sun. It mostly consists of electrons and protons. These particles can escape the Sun's gravity because of their high kinetic energy and the high temperature of the corona. The solar wind creates the heliosphere, a vast bubble in the interstellar medium that surrounds the solar system. Other phenomena include geomagnetic storms that can knock out power grids on Earth, the aurorae (northern and southern lights), and the plasma tails of comets that always point away from the Sun

The solar wind flows outward from the Sun until encountering the termination shock, where motion slows abruptly. The heliosheath is the region of the heliosphere beyond the termination shock. The outermost edge of the heliosphere is called the heliopause

A coronal mass ejection (CME) is a massive burst of solar wind, other light isotope plasma, and magnetic fields rising above the solar corona or being released into space

1859 Solar Superstorm, or the Carrington Event, was a powerful geomagnetic solar storm in. A solar flare and/or coronal mass ejection hit Earth's magnetosphere and induced the largest known solar storm

Solar mass, 1.989 x 10³⁰ kg, is a standard way to express mass in astronomy, used to describe the masses of other stars and galaxies. It is equal to the mass of the Sun

Gravity at the surface of the Sun is 28 times higher than the gravity on earth

Sun’s radiation – ultraviolet A, ultraviolet B, ultraviolet C

Helioseismology – the study of the propagation of wave oscillations, particularly acoustic pressure waves, in the Sun

Galileo discovered that Sun rotated by tracking movement of sunspots

Fred Hoyle realised that the Sun would become a red giant, then a white dwarf

Yarkovsky effect – photons from the Sun affect path of asteroids

Helioscope – an instrument used in observing the Sun. The helioscope was first used by Benedetto Castelli and refined by Galileo. The method involves projecting an image of the Sun onto a white sheet of paper suspended in a darkened room with the use of a telescope

George Hale is is known for inventing the spectroheliograph, with which he made his discoveries of the solar vortices and magnetic fields of sunspots

Stars

A star is a massive, luminous sphere of plasma held together by its own gravity. The nearest star to Earth is the Sun

Stars contain 11% of atoms in the universe

Most elements up to atomic number 26 (iron) were made inside stars

Magnitude – a measure of a star’s brightness; the higher the number, the fainter the star

Apparent magnitude – a measure of its brightness as seen by an observer on Earth, normalized to the value it would have in the absence of the atmosphere

Absolute magnitude – the apparent magnitude an object would have if it were at a standard luminosity distance (10 parsecs, or 1 AU, depending on object type) away from the observer

Dwarf – a small, low-luminosity star

White dwarfs are very dense; a white dwarf's mass is comparable to that of the Sun and its volume is comparable to that of the Earth

Giant – a large and luminous star with a low density

A white dwarf is produced when a low or medium mass star dies. These stars are not heavy enough to generate the core temperatures required to fuse carbon in nucleosynthesis reactions

Chandrasekhar limit – the theoretical upper limit to the mass of a stable white dwarf star, 2.8 x 10³⁰ kg, i.e. 1.4 times the mass of the Sun. Above the Chandrasekhar limit, white dwarfs collapse into neutron stars. Only stars more than three times the Sun’s mass can shrink further to produce black holes

A red giant is a star of 0.4–10 times the mass of the Sun which has exhausted the supply of hydrogen in the core and switched to fusing hydrogen in a shell outside the core. They have radii tens to hundreds of times larger than that of the Sun. However, their outer envelope is lower in temperature, giving them an orange hue

Red giants produce carbon and oxygen. Become white dwarves

Sun will become a red giant in c. five billion years

Red dwarf stars are small, cool, faint stars on the main sequence that constitute the vast majority of stars in the Milky Way and have a diameter and mass of less than one-third that of the Sun. Proxima Centauri and Barnard’s Star are both red dwarf stars

Proxima Centauri is rich in magnesium. Will become a white dwarf

Neutron star – the collapsed core of a massive star remaining after a supernova explosion. Such stars are composed almost entirely of neutrons

Magnetar – type of neutron star with an extremely powerful magnetic field, the decay of which powers the emission of copious amounts of high-energy electromagnetic radiation, particularly X-rays and gamma rays

Supergiant – a star of enormous size, low density and great luminosity

Black dwarf is a hypothetical stellar remnant, created when a white dwarf becomes sufficiently cool to no longer emit significant heat or light

Brown dwarfs are sub-stellar objects which are too low in mass to sustain stable hydrogen fusion

The Sun is a G2V star – a main-sequence dwarf star of moderate temperature (yellow dwarf star of spectral type G and luminosity class V)

Most stars are currently classified using the letters O, B, A, F, G, K, and M, where O stars are the hottest (blue) and the letter sequence indicates successively cooler stars up to the coolest M class (red)

Double star – a pair of stars that appear close to each other in the sky as seen from Earth when viewed through an optical telescope

Cepheid – pulsating variable star, used to measure distances to other galaxies. Henrietta Leavitt's discovery of the period-luminosity relation of Cepheid variables radically changed the theory of modern astronomy, an accomplishment for which she received almost no recognition during her lifetime. Her work was the basis for the work of astronomer Edwin Hubble

Compact star – used to refer collectively to white dwarfs, neutron stars, other exotic dense stars, and black holes. Most compact stars are at the endpoint of stellar evolution and are thus called stellar remnants. A compact star which is not a black hole, may be called a degenerate star

Nova – a cataclysmic nuclear explosion caused by the accretion of hydrogen onto the surface of a white dwarf star, which ignites and starts nuclear fusion in a runaway manner

Supernova – a stellar explosion that is more energetic than a nova. Supernovae are extremely luminous and cause a burst of radiation that often briefly outshines an entire galaxy. It may undergo sudden gravitational collapse into a neutron star or black hole

SN 1572 (Tycho's Supernova) was a supernova of Type Ia in the constellation Cassiopeia, one of about eight supernovae visible to the naked eye in historical records. It burst forth in 1572

Supernovae can be triggered in one of two ways: by the sudden reignition of nuclear fusion in a degenerate star; or by the collapse of the core of a massive star

In 1934 Fritz Zwicky and Walter Baade coined the term ‘supernova’ and hypothesized that they were the transition of normal stars into neutron stars, as well as the origin of cosmic rays

If a supernova's spectrum contains lines of hydrogen (known as the Balmer series in the visual portion of the spectrum) it is classified Type II; otherwise it is Type I

Type Ia supernovas all explode with the same brightness, so they are used to calculate distances from Earth

Hertzsprung–Russell diagram is a scatter graph of stars showing the relationship between the stars' absolute magnitudes or luminosity versus their spectral types or classifications and effective temperatures. Stars on this band are known as main-sequence stars or ‘dwarf’ stars. Stars tend to fall only into certain regions of the diagram. The most prominent is the diagonal, going from the upper-left (hot and bright) to the lower-right (cooler and less bright), called the main sequence. In the lower-left is where white dwarfs are found, and above the main sequence are the subgiants, giants and supergiants

