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Astronomy glossary

69 terms used in Celestial Atlas, defined in one or two sentences each. Terms link to the pages where they appear in context.

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Albedo: The fraction of incoming sunlight a surface reflects. Earth's is about 0.30, Venus's about 0.76. It sets a planet's equilibrium temperature.
Altitude and azimuth: A position in the sky measured from your horizon: altitude is the angle above the horizon (0° to 90°), azimuth the compass bearing (N 0°, E 90°, S 180°, W 270°). Used in the Sky view.
Annular eclipse: A solar eclipse in which the Moon is too far from Earth to cover the Sun completely, leaving a bright ring (“ring of fire”). See eclipses.
Aphelion and perihelion: The farthest and nearest points of a body's orbit around the Sun. Earth's perihelion is about 147.1 million km in early January; its aphelion about 152.1 million km in early July.
Apogee and perigee: The farthest and nearest points of the Moon's orbit around Earth (about 405,500 km and 363,300 km).
Arcsecond: 1/3,600 of a degree. The Moon's disc is about 1,800 arcseconds wide; the accuracy of the atlas's planet positions is a few arcseconds.
Asymptotic giant branch (AGB): The final giant phase of a Sun-like star, after helium burning, when it swells again and sheds its outer layers. See the Sun's life.
Astronomical unit (AU): The average distance between Earth and the Sun, about 149.6 million km; light needs 499 seconds to cross it.
Barred spiral galaxy: A spiral galaxy with a straight bar of stars across its centre. The Milky Way, NGC 1300 and the Large Magellanic Cloud have bars.
Black dwarf: The hypothetical end state of a white dwarf after it has cooled completely, expected to take far longer than the age of the Universe.
Bloom: A graphic effect that adds a soft glow around very bright objects. It can be switched off in Options.
Bulge (galactic): The central, roughly spheroidal swelling of old stars in a spiral galaxy; the Milky Way's is boxy or peanut-shaped.
Celestial sphere: The imaginary sphere on which stars appear to lie, centred on the observer, used to describe directions in the sky.
Chandrasekhar dynamical friction: The gravitational drag a massive body feels as it moves through a sea of lighter particles such as a dark-matter halo, causing galaxies on close orbits to lose energy and merge. Used in the collision simulation.
Civil, nautical and astronomical twilight: Stages of dusk and dawn when the Sun is 0–6°, 6–12° and 12–18° below the horizon. See Sky view.
Climatology: The statistical average of weather for a place, date and time of day over many years. Shown instead of a forecast when none exists.
Conjunction: When two bodies appear at the same ecliptic longitude as seen from Earth; a planet is in conjunction with the Sun when it is in line with it.
Corona: The Sun's outer atmosphere, visible as a white halo only during a total solar eclipse.
Cosmic microwave background (CMB): The faint microwave glow left over from the early Universe. Its slight dipole reveals our motion: the Sun moves at about 370 km/s relative to it.
Dark matter: Invisible matter detected only through its gravity, which makes up most of every galaxy's mass. In the collision simulation it is the halo that does the pulling.
Dwarf spheroidal galaxy: A faint, round or elliptical galaxy with few stars and little gas, such as Draco, Sculptor and Fornax around the Milky Way.
Ecliptic: The plane of Earth's orbit around the Sun, and the apparent yearly path of the Sun among the stars. Planets lie close to it.
Elliptical galaxy: A smooth, roughly ellipsoidal galaxy of old stars with little gas and no spiral arms; the likely product of a major merger.
Elongation: The angle between a planet and the Sun as seen from Earth. Mercury's maximum is about 28°, Venus's about 47°.
ERA5: A global hourly reanalysis of the weather since 1940 produced by ECMWF/Copernicus, used for historical weather in the atlas.
Equilibrium temperature: The temperature a planet would have if it balanced absorbed sunlight against emitted heat, ignoring any greenhouse effect: T = 278.5 K × (1−A)^¼ L^¼ / √a, with albedo A, luminosity L in solar units and distance a in AU.
Equinox and solstice: Moments when the Sun is over the equator (equinox, near 20 March and 22–23 September) or at its farthest north or south (solstice, near 21 June and 21 December).
Gaia: The European Space Agency's astrometry mission (launched 2013) that measures the positions, distances and motions of over a billion stars. See Gaia stars.
Galactic year: One orbit of the Sun around the Galactic Centre, about 223 million years.
Gould Belt: A tilted ring of young stars and star-forming clouds about 700 pc across around the Sun, including Orion and Scorpius–Centaurus.
Great Attractor: A massive concentration of galaxies in the Norma region, about 220 million light-years away, toward which our local supercluster flows.
Habitable zone: The range of distances from a star where a rocky planet could keep liquid water on its surface. Today 0.95–1.68 AU for the Sun by the simple limits used in the atlas.
