Glossary of Astronomical Terms

Angle – The opening between two straight lines that meet at a point.

Angular diameter (angular size) – The arc angle across an object.

Annular eclipse – An eclipse of the Sun in which the Moon is too distant to cover the sun completely so that a ring of sunlight is seen around the Moon at mid-eclipse.

Arc angle – The measurement of the angle between two objects or two parts of the same object.

Asterism – In astronomy, an asterism is a pattern of stars recognized on Earth’s night sky. It may form part of an official constellation, or be composed of stars from more than one. Like constellations, asterisms are in most cases composed of stars which, while they are visible in the same general direction, are not physically related, often being at significantly different distances from Earth. The mostly simple shapes and few stars make these patterns easy to identify, and thus particularly useful to those learning to familiarize themselves with the night sky.

Asteroid – Any of the rocky objects larger than a few hundred meters in diameter (and not classified as a planet or moon) that orbits the Sun.

Astronomical unit (AU, or au), a unit of length effectively equal to the average, or mean, distance between Earth and the Sun, defined as 149,597,870.7 km (92,955,807.3 miles). Alternately, it can be considered the length of the semi-major axis—i.e., the length of half of the maximum diameter—of Earth’s elliptical orbit around the Sun. The astronomical unit provides a convenient way to express and relate distances of objects in the solar system and to carry out various astronomical calculations. For example, stating that the planet Jupiter is 5.2 AU (5.2 Earth distances) from the Sun and that Pluto is nearly 40 AU gives ready comparisons of the distances of all three bodies.

Autumnal equinox – The intersection of the ecliptic and celestial equator where the Sun crosses the equator moving from north to south. The beginning of autumn (around September 23).

Black hole – An object whose gravity is so strong that the escape velocity from it exceeds the speed of light.

Celestial equator – A great circle on the celestial sphere 90° from the celestial poles.

Celestial sphere – A hypothetical sphere of very large radius centered on the observer; the apparent sphere of the night sky.

Circumpolar star – All the stars that never set at a given latitude; all the stars between Polaris and the northern horizon.

Comet – A small body of ice and dust in orbit about the Sun. While passing near the Sun, a comet’s vaporized ices give rise to a coma, tails, and a hydrogen envelope.

Conjunction – A phenomenon which occurs when two astronomical objects have either the same right ascension or the same ecliptical longitude, usually when observed from the Earth.

Constellation – Any of the 88 contiguous regions that cover the entire celestial sphere, including all the objects in each region; also, a configuration of stars often named after an object, a person, or an animal.

Declination – The coordinate on the celestial sphere exactly analogous to latitude on Earth; measured north and south of the celestial equator.

Degree – A unit of angular measure (there are 360 degrees in a circle), or a unit of temperature.

Diurnal motion – Cyclic motion with a 1-day period.

Eclipse path – The track of the tip of the Moon’s shadow along Earth’s surface during a total or annular solar eclipse.

Ecliptic – The annual path of the Sun on the celestial sphere; the plane of Earth’s orbit around the Sun.

Equinox – Either of the two days of the year when the Sun crosses the celestial equator and is therefore directly over Earth’s equator.

Gravitation – The tendency of all matter to attract all other matter.

Lambda – Lambda is another name for wavelength

Light year – A light-year, also light year or light year (symbol: ly), is a unit of length equal to just under 10 trillion kilometers (or about 6 trillion miles). As defined by the International Astronomical Union (IAU), a light-year is the distance that light travels in a vacuum in one Julian year. The light-year is mostly used to measure distances to stars and other distances on a galactic scale, especially in non-specialist and popular science publications. The preferred unit in astrometry is the parsec (approximately 3.26 light-years), because it can be more easily derived from, and compared with, observational data. Note that the light-year is a measure of distance (rather than, as is sometimes misunderstood, a measure of time)

Line of nodes – The line along which the plane of the Moon’s orbit intersects the plane of the ecliptic.

Lunar eclipse – An eclipse during which Earth blocks light that would have struck the moon.

Lunar phases – The names given to the apparent shapes of the Moon as seen from Earth.

Meteoroid – A small rock in interplanetary space.

Neutron star – A neutron star is a type of stellar remnant that can result from the gravitational collapse of a massive star during a Type II, Type Ib or Type Ic supernova event.

North Celestial Pole (NCP) – The location on the celestial sphere directly above Earth’s northern rotation pole.

Opposition – A phenomenon which occurs when the right ascension or ecliptical longitude of two astronomical objects differs by 180°, usually when observed from the Earth.

Parsec – The parsec (symbol: pc) is a unit of length used in astronomy. It is about 3.26 light-years, which is about 30.9 trillion (3.09×1013) kilometers or about 19.2 trillion (1.92×1013) miles. The name parsec is “an abbreviated form of ‘a distance corresponding to a parallax of one second’.”It was coined in 1913 at the suggestion of British astronomer Herbert Hall Turner. A parsec is the distance from the Sun to an astronomical object which has a parallax angle of one arc-second.

Partial eclipse – A lunar or solar eclipse in which the eclipsed object does not appear completely covered.

Penumbra – The portion of a shadow in which only part of the light source is covered by the shadow-making body.

Penumbral eclipse – A lunar eclipse in which the Moon passes only through Earth’s penumbra.

Precession (of Earth) – A slow, conical motion of Earth’s axis of rotation caused by the gravitational pull of the Moon and Sun on Earth’s equatorial bulge.

Precession of the equinoxes – The slow westward motion of the equinoxes along the ecliptic because of Earth’s precession.

Revolution – The orbit of one body about another.

Right Ascension – The celestial coordinate analogous to longitude on Earth and measured around the celestial equator from the vernal equinox.

Rotation – The spinning of a body about an axis passing through it.

Scientific Notation – The style of writing large and small numbers using powers of ten.

Sidereal month – The period of the Moon’s revolution about Earth measured with respect to the Moon’s location among the stars; 27 1/3 Earth days.

Sidereal period – The orbital period of one object about another measured with respect to the stars.

Solar corona – The Sun’s outer atmosphere.

Solar day – From noontime to the next noontime; for Earth it is 24 hours.

Solar eclipse – An eclipse during which the Moon blocks the Sun.

South celestial pole – The location on the celestial sphere directly above Earth’s south rotation pole.

Summer solstice – The point on the ecliptic where the Sun is farthest north of the celestial equator; the day with the largest number of daylight hours in the northern hemisphere, around June 21.

Synodic month (lunar month) – The period of revolution of the Moon with respect to the Sun; the length of one cycle of lunar phases; 29 ½ Earth days.

Terminator – The line dividing day and night on the surface of any body orbiting the Sun; the line of sunset or sunrise.

Time zone – One of 40 divisions of Earth’s surface separated by geographical and political longitudinal lines.

Total eclipse – A solar eclipse during which the Sun is completely hidden by the Moon, or a lunar eclipse during which the Moon in completely immersed in Earth’s umbra.

Umbra – The central, completely dark portion of a shadow.

Vernal equinox – The point on the ecliptic where the Sun crosses the celestial equator from south to north; the beginning of spring (in the Northern Hemisphere) around March 21.

Winter solstice – The point on the ecliptic where the Sun is farthest south of the celestial equator; fewest hours of dayligh t in the northern hemisphere, around December 21.

Zenith – The point on the celestial sphere directly overhead.