Pulsars are highly magnetized, rotating neutron stars that emit a beam of electromagnetic radiation in the form of radio waves. The radiation can only be observed when the beam of emission is pointing towards the Earth. This is called the lighthouse effect and gives rise to the pulsed nature that gives pulsars their name

The first pulsar was discovered in 1967, by Jocelyn Bell Burnell and Antony Hewish of the University of Cambridge. While using a radio array to study the scintillation of quasars, they found a very regular signal, consisting of pulses of radiation at a rate of one in every few seconds. Initially baffled as to the seemingly unnatural regularity of its emissions, the pair dubbed their discovery LGM-1, for ‘little green men’

Crab Pulsar is a relatively young neutron star. The star is the central star in the Crab Nebula, a remnant of the supernova which was observed on Earth in the year 1054. Discovered in 1968, the pulsar was the first to be connected with a supernova remnant

Quasar (contraction of QUASi-stellAR radio source) is an astronomical source of electromagnetic energy, including radio waves and visible light. A compact region in the centre of a massive galaxy that surrounds its central supermassive black hole

The first quasars were discovered with radio telescopes in the late 1950s

The term quasar was coined by Chinese-born U.S. astrophysicist Hong-Yee Chiu in 1964

Donald Lynden-Bell is an English astrophysicist, best known for his theories that galaxies contain massive black holes at their centre, and that such black holes are the principal source of energy in quasars

Wolf-Rayet (WR) stars are evolved, massive stars (over 20 solar masses initially), which are losing mass rapidly by means of a very strong stellar wind. Wolf–Rayet stars are extremely hot

An open cluster is a group of up to a few thousand stars that were formed from the same giant molecular cloud and have roughly the same age

A globular cluster is a spherical collection of stars that orbits a galactic core as a satellite. Globular clusters are very tightly bound by gravity

Population III, or metal-free stars, are a hypothetical extinct population of extremely massive and hot stars with virtually no surface metals

A molecular cloud, sometimes called a stellar nursery (if star formation is occurring within), is a type of interstellar cloud, the density and size of which permit the formation of molecules, most commonly molecular hydrogen

One of James Jeans' major discoveries, named Jeans length, is a critical radius of an interstellar cloud in space

Sirius is the brightest star in the night sky, almost twice as bright as Canopus, the next brightest star. It is known colloquially as the ‘Dog Star’, reflecting its prominence in its constellation, Canis Major

Sirius is a binary star system consisting of two stars (Sirius A and Sirius B) orbiting each other with a separation of about 20 astronomical units

Sirius B is a white dwarf. This star is primarily composed of a carbon–oxygen mixture that was generated by helium fusion

Apparent magnitudes – Sun -26.8, Sirius A -1.46, Canopus -0.72, Arcturus -0.06

Absolute magnitudes – Sun 4.83, Canopus – 5.53

Alpha Centauri is the closest star system to our own solar system at 4.37 light-years distant. Proxima Centauri, usually regarded as part of the system, is the closest star (apart from the Sun) at 4.22 light-years distant

Proxima Centauri – apparent magnitude 11

Alpha Centauri – apparent magnitude 0

Regulus is the brightest star in Leo

Barnard’s Star – six light-years away from Earth in the constellation of Ophiuchus

Vega is the brightest star in the constellation Lyra, the fifth brightest star in the night sky and the second brightest star in the northern celestial hemisphere, after Arcturus

Altair is the largest star in the constellation Aquila

Vega will become a Pole Star

Summer Triangle of stars – Vega, Altair and Deneb

Deneb is the brightest star in the constellation Cygnus. A blue-white supergiant, Deneb is also one of the most luminous nearby stars

Winter Hexagon has vertices at Rigel, Aldebaran, Capella, Pollux, Procyon, and Sirius. Smaller and more regularly shaped is the Winter Triangle, an approximately equilateral triangle that shares two vertices (Sirius and Procyon) with the larger asterism. The third vertex is Betelgeuse

Lucy is a diamond star in constellation Centaurus

Antares is a red supergiant star in Scorpius constellation

Rigel is the brightest star in the constellation Orion

Betelgeuse is in the constellation Orion

Betelgeuse is a red supergiant. It is likely that the star will continue to burn and fuse elements until its core is iron, at which point Betelgeuse will explode as a type II supernova. During this event the core will collapse, leaving behind a neutron star

Arcturus is the brightest star in the constellation Bootes. Red giant

Pleiades, or Seven Sisters (Messier object 45 or M45), is an open star cluster containing middle-aged hot B-type stars located in the constellation of Taurus

Hyades is a star cluster in Taurus

Aldebaran is an orange giant star in Taurus. Known as the ‘eye’ of Taurus

Almach is a double star in Andromeda

Plough or the Big Dipper is an asterism of seven stars that has been recognized as a distinct grouping in many cultures from time immemorial. The comprising stars are the seven brightest of the formal constellation Ursa Major

Thuban – Pole Star in Egyptian times

Gamma Cephei will succeed Polaris as the Earth's northern pole star, c. 5200, due to the precession of the equinoxes

Nebulae

A nebula is an interstellar cloud of dust, hydrogen, helium and other ionized gases

Orion nebula – the closest region of massive star formation to Earth

Nebulae are often star-forming regions. In these regions the formations of gas, dust, and other materials clump together to form larger masses, which attract further matter, and eventually will become massive enough to form stars

Pillars of Creation is a photograph taken by the Hubble Telescope of elephant trunks (formations of interstellar matter found in space) of interstellar gas and dust in the Eagle Nebula, some 7000 light years from Earth

Coalsack Dark Nebula is the most prominent dark nebula in the skies, easily visible to the naked eye as a dark patch silhouetted against the southern Milky Way. It was observed by Vincente Yanez Pinzon in 1499. The Coalsack is located at a distance of approximately 600 light years away from Earth, in the Crux constellation

Crab nebula – a supernova remnant and pulsar in the constellation of Taurus. The nebula was first observed in 1731 by John Bevis

Rosette nebula – in Monoceros constellation

The Horsehead Nebula is a dark nebula in the Orion constellation. The nebula is located just below Alnitak, the easternmost star of Orion's Belt, and is part of the much larger Trapezium, or Orion Trapezium Cluster is a group of stars in the heart of the Orion Nebula, in the constellation of Orion. It was discovered by Galileo

Tarantula Nebula is in the Large Magellanic Cloud. Tarantula Nebula is the most active starburst region known in the Local Group of galaxies

Nebular hypothesis – the most widely accepted model explaining the formation and evolution of the Solar System. According to the nebular hypothesis, stars form in massive and dense clouds of molecular hydrogen – giant molecular clouds (GMC). They are gravitationally unstable, and matter coalesces to smaller denser clumps within, which then proceed to collapse and form stars

