Astronomical dictionary

As with almost anything, astronomy is full of language and terms which are designed to frustrate people who do not have a clue what astronomers are talking about. This is completely intentional. The purpose of it is to make astronomers feel better about themselves in that they know a foreign language. Unfortunately, it also gets in the way of people trying to start out, and can be discouraging.

Dark Skies tackles this problem in the spirit of science, by providing a reference full of everything except what you want to know. :-) This dictionary is growing as I strike words and terms which aren't quite as obvious as they could be. If there is a term missing which you think should be included, please email me.

absolute zero: The coldest anything can become. This is because temperature is a result of atomic vibration. Absolute zero is when atoms are not vibrating. See also: kelvin.

altitude: The vertical angle between the horizon of an observer, and an object in the sky. See also: azimuth.

aphelion: The point in the Earth's orbit which is farthest from the Sun. See also: perihelion, apogee, perigee.

apogee: The point in the Moon's orbit which is farthest from the Earth. See also: perigee, aphelion, perehilion.

aperture: The diameter of a lens or mirror in a telescope, binoculars, or photography equipment.

asteroid: Rocky objects orbiting the Sun not large enough to be considered planets. Most are located between the orbits of Mars and Jupiter. Also called meteoroids and minor planets. See also: comet, meteor, meteorite, planet.

A.U.
astronomical unit: Distance measurement. One astronomical unit is equivalent to the mean distance between the Earth and the Sun, 149.6 million kilometres. This term is used primarily to measure distances within, or near our Solar System. See also: light year, parsec.

astrology: Excercise in fortelling events by studying the motions of stars and planets. (Unrelated with astronomy, but often confused.)

astronomy: You poor misguided soul. Now would be a good time to look at the About Astronomy socklet.

azimuth: Angular measurement around the horizon where North=0 degrees, East=90 degrees, South=180 degrees, and West=270 degrees. See also: altitude.

caffeine: Stimulating alkaloid found in coffee plants, and commonly consumed through drinking coffee. Often used to stay awake in times of crisis.

celestial pole: Either of the two points in the sky directly above the North and South poles. These points remain stationary as the rest of the sky appears to revolve around them as the Earth turns on it's axis. See also: celestial equator, ecliptic, meridian.

celestial equator: The imaginary line projected in the sky which is directly above the Earth's equator. At the equator, this line goes directly overhead, through the zenith. At either the north or south pole, this line forms a circle at the level of the horizon. See also: celestial pole, ecliptic, meridian.

celsius: A measurement of temperature which is derived from the freezing and boiling points of water. 0 degrees celsius is the freezing point of pure water at sea level, and 100 degrees celsius is the boiling point of pure water at sea level. The scale increments in consistent steps before, between, and after these two points. See also: kelvin.

cluster: A group of stars which exist near each other. See: globuler cluster, open cluster.

coffee: Yummy hot beverage used by intelligent astronomers to stimulate braincells and stay awake. Mmmmmmm, coffee! See also: caffeine.

comet: An object orbiting the Sun, often made of ice (although not necessarily water ice.) Comets are usually seen when they enter the inner Solar System, and heat from the Sun causes gas and dust to stream out in several tails. Many comets have a straight gas tail and a curved dust tail. The tail(s) will always be pointed away from the Sun, regardless of the comet's