Helium flash: The sudden ignition of helium fusion in the degenerate core of a red giant, at the tip of the red-giant branch.
Hubble Space Telescope: NASA/ESA's orbiting observatory, launched 1990, source of the photographs of M51, M104, NGC 1300, NGC 4414 and the Antennae used in the atlas.
Julian day: A continuous count of days used by astronomers; the atlas converts every date to a Julian day (and UTC) for its calculations.
Kiloparsec (kpc): 1,000 parsecs, about 3,262 light-years. The Sun is 8.18 kpc from the Galactic Centre.
Laniakea: The supercluster of about 100,000 galaxies, 520 million light-years across, that contains the Milky Way. See Laniakea.
Libration: The apparent wobble of the Moon that lets us see about 59% of its surface over time.
Light-year: The distance light travels in a year: 9.46 trillion km (63,241 AU).
Limiting magnitude: The faintest star visible under given conditions; about 6.5 in a dark sky.
Local Bubble: A cavity about 100 parsecs wide of hot, thin gas around the Sun, carved by supernovae.
Local Group: The gravitationally bound group of about 80 galaxies including the Milky Way, Andromeda and Triangulum.
Magnitude: A logarithmic brightness scale in which smaller numbers are brighter: the Sun −26.7, the full Moon −12.7, Venus up to −4.9, the faintest naked-eye stars about +6.5.
Main sequence: The long phase in a star's life when it fuses hydrogen to helium in its core. The Sun has been on it for 4.57 billion years and will remain for about 5.4 billion more.
Milkomeda: A nickname for the single elliptical galaxy that the Milky Way and Andromeda may form. See the collision.
Moist greenhouse: A climate state in which a warming planet's stratosphere fills with water vapour, which escapes to space; the first step in losing oceans. It begins at roughly 1.1 times Earth's present sunlight.
Node (lunar): One of the two points where the Moon's orbit crosses the ecliptic. Eclipses occur when New or Full Moon happens near a node.
Obscuration: The fraction of the Sun's disc area covered by the Moon during a solar eclipse.
Parallax: The apparent shift of a nearby star against distant ones as Earth orbits the Sun; a star with 1 arcsecond parallax is 1 parsec away.
Parsec (pc): 3.26 light-years; the distance at which one AU subtends one arcsecond.
Peculiar velocity: A body's motion relative to its local rest frame, such as the Sun's 18–20 km/s relative to nearby stars or a galaxy's flow relative to the Hubble expansion.
Penumbra: The partial shadow where only part of the Sun is hidden. In a lunar eclipse, the faint outer part of Earth's shadow.
Planetary nebula: A shell of gas ejected by a dying Sun-like star and lit by its hot core; unrelated to planets.
Prograde and retrograde: Moving or spinning in the same sense as an orbit (prograde) or the opposite sense (retrograde).
Proper motion: A star's apparent change of position on the sky over time, in arcseconds per year. Sirius moves 1.33″ per year.
Red giant: A star that has used up the hydrogen in its core and swollen to tens or hundreds of times its former radius, with a cool, red surface.
Saros: An eclipse cycle of 6,585.3 days (18 years 11 days 8 hours) after which Sun, Moon and nodes return to almost the same geometry, so similar eclipses recur.
Sidereal day: The time Earth takes to rotate once relative to the stars: 23 h 56 min 4 s.
Sidereal and synodic month: The Moon's orbit relative to the stars (27.32 days) and its cycle of phases (29.53 days).
Spiral arm: A curved band of stars, gas and dust in a spiral galaxy. The Milky Way has four main arms; the Sun lies in the Orion Spur between two of them.
Supercluster: A vast region containing many galaxy groups and clusters, tens to hundreds of millions of light-years across.
Supermassive black hole: A black hole of millions to billions of solar masses at the centre of a galaxy. The Milky Way's, Sagittarius A*, has about 4 million.
Synodic period: The time between successive similar alignments, such as the same Moon phase or a planet's return to opposition.
Tidal tail: A long stream of stars pulled out of a galaxy by the gravity of a close neighbour during an encounter.
Totality: The phase of a total solar eclipse when the Sun is completely covered, lasting up to about 7½ minutes.
True scale: A display mode in which bodies and distances are drawn at their real relative sizes (1 scene unit = 1,000 km in the atlas).
Umbra: The darkest part of a shadow, where the light source is fully hidden. Observers in the Moon's umbra see a total solar eclipse.
White dwarf: The dense, Earth-sized remnant of a Sun-like star that has shed its outer layers; the Sun will become one of about 0.54 solar masses.
ΔT (delta T): The difference between uniform time and time based on Earth's irregular rotation; it limits how precisely eclipses far in the past or future can be timed.