Dark nebula – a type of interstellar cloud that is so dense that it obscures the light from the background emission or reflection nebula (e.g. the Horsehead Nebula) or that it blocks out background stars (e.g., the Coalsack Nebula). The extinction of the light is caused by interstellar dust grains located in the coldest, densest parts of larger molecular clouds

Frost line, also known as the snow line or ice line, refers to a particular distance in the solar nebula from the central sun where it is cold enough for hydrogen compounds such as water, ammona, and methane to condense into solid ice grains. The frost line of the Solar System is around 5 AU

Constellations

A constellation is a group of stars into which the sky is divided for the purposes of identifying and naming celestial objects. There are 88 constellations. The largest constellation is Hydra (the water snake), the smallest constellation is Crux (the southern cross)

Apus – Bird of Paradise

Auriga – Charioteer

Bootes – herdsman

Camelopardalis – Giraffe

Canes Venatici – Hunting Dogs

Cetus – Whale

Circinus – Compasses

Columba – Dove

Cygnus – Swan

Delphinus – Dolphin

Dorado – Swordfish

Draco – Dragon

Grus – Crane

Lacerta – Lizard

Lepus – Hare

Monoceros – Unicorn

Musca – Fly

Ophiuchus – Serpent-bearer

Pavo – Peacock

Pyxix – Mariner’s Compass

Reticulum – Net

Scutum – Shield

Vela – Sails

Volans – Flying Fish

Vulpecala – Fox

Cancer the crab – faintest constellation

Alpha – brightest star in a constellation

Cassiopeia – W shaped constellation, formed by five bright stars

Zodiac – the ring of constellations that lines the ecliptic, which is the apparent path of the Sun across the sky over the course of the year. The Moon and planets also lie within the ecliptic, and so are also within the constellations of the zodiac

Ursa Minor – constellation that is the location of the north celestial pole, that currently has nearly the same coordinates as the bright star Polaris

Polaris (commonly North(ern) Star or Pole Star) is the brightest star in the constellation Ursa Minor

The constellation Octans is the location of the south celestial pole

Virgo is the second largest constellation in the sky. It can be easily found through its brightest star, Spica

Orion Molecular Cloud Complex. It is approximately 1500 light years from Earth

Hunting dogs of Orion – Canis Major and Canis Minor

Procyon is the brightest star in the constellation Canis Minor

Stars in Orion’s belt – Alnitak (zeta), Alnilam (epsilon), and Mintaka (delta)

Hubble Ultra Deep Field is an image of a small region of space in the constellation Fornax. It is the deepest image of the universe ever taken, looking back approximately 13 billion years

Constellation of Ophiuchus occupies most of the sign of Sagittarius

Galaxies

Galaxy – large aggregation of stars, gas, and dust, typically containing billions of stars

Starburst galaxy – a galaxy undergoing an exceptionally high rate of star formation

Active galactic nucleus (AGN) is a compact region at the centre of a galaxy that has a much higher than normal luminosity over at least some portion, and possibly all, of the electromagnetic spectrum. A galaxy hosting an AGN is called an active galaxy

Local Group – the group of galaxies that includes Earth's galaxy, the Milky Way. The group comprises more than 30 galaxies (including dwarf galaxies), with its gravitational centre located somewhere between the Milky Way and the Andromeda Galaxy

Andromeda is the largest galaxy of the Local Group

Milky Way has 200 billion stars

Andromeda – nearest galaxy to the Milky Way

Andromeda is 2.5 million light years away

Milky Way and Andromeda are locked by gravity

Hubble discovered that Andromeda was a separate galaxy

Pinwheel Galaxy is in the constellation Ursa Major

Antennae Galaxies are a pair of interacting galaxies in the constellation Corvus. They are currently going through a phase of starburst. They were discovered by William Herschel in 1785

Seyfert galaxies are a class of galaxies with nuclei that produce spectral line emission from highly ionized gas. Named after Carl Seyfert

Large Magellanic Cloud (LMC) is a nearby irregular galaxy, and is a satellite of the Milky Way. It is the third closest galaxy to the Milky Way

Small Magellanic Cloud (SMC) is a dwarf galaxy. It contains several hundred million stars

Radio galaxies are types of active galaxy that are very luminous at radio wavelengths. The radio emission is due to the synchrotron process. Cygnus A is one of the most famous radio galaxies

The Hubble sequence is a classification of galaxy types developed by Edwin Hubble in 1925. It is also called the tuning-fork diagram due to the shape of its graphical representation. Types of galaxy – elliptical (most galaxies), spiral (e.g. the Milky Way, which is a barred spiral) and irregular

Dwarf spheroidal galaxies are low luminosity galaxies that are companions to the Milky Way and to the similar systems that are companions to the Andromeda Galaxy

Dwarf elliptical galaxies elliptical galaxies that are much smaller than other galzxies

Radio galaxy – intense source of cosmic radio waves

Whirlpool Galaxy is a spiral galaxy that is estimated to be 23 million light-years from the Milky Way

Sombrero Galaxy is an unbarred spiral galaxy in the constellation Virgo

Jacobus Kapteyn was a Dutch astronomer, best known for his extensive studies of the Milky Way and as the first discoverer of evidence for galactic rotation in 1904

Eclipses

The Sun's distance from the Earth is about 390 times the Moon's distance, and the Sun's diameter is about 400 times the Moon's diameter. Because these ratios are approximately the same, the Sun and the Moon as seen from Earth appear to be approximately the same size in a solar eclipse

A total eclipse occurs when the Sun is completely obscured by the Moon. The intensely bright disk of the Sun is replaced by the dark outline of the Moon, and the much fainter corona is visible. During any one eclipse, totality is visible only from at most a narrow track on the surface of the Earth.

Next UK total eclipse will be in 2090

An annular eclipse occurs when the Sun and Moon are exactly in line, but the apparent size of the Moon is smaller than that of the Sun. Hence the Sun appears as a very bright ring, or annulus, surrounding the outline of the Moon

A hybrid eclipse shifts between a total and annular eclipse

Solar eclipse can only occur during a new moon

Total lunar eclipse occurs when Moon passes into Earth's shadow

Danjon Scale of lunar eclipse brightness is a five-point scale useful for measuring the appearance and luminosity of the Moon during a lunar eclipse. An eclipse's rating on the Danjon Scale is denoted by the letter L (0 to 4)

Saros – a period of 223 synodic months that can be used to predict eclipses of the Sun and Moon

The umbra (Latin for ‘shadow’) is the darkest part of the shadow, where the light source is completely blocked by the occluding body. An observer in the umbra experiences a total eclipse

The penumbra (from the Latin paene ‘almost, nearly’ and umbra ‘shadow’) is the region in which only a portion of the light source is obscured by the occluding body. An observer in the penumbra experiences a partial eclipse

A penumbral eclipse occurs when the Moon passes through the Earth’s penumbra. The penumbra causes a subtle darkening of the Moon's surface

The antumbra is the region from which the occluding body appears entirely contained within the disc of the light source. If an observer in the antumbra moves closer to the light source, the apparent size of the occluding body increases until it causes a full umbra. An observer in this region experiences an annular eclipse

A selenelion or selenehelion occurs when both the Sun and the eclipsed Moon can be observed at the same time

Black holes

John Wheeler is widely credited with coining the term black hole in 1967. Originally a black hole was known as a frozen star. John Wheeler also worked with Niels Bohr in explaining the basic principles behind nuclear fission

Event horizon – boundary of a black hole

Singularity – the zero-volume, infinitely dense region at the centre of a black hole, where matter is crushed to infinite density, the pull of gravity is infinitely strong, and spacetime has infinite curvature

Rotating black holes have an ergosphere, a region bounded by: on the outside, an oblate spheroid which coincides with the event horizon at the poles and is noticeably wider around the ‘equator’. This boundary is sometimes called the ‘ergosurface’, but it is just a boundary and has no more solidity than the event horizon. At points exactly on the ergosurface, space-time is dragged around at the speed of light

Rotating black holes were first described by Roy Kerr

There is a supermassive black hole at the centre of every galaxy

Sagittarius A* is a bright and very compact astronomical radio source at the centre of the Milky Way Galaxy, near the border of the constellations Sagittarius and Scorpius. It is believed to be the location of a supermassive black hole

Stellar mass black hole – a black hole formed by the gravitational collapse of a massive star (more massive than about 20 solar masses) at the end of its lifetime. The process is observed as a supernova explosion or as a gamma ray burst

Hawking radiation – thermal radiation thought to be emitted by black holes due to quantum effects. It is named after Stephen Hawking who worked out the theoretical argument for its existence in 1974, becoming the first insight into quantum gravity

S = A/4 – Hawking entropy equation. The black hole entropy (S) is proportional to the area of its event horizon (A)

Spaghettification – a body falling into a black hole would be stretched into long thin shapes like spaghetti

Cygnus X-1 was the first X-ray source widely accepted to be a black hole candidate. Discovered by Paul Murdin

Schwarzschild radius – the radius of a sphere in space, that if containing a correspondingly sufficient amount of mass (and therefore, reaches a certain density), the force of gravity from the contained mass would be so great that no known force could stop the mass from continuing to collapse in volume into a point of infinite density: a gravitational singularity (a black hole)

Accretion disc – a structure formed by diffuse material in orbital motion around a central body. The central body is typically a young star, a white dwarf, a neutron star, or a black hole. Gravity causes material in the disc to spiral inward towards the central body

Cosmology

Current interpretations of astronomical observations indicate that the age of the Universe is 13.8 billion years

Big Bang Theory – universe came out of a singularity. Quarks and leptons, together with their antiparticles, dominated the universe less than a second after its beginning. A slight imbalance between quarks and antiquarks then resulted in the annihilation of most antiparticles, creating an immense amount of electromagnetic radiation. After a microsecond the universe cooled sufficiently for the quarks to combine to form protons and neutrons

Big Bang nucleosynthesis – the production of nuclei other than those of the lightest isotope of hydrogen during the early phases of the universe. Primordial nucleosynthesis is believed to have taken place between approximately 10 seconds until 20 minutes after the Big Bang, and is calculated to be responsible for the formation of most of the universe's helium as isotope He-4, along with small amounts of deuterium (H-2 or D), the helium isotope He-3, and a very small amount of the lithium isotope Li-7

Cosmic inflation – the theorized exponential expansion of the universe at the end of the grand unification epoch, 10⁻³⁶ seconds after the Big Bang. The inflationary hypothesis was originally proposed in 1980 by American physicist Alan Guth

Inflation replaced the hot big bang model

300,000 years after the universe was formed, at the end of the photon epoch, the ambient temperature had fallen below 10,000 degrees. Electrons and positively-charged atomic nuclei combined to form neutral atoms. Electromagnetic radiation was set free and light could roam across the universe

Hydrogen and helium atoms began to form as the density of the universe falls. This is thought to have occurred about 377,000 years after the Big Bang

Baryon asymmetry problem – the apparent fact that there is an imbalance in baryonic matter and antibaryonic matter in the universe. Neither the standard model of particle physics, nor the theory of general relativity provide an obvious explanation for why this should be so; and it is a natural assumption that the universe be neutral with all conserved charges. The Big Bang should have produced equal amounts of matter and antimatter, as such, there should have been total cancellation of both. In other words, protons should have cancelled with antiprotons, electrons with antielectrons (positrons), neutrons with antineutrons, and so on for all elementary particles. This would have resulted in a sea of photons in the universe with no matter. Since this is evidently not the case, after the Big Bang, some physical laws must have acted differently for matter and antimatter

Baryogenesis – generic term for hypothetical physical processes that produced an asymmetry between baryons and antibaryons in the very early universe, resulting in the substantial amounts of residual matter that make up the universe today

Steady State theory (also known as the Infinite Universe theory or continuous creation) is a model, now discredited, developed in 1948 by Fred Hoyle, Thomas Gold, Hermann Bondi and others as an alternative to the Big Bang theory

Georges Lemaitre was the first person to propose the theory of the expansion of the Universe, widely misattributed to Edwin Hubble. Lemaitre also proposed what became known as the Big Bang theory of the origin of the Universe, which he called his 'hypothesis of the primeval atom'

In physical cosmology, the Big Crunch is one possible scenario for the ultimate fate of the universe, in which the metric expansion of space eventually reverses and the universe recollapses, ultimately ending as a black hole singularity

Big Freeze – Observations suggest that the expansion of the universe will continue forever. If so, the universe will cool as it expands, eventually becoming too cold to sustain life. For this reason, this future scenario is popularly called the Big Freeze. Also known as Heat death of the universe, its final thermodynamic state in which it has diminished to a state of no thermodynamic free energy to sustain motion or life. It has reached maximum entropy

Big Rip – the matter of the universe, from stars and galaxies to atoms and subatomic particles, is progressively torn apart by the expansion of the universe at a certain time in the future

Big Bounce – Big Crunch followed by a Big Bang

Dark energy – a hypothetical form of energy which permeates all of space and has strong negative pressure. According to the theory of relativity, the effect of such a negative pressure is qualitatively similar to a force acting in opposition to gravity at large scales. Invoking such an effect is currently the most popular method for explaining recent observations that the universe appears to be expanding at an accelerating rate, as well as accounting for a significant portion of the missing mass in the universe. Term was coined by Michael Turner. In the standard model of cosmology, dark energy currently accounts for 74% of the total mass-energy of the universe

Quintessence – a hypothetical form of dark energy postulated as an explanation of observations of an accelerating universe. It has been proposed by some physicists to be a fifth fundamental force

