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Glossary of Astronomical Terms

A

albedo: surface reflectivity
altitude: the angular distance of an object above or below the horizon
angstrom (Å): commonly used to measure the wavelength of light; equal to 10^-10 meters
aphelion: the outermost point in a solar orbit
apogee: the outermost point in a terrestrial orbit
appulse: the apparent closest approach of two celestial bodies
asteroid: now called a dwarf planet or small solar-system body, depending on its characteristics
astrology: a belief system which claims that the positions of celestial objects can affect life on Earth
astronomical unit (AU): a unit of measurement often used within the solar system; approximately equal to 93,000,000 miles or 8 light minutes, the average distance between the Earth and the Sun
astronomy: the scientific study of celestial objects
azimuth: the angular distance of an object around or parallel to the horizon from a predefined zero point

B

binary star: two stars close together in the sky; they may be gravitationally bound objects or simply a projection effect on the sky
blue moon: 1. the second full moon to fall within a calendar month
2. the third full moon in a season containing four
bolide: an exceptionally bright meteor

C

celestial equator: the Earth's equator projected onto the sky
celestial poles: the Earth's north and south poles projected onto the sky
comet: a small body comprised of a nucleus of dirt and ice; when it nears the Sun, the ice melts, releasing gas and dust, forming a glowing coma around the nucleus and usually a tail pointing away from the Sun.
conjunction: when two bodies appear close together in the sky
constellation: a grouping of stars in the sky to form some kind of pattern; there are 88 officially recognised constellations

D

declination

the angular distance north or south of the celestial equation and measured in degrees °, minutes ' and seconds "

direct motion

eastward motion in the sky

dwarf planet

a celestial body that

is in orbit around the Sun,
has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic (nearly round) shape,
has not cleared the neighbourhood around its orbit, and
is not a satellite

(official IAU definition dated 24 August 2006)

E

eccentricity: a parameter that describes the shape of an orbit; the closer the eccentricity is to zero, the more circular the orbit
ecliptic: the path the Sun takes against the background stars; the apparent positions of the Moon and the planets are usually quite close to the ecliptic
elongation: the angular distance of a body from the Sun as seen from the Earth; a planet at greatest eastern elongation is seen in the evening sky and a planet at greatest western elongation is seen in the morning sky
ephemeris: a table of data arranged by date; typically tables of the positions of the Sun, Moon, planets and other solar system objects
equinox: the two points where the ecliptic intersects the celestial equator; when the Sun appears on the celestial equator

F

fireball: bolide

G

galaxy: a gravitationally bound collection of millions or billions of stars, gas, and dust
geocentric: having the Earth as the centre
globular cluster: a spherical cluster of older stars found in the halos of galaxies

H

heliocentric: having the Sun as the centre

I

inferior conjuction: when an inferior planet passes between the Sun and the Earth so that they are lined up in the order Sun, inferior planet, Earth
inferior planet: a planet whose orbit is inside the Earth's orbit around the Sun: Mercury and Venus

L

light year (ly): a unit of measurement often used for interstellar or intergalactic distances; the distance light travels in a vacuum during a year; approximately equal to 6 × 10¹² miles
limb: the edge of the apparent disc of a celestial object
lunar eclipse: occurs when the Moon passes through the shadow cast by the Earth; the three types of lunar eclipses are partial, penumbral and total
lunation: the cycle from new Moon to new Moon

M

magnitude: brightness scale; the smaller the magnitude number, the brighter the object so that the Sun (magnitude -26) is much brighter than Uranus (magnitude +6)
meridian: an imaginary line connecting a point due south on the horizon with a point due north on the horizon and going through the zenith
meteor: the trail of light left when a meteoroid enters the Earth's atmosphere and burns up
meteorite: a meteoroid that enters the Earth's atmosphere but survives to hit the ground
meteoroid: a chunk of space debris
Milky Way: the galaxy in which we reside
minor planet: now called a dwarf planet or small solar-system body, depending on its characteristics

N

nadir: the point directly underneath
nebula: a collection of gas and/or dust

O

obliquity: the angle of tilt of a planet's axis of rotation
open cluster: a collection of young stars that formed together; they may or may not be still gravitationally bound; the youngest clusters are still embedded in the gas and dust out of which they formed
opposition: when the Earth passes between the Sun and a superior planet so that they are lined up in the order Sun, Earth, superior planet
orbit: the path of a celestial body through space

P

parallax

the difference in apparent direction of an object as seen from two different vantage points

parsec (pc)

a unit of measurement often used for interstellar or intergalactic distances; approximately equal to 3.2616 light years

penumbra

the incomplete part of a shadow surrounding the umbra

perigee

the innermost point of a terrestrial orbit

perihelion

the innermost point of a solar orbit

planet

a celestial body that

is in orbit around the Sun,
has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and
has cleared the neighbourhood around its orbit

(official IAU definition dated 24 August 2006)

plutoid

a celestial body that

is in orbit around the Sun at a semi-major axis greater than that of the planet Neptune,
has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic (nearly round) shape,
has not cleared the neighbourhood around its orbit, and
is not a satellite of a plutoid

(official IAU definition dated 11 June 2008)

prograde

direct motion

proper motion

the apparent angular motion across the sky of an object relative to the solar system

Q

quadrature: when a superior planet appears at right angles to the Sun as seen from the Earth

R

radiant: the point in the sky from which meteor showers appear to originate
retrograde: westward motion in the sky
revolution: the movement in an orbit around another body
right ascension: the angular distance around the sky parallel to the celestial equator; measured in hours h, minutes m and seconds s
rotation: the spin on a body's axis

S

satellite: a small body, either natural or artificial, in orbit around a planet
sidereal: with respect to the stars
small solar-system body: any object except a planet, dwarf planet or satellite which is orbiting the Sun; examples include most asteroids, trans-Neptunian objects and comets (official IAU definition dated 24 August 2006)
solar eclipse: occurs when the Moon occults the Sun and casts a shadow on the Earth; the three types of solar eclipses are annular, partial and total
solstice: when the Sun appears furthest north or south of the celestial equator
star: a ball of gas which radiates energy
superior conjuction: when a planet passes behind the Sun as seen from the Earth so that they are lined up in the order planet, Sun, Earth; the term conjunction is often used when the planet involved is a superior planet
superior planet: a planet whose orbit is outside the Earth's orbit around the Sun: Mars, Jupiter, Saturn, Uranus and Neptune

T

terminator: the boundary between the illuminated and dark areas of the Moon or other planetary body
trans-Neptunian object: dwarf planets or small solar-system bodies orbiting the Sun beyond the planet Neptune; Pluto was the first one discovered
transit: 1. the crossing of a celestial body over the meridian
2. the crossing of a celestial body in front of another

U

umbra: the total part of a shadow
Universal Time (UT): local time on the Greenwich meridian

Z

zenith: the point directly overhead
zodiac: the twelve constellations (not including Ophiuchus) through which the ecliptic passes: Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpius, Sagittarius, Capricornus, Aquarius and Pisces

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