Dark matter – a conjectured form of matter that is undetectable by its emitted electromagnetic radiation, but whose presence can be inferred from gravitational effects on visible matter and background radiation

Dark matter is estimated to constitute 84% of the matter in the universe and 23% of the mass-energy

Dark matter was postulated by Fritz Zwicky in 1934 to account for evidence of ‘missing mass’ in the orbital velocities of galaxies in clusters

Vera Rubin uncovering the discrepancy between the predicted angular motion of galaxies and the observed motion. This phenomenon became known as the galaxy rotation problem. This discrepancy can be accounted for by a large amount of dark matter that permeates the galaxy

Dark flow – a net motion of galaxy clusters with respect to the cosmic microwave background radiation which was found in a 2008 study. May indicate a multiverse

When a galaxy or a planetary system forms, its material takes the shape of a disk. Most of the material orbits and rotates in one direction. This uniformity of motion is due to the collapse of a gas cloud. The nature of the collapse is explained by the principle called conservation of angular momentum

Cosmic microwave background radiation (most often abbreviated CMB, also referred to as relic radiation) is a form of electromagnetic radiation discovered in 1965 that fills the entire universe. The high degree of uniformity throughout the observable universe and its faint but measured anisotropy lend strong support for the Big Bang model in general

COsmic Background Explorer (COBE), also referred to as Explorer 66, was a satellite dedicated to cosmology. Its goals were to investigate the cosmic microwave background radiation of the universe. This work provided evidence that supported the Big Bang theory of the universe: that the CMB was a near-perfect black-body spectrum and that it had very faint anisotropies

Wilkinson Microwave Anisotropy Probe (WMAP), also referred to as Explorer 80, is a spacecraft which measures differences in the temperature of the Big Bang's remnant radiant heat, the Cosmic Microwave Background Radiation , across the full sky

Planck is a space observatory launched in 2009 by the European Space Agency (ESA) and designed to observe the anisotropies of the cosmic microwave background (CMB) over the entire sky, at a high sensitivity and angular resolution. Planck is the successor to WMAP

BICEP (Background Imaging of Cosmic Extragalactic Polarization) and the Keck Array are a series of CMB experiments. They aim to measure the polarization of the CMB; in particular, measuring the B-mode of the CMB. In 2014, the collaboration announced that BICEP2 had made the first detection of so-called B-mode polarization, a possible signature of inflation (gravitational waves) in the very early Universe. Data from the Planck spacecraft showed that 75% of the signal was due to galactic dust

Temperature of Cosmic Microwave Background Radiation is 3^oK

The multiverse is the hypothetical set of multiple possible universes that together comprise everything that exists and can exist: the entirety of space, time, matter, and energy as well as the physical laws and constants that describe them. The term was coined in 1895 by the American philosopher and psychologist William James. The various universes within the multiverse are sometimes called parallel universes

Hydrogen line, 21 centimetre line or HI line refers to the electromagnetic radiation spectral line that is created by a change in the energy state of neutral hydrogen atoms. The line is of great interest in big bang cosmology because it is the only known way to probe the ’dark ages’ from recombination to reionization

Hubble's law is the statement in physical cosmology that the redshift in light coming from distant galaxies is proportional to their distance. Galaxies recede from us with speeds that increase with their distance. The law is often expressed by the equation v = H₀D, with H₀ the constant of proportionality (the Hubble constant) between the distance D to a galaxy and its velocity v

Hubble constant – 70 (km/s)/Mpc (Megaparsec)

Hubble's law can be easily depicted in a Hubble Diagram in which the velocity (assumed approximately proportional to the redshift) of an object is plotted with respect to its distance from the observer

The cosmological constant (usually denoted by the Greek capital letter lambda: Λ) is the value of the energy density of the vacuum of space. It was proposed by Albert Einstein as a modification of his original theory of general relativity to achieve a stationary universe. Einstein abandoned the concept after the observation of the Hubble redshift indicated that the universe might not be stationary, as he had based his theory on the idea that the universe is unchanging

ΛCDM (Lambda Cold Dark Matter) or Lambda-CDM model is a parametrization of the Big Bang cosmological model in which the universe contains a cosmological constant, denoted by Lambda, associated with dark energy, and cold dark matter (abbreviated CDM). It is frequently referred to as the standard model of Big Bang cosmology

WIMPs (Weakly Interacting Massive Particles) are hypothetical particles serving as one possible solution to the dark matter problem. These particles interact through the weak nuclear force and gravity, and possibly through other interactions no stronger than the weak force. Because they do not interact with electromagnetism they cannot be seen directly, and because they do not interact with the strong nuclear force they do not react strongly with atomic nuclei

MACHO (Massive Astrophysical Compact Halo Object) is the general name for any kind of astronomical body that might explain the apparent presence of dark matter in galaxy halos. The term was chosen whimsically by contrast with WIMP

DarkSide collaboration is an international affiliation of universities and labs seeking to directly detect dark matter in the form of WIMPs

Gamma-ray bursts – (GRB) the most luminous events known in the universe since the Big Bang. They are flashes of gamma rays coming from seemingly random places in deep space at random times

Roger Penrose won the 1988 Wolf Prize for physics, which he shared with Stephen Hawking for their contribution to our understanding of the universe

Most abundant elements in the universe – hydrogen, helium, oxygen

Tables and catalogues

Alfonsine Tables provided data for computing the position of the Sun, Moon and planets relative to the fixed stars. They are named for Alfonso X of Castile, upon whose order they were prepared in Toledo, Spain around 1252 to 1270

The Rudolphine Tables consist of a star catalog and planetary tables published by Johannes Kepler in 1627. Named after Emperor Rudolf II, they contain positions for the 1006 stars measured by Tycho Brahe, and 400 and more stars from Ptolemy and Johann Bayer, with directions and tables for locating the planets of the solar system

Bayer designation is a stellar designation in which a specific star is identified by a Greek letter, followed by the genitive form of its parent constellation's Latin name. The original list of Bayer designations contained 1564 stars. Most of the brighter stars were assigned their first systematic names by the German astronomer Johann Bayer in 1603, in his star atlas Uranometria

Messier objects are a set of astronomical objects first listed by French astronomer Charles Messier in his Catalogue of Nebulae and Star Clusters published in 1771

There are 110 Messier objects, including M1 (Crab nebula), M31 (Andromeda galaxy), M42 (Orion nebula) and M45 (Pleiades)

New General Catalogue of Nebulae and Clusters of Stars (abbreviated as NGC) is a catalogue of 7840 deep-sky objects in astronomy compiled by John Louis Emil Dreyer in 1888, as a new version of John Herschel's Catalogue of Nebulae and Clusters of Stars. NGC 1 is a spiral galaxy located 190 million light-years away in the constellation Pegasus

The Gliese catalogue attempts to list all stars within 20 parsecs of Earth ordered by right ascension

Catalogue of Galaxies and of Clusters of Galaxies was compiled by Fritz Zwicky in 1961 – 1968

Carte du Ciel (literally, 'Map of the Sky') and the Astrographic Catalogue were two distinct but connected components of a massive international astronomical project, initiated in the late 19th century, to catalogue and map the positions of millions of stars

Henry Draper Catalogue is a star catalogue published between 1918 and 1924, giving spectroscopic classifications for 225,300 stars

Father Angelo Secchi was a pioneer in astronomical spectroscopy, and was one of the first scientists to state authoritatively that the Sun is a star. Published La Stelle (‘The Stars’) in 1877

Observation

Charles II founded the Royal Greenwich Observatory in 1675 and appointed the first Astronomer Royal John Flamsteed. Astronomer Royal was originally the title of the director of the Royal Greenwich Observatory, but since 1972 it has simply been an honorary title. Edmond Halley was the second Astronomer Royal. Martin Rees is the current Astronomer Royal

Edmond Halley edited Newton’s Principia Mathematica (1687)

John Flamsteed was refusing to publish work that had been commissioned by the king, and in 1712 Newton and Edmond Halley published a preliminary version of Flamsteed's Historia Coelestis Britannica without crediting the author. Some years later, Flamsteed managed to buy many copies of the book, and publicly burnt them in front of the Royal Observatory

Nevil Maskelyne was the fifth English Astronomer Royal, from 1765 to 1811. Helped to measure longitude, latitude, and calculated the density of the earth

George Airy was Astronomer Royal from 1835 to 1881. He established Greenwich as the location of the prime meridian

Uraniborg was an astronomical observatory operated by Tycho Brahe; built c. 1580 on Hven, an island in the Oresund between Zealand and Scania, at that time belonging to Denmark

During the course of his career, Herschel constructed more than four hundred telescopes. The largest and most famous of these was a reflecting telescope with a 49+ inches diameter primary mirror and a 40 feet focal length

Leviathan of Parsonstown is the unofficial name of the Rosse six foot telescope. This is a historic reflecting telescope of 72 in (1.8 m) aperture, which was the largest telescope in the world from 1845 until the construction of the 100 in (2.5 m) Hooker Telescope in 1917. The Rosse six foot telescope was built by William Parsons, 3rd Earl of Rosse, on his estate, Birr Castle, at Parsonstown (now Birr in County Offaly, Ireland)

Hooker telescope is at Mount Wilson Observatory, California. Hubble determined that some nebulae were actually galaxies outside our own Milky Way, and discovered the presence of the redshift that indicated the universe is expanding

Hale telescope (60 in) at Mount Wilson Observatory built to test Hubble’s theory that the universe was expanding. Mirror manufactured in New York and transported to California by train

Yerkes Observatory is operated by the University of Chicago in Williams Bay, Wisconsin. The Observatory has the world's largest refracting telescope

Bolshoi telescope is in the Caucasus. Largest telescope in the world when it opened in 1975

Large Binocular Telescope (LBT) is at Mount Graham, Arizona. Largest telescope. 2 x 8.4m mirrors

James Webb Space Telescope – replacement for Hubble Space Telescope. Due to be launched in 2018. It will have a large and very cold 6.5 m diameter mirror covered in gold, an observing position far from Earth, orbiting the Earth–Sun L₂ point

Stratospheric Observatory for Infrared Astronomy (SOFIA) is a joint project of NASA and the German Aerospace Center (DLR) to construct and maintain an airborne observatory. SOFIA is based on a Boeing 747 wide-body aircraft that has been modified to include a large door in the aft fuselage that can be opened in flight to allow a reflecting telescope access to the sky

ANTARES is the name of a neutrino detector residing 2.5 km under the Mediterranean Sea off the coast of Toulon. It is designed to be used as a directional Neutrino Telescope to locate and observe neutrino flux from cosmic origins in the direction of the Southern Hemisphere of the Earth, a complement to the northern hemisphere neutrino detector IceCube

IceCube is a neutrino telescope constructed at the Amundsen-Scott South Pole Station in Antarctica. Similar to its predecessor, the Antarctic Muon And Neutrino Detector Array (AMANDA)

Lovell Telescope is a radio telescope at Jodrell Bank Observatory, near Goostrey, Cheshire. When it was constructed in the mid 1950s, the telescope was the largest steerable dish radio telescope in the world. Bernard Lovell was the first Director of Jodrell Bank

Multi-Element Radio Linked Interferometer Network (MERLIN) is an interferometer array of radio telescopes spread across England. The array is run from Jodrell Bank Observatory

Lowell Observatory Near-Earth Object Search (LONEOS) was a project designed to discover asteroids and comets that orbit near the Earth that ran until 2008

The Very Large Array (VLA) is a radio astronomy observatory located on the Plains of San Augustin, 50 miles west of Socorro, New Mexico

The Very Large Telescope Project (VLT) is a system of four separate optical telescopes organized in an array formation. The project is organized by the European Southern Observatory. VLT is located at the Paranal Observatory on Cerro Paranal, a mountain in the Atacama Desert in northern Chile

Palomar Observatory is located in San Diego County, California, 90 miles southeast of Mount Wilson Observatory, on Palomar Mountain. It is owned and operated by the California Institute of Technology (Caltech). The observatory currently consists of four main instruments, including the 200 inch Hale Telescope

The Arecibo Observatory in Puerto Rico is operated by Cornell University under cooperative agreement with the National Science Foundation. The observatory's 305m radio telescope is the largest single-aperture telescope ever constructed. It was featured in the film GoldenEye

The Atacama Large Millimetre Array (ALMA) is an international astronomy project that consists of an astronomical interferometer formed from an array of radio telescopes, located in the Atacama Desert in northern Chile

The Galaxy Evolution Explorer (GALEX) is an orbiting ultraviolet space telescope that was launched in 2003. The first observation (called ‘First Light’) was dedicated to the crew of the Space Shuttle Columbia

The Lick Observatory is an astronomical observatory, owned and operated by the University of California. It is situated on the summit of Mount Hamilton

Along with the Faulkes Telescope North and the Faulkes Telescope South, the Liverpool Telescope is available for use by school children around the world over the internet

The Observatories at Mauna Kea, (MKO), are an independent collection of astronomical research facilities located on the summit of Mauna Kea on the Big Island of Hawaii. The facilities are located in a 500-acre special land use zone known as the ‘Astronomy Precinct’

James Clerk Maxwell Telescope (JCMT) is an infrared telescope with a primary mirror diameter of 15 metres. It is located at Mauna Kea Observatory in Hawaii. It is the largest astronomical telescope in the world designed specifically to operate in the submillimetre regime (between the far-infrared and the microwave regions of the electromagnetic spectrum)

UKIRT, the United Kingdom Infra-Red Telescope, is a 3.8 metre infrared reflecting telescope, the largest dedicated infrared telescope in the world. It is located on Mauna Kea, Hawaii as part of Mauna Kea Observatory

Southern African Large Telescope (SALT) is a 66m² area optical telescope. It is located close to the town of Sutherland in the semi-desert region of the Karoo

Square Kilometre Array (SKA) is a radio telescope project to be built in Australia and South Africa which would have a total collecting area of approximately one square kilometre. It will operate over a wide range of frequencies and its size will make it 50 times more sensitive than any other radio instrument. Construction of the SKA is scheduled to begin in 2018

Herschel Space Observatory is a European Space Agency space observatory sensitive to the far infrared and submillimetre wavebands. It is the largest space telescope ever launched, carrying a single mirror of 3.5 metres in diameter. The observatory was carried into orbit in May 2009, reaching the second Lagrangian point (L₂) of the Earth-Sun system, 1,500,000 kilometres (930,000 mi) from the Earth, two months later

LOFAR is the LOw Frequency ARray for radio astronomy. LOFAR will be the largest radio telescope ever built, using a new concept based on a vast array of omni-directional antennas. The project is based on an interferometric array of radio telescopes using 15,000 small antennas and 77 larger stations, mostly in the Netherlands

Solar and Heliospheric Observatory (SOHO) is a spacecraft built Astrium that was launched in 1995 to study the Sun, and has discovered over 2700 comets

Solar Dynamics Observatory (SDO) is a NASA mission which will observe the Sun for over five years. Launched in 2010, the observatory is part of the Living With a Star (LWS) program

Space Weather Prediction Center (SWPC), formerly the Space Environment Center (SEC), is a laboratory and service center of the National Oceanic and Atmospheric Administration (NOAA) located in Boulder, Colorado. SWPC continually monitors and forecasts Earth's space environment, providing solar-terrestrial information

The Corona program was a series of American strategic reconnaissance satellites. The Corona satellites were used for photographic surveillance of the Soviet Union, the People's Republic of China, and other areas beginning in 1959 and ending in 1972

MAGIC (Major Atmospheric Gamma-ray Imaging Cherenkov Telescopes) is a system of two telescopes situated at the Roque de los Muchachos Observatory on La Palma, one of the Canary Islands. MAGIC detects particle showers released by gamma rays, using the Cherenkov radiation, i.e., faint light radiated by the charged particles in the showers

Kepler space observatory was launched in 2009. Kepler mission is ‘specifically designed to survey a portion of our region of the Milky Way galaxy to discover dozens of Earth-size planets in or near the habitable zone and determine how many of the billions of stars in our galaxy have such planets’

European Extremely Large Telescope (E-ELT) is currently being built by the European Southern Observatory (ESO) on top of Cerro Armazones in the Atacama Desert of northern Chile

Giant Magellan Telescope (GMT) is a ground-based extremely large telescope in Chile planned for completion in 2020

CryoSat is an ESA programme which will monitor variations in the extent and thickness of polar ice through use of a satellite in low Earth orbit

Minor Planet Center operates at the Smithsonian Astrophysical Observatory in Cambridge, Massachusetts. It is the official organization in charge of collecting observational data for minor planets and comets

Sentinel is a space observatory currently being developed by Ball Aerospace for the private philanthropic B612 Foundation. The foundation is dedicated to the protection of Earth from asteroid strikes

National Solar Observatory operates facilities at two locations – one at Sacramento Peak near Sunspot in New Mexico, and the other at Kitt Peak in Arizona

Liverpool Telescope – fully robotic telescope on La Palma

Isaac Newton Group of Telescopes operates the William Herschel Telescope, Isaac Newton Telescope and Jacobus Kapteyn Telescope optical telescopes on La Palma

Most large telescopes have spectrometers, which are used either to measure the chemical composition and physical properties of astronomical objects or to measure their velocities from the Doppler shift of their spectral lines

Balloon satellite (also referred to as a satelloon) is a satellite that is inflated with gas after it has been put into orbit

Sloan Digital Sky Survey is a redshift survey which aims to map 25% of the sky and obtain observations on around 100 million objects and spectra for 1 million objects

Gaia is a space observatory of the European Space Agency. The mission aims to construct a 3D space catalog of approximately 1 billion astronomical objects. It has a billion pixel camera

The Wow! signal was a strong narrowband radio signal detected by Jerry Ehman in 1977, while working on a SETI project at The Big Ear radio telescope of The Ohio State University. The signal bore expected hallmarks of potential non-terrestrial and non-solar system origin. It lasted for the full 72 second duration that Big Ear observed it, but has not been detected again

Glossary

Aberration of light – (also referred to as astronomical aberration or stellar aberration) is an astronomical phenomenon which produces an apparent motion of celestial objects about their locations dependent on the velocity of the observer

Absorption spectroscopy – spectroscopic techniques that measure the absorption of radiation, as a function of frequency or wavelength. Shows what elements exist in a star or the atmosphere of an exoplanet

Albedo – the ratio of the outgoing solar radiation reflected by an object to the incoming solar radiation incident upon it

Aphelion – point where a planet is furthest from the Sun

Apogee – point in the orbit of the moon or of an artificial satellite most distant from the centre of the earth

Apparition – period of time a planet is visible

Asterism – a pattern of stars seen in Earth's sky which is not an official constellation

Astronomical unit – (AU) standard unit of measure to represent the distances of objects within our solar system from the Sun. One AU is 149,597,870 kilometers, c. 93 million miles, (the mean distance between the Earth and the Sun over one Earth orbit)

Barycentre – the point between two celestial objects where they balance each other

Bolide – an exploding meteor, or fireball

Celestial equator – a great circle on the imaginary celestial sphere, in the same plane as the Earth's equator. It is a projection of the terrestrial equator out into space. As a result of the Earth's axial tilt, the celestial equator is inclined by 23.4° with respect to the ecliptic plane

Celestial mechanics – the branch of astronomy that deals with the motions of celestial objects

Celestial sphere – an imaginary rotating sphere of ‘gigantic radius’, concentric and coaxial with the Earth. All objects in the sky can be thought of as lying upon the sphere

Conjunction – as seen from some place (usually the Earth), two celestial bodies appear near one another in the sky

Cosmic dawn – the time, a few hundred million years after the Big Bang, when the first stars and galaxies burst into existence

Cosmic rays – energetic particles originating from outer space that impinge on Earth's atmosphere. About 89% of all the incoming cosmic ray particles are simple protons, with nearly 10% being helium nuclei (alpha particles), and slightly under 1% are heavier elements; electrons (beta particles) constitute about 1% of galactic cosmic rays. The term ‘ray’ is a misnomer, as cosmic particles arrive individually, not in the form of a ray or beam of particles

Declination – the celestial sphere equivalent of terrestrial latitude

Earthshine – reflected earthlight visible on the Moon's night side. It is also known as the Moon's ashen glow or as ‘the old Moon in the new Moon's arms’

Ecliptic – the apparent path the Sun traces out along the sky, independent of Earth's rotation in the course of the year

Elongation – the angular distance between the Sun and any other object in the sky

Equinox – from the Latin for ‘equal’ and ‘night’. Vernal equinox in March, autumn equinox in September, in the northern hemisphere

First light – when a telescope (or, in general, a new instrument) is first used to take an astronomical image after it has been constructed

Geostationary orbit – a geosynchronous orbit directly above the Earth's equator (0° latitude), with a period equal to the Earth's rotational period and an orbital eccentricity of approximately zero. A geostationary satellite has an altitude of 22,200 miles above the Earth’s surface

Geosynshronous orbit – an orbit around a planet or moon with an orbital period that matches the planet or moon's sidereal rotation period

Heliometer – an instrument originally designed for measuring the variation of the Sun's diameter at different seasons of the year

Inclination – of the orbit of a planet is defined as the angle between the plane of the orbit of the planet and the ecliptic, which is the plane containing Earth's orbital path

Interstellar cloud – the generic name given to an accumulation of gas, plasma and dust in our and other galaxies

Lagrangian points – the five positions in an orbital configuration where a small object affected only by gravity can theoretically be stationary relative to two larger objects (such as a satellite with respect to the Earth and Moon). The Lagrange points mark positions where the combined gravitational pull of the two large masses provides precisely the centripetal force required to orbit with them

Lamb shift, named after Willis Lamb, is a small difference in energy between two energy levels of the hydrogen atom. Used by astronomers to detect minor wobbles in a stars’ position

Libration – an oscillating motion of orbiting bodies relative to each other, notably including the motion of the Moon relative to Earth

Light year – the distance travelled by light in space during a year. 5.88 x 10¹² miles

Luminosity – measures the total amount of energy emitted by a star or other astronomical object per unit time. The Sun has 1.00 solar luminosity

Lunation – the mean time for one lunar phase cycle (i.e., the synodic period of the Moon)

Magnetosphere – the area of space near an astronomical object in which charged particles are controlled by that object's magnetic field. Near the surface of the object, the magnetic field lines resemble those of a magnetic dipole

Meteor – a bright trail or streak that appears in the sky when a meteoroid is heated to incandescence by friction with the earth's atmosphere. Also known as a shooting star

Meteor stream – a transient shower of meteors when a meteor swarm enters the earth's atmosphere

Meteor swarm – a very powerful and spectacular spell that is similar to fireball

Meteorite – stony or metallic object that is the remains of a meteoroid that has reached the earth's surface

Meteoroid – any of the small solid extraterrestrial bodies that hits the earth's atmosphere

Metonic cycle – moon returns to the same position amongst the stars every 19 years

Nadir – a point on the celestial sphere directly below the observer

Observable universe – the galaxies and other matter that we can in principle observe from Earth in the present day, because light (or other signals) from those objects has had time to reach us since the beginning of the cosmological expansion

Occultation – an event that occurs when one celestial object is hidden by another celestial object that passes between it and the observer

Olbers' paradox – the argument that the darkness of the night sky conflicts with the assumption of an infinite and eternal static universe. It is one of the pieces of evidence for a non-static universe such as the current Big Bang model. Thomas Digges was the first to expound the Copernican system in English in 1576 but discarded the notion of a fixed shell of immoveable stars to postulate infinitely many stars at varying distances; he was also first to postulate the ‘dark night sky paradox’ (now known as Olbers’ paradox)

Opposition – celestial bodies are said to be in opposition when they are on opposite sides of the sky, viewed from a given place

Orbital eccentricity – a parameter that determines the amount by which its orbit around another body deviates from a perfect circle. A value of 0 is a circular orbit, values between 0 and 1 form an elliptical orbit, 1 is a parabolic escape orbit, and > 1 is a hyperbolic escape orbit

Orbital resonance – occurs when two orbiting bodies exert a regular, periodic gravitational influence on each other

Orbital velocity – the speed of a satellite in orbit around a planet

Parallax – an apparent displacement or difference in the apparent position of an object viewed along two different lines of sight, and is measured by the angle or semi-angle of inclination between those two lines. Friedrich Bessel is credited with being the first to use parallax in calculating the distance to a star

Parsec – the distance from the Sun which would result in a parallax of 1 second of arc as seen from Earth. From parallax and second, 3.25 light years

Perigee – point in the orbit of the moon or of an artificial satellite closest to the centre of the earth

Perihelion – point where a planet is nearest to the Sun

Planisphere – a star chart analog computing instrument in the form of two adjustable disks that rotate on a common pivot. It can be adjusted to display the visible stars for any time and date

Precession of the equinoxes – westward motion of the equinoxes along the ecliptic, discovered by Hipparchus. Variation in the direction of the Earth’s axis of rotation. Also known as axial precession

Prograde – all the planets and almost all of the other objects that revolve in orbit around the Sun, with the exception of many comets, do so in the ‘prograde’ direction, i.e the same sense as the rotation of the Sun. Most planets also rotate in the same prograde direction, the exceptions being Venus and Uranus, which have retrograde rotations

Quadrature – that aspect of a heavenly body in which it makes a right angle with the direction of the Sun

Redshift – change in the wavelength of light, first observed by Hubble

Sidereal period – the time that it takes the object to make one full orbit around the Sun, relative to the stars. This is considered to be an object's true orbital period

Right ascension – the celestial sphere equivalent of terrestrial longitude

Roche limit – the distance within which a celestial body, held together only by its own gravity, will disintegrate due to a second celestial body's tidal forces exceeding the first body's gravitational self-attraction. Typically, the Roche limit applies to a satellite disintegrating due to tidal forces induced by its primary, the body about which it orbits

Scintillation or twinkling – generic terms for rapid variations in apparent brightness or colour of a distant luminous object viewed through a medium, most commonly the atmosphere

Solstice – either of the two times of the year when the Sun is at its greatest distance from the celestial equator

Sun dog or parhelion – an atmospheric phenomenon that creates bright spots of light in the sky, often on a luminous ring or halo on either side of the sun

Synodic – conjunction of two bodies, as seen from the Sun

Synodic period – the time that it takes for the object to reappear at the same point in the sky, relative to the Sun, as observed from Earth; i.e. returns to the same elongation

Syzygy – the alignment of three celestial bodies in the same gravitational system along a straight line

Vacuum energy – an underlying background energy that exists in space throughout the entire universe

White hole – a hypothetical region of spacetime which cannot be entered from the outside; but matter and light can escape from it. The reverse of a black hole

Zenith – a point on the celestial sphere directly above the observer

Zodiacal light – a faint, roughly triangular, whitish glow seen in the night sky which appears to extend up from the vicinity of the Sun along the ecliptic or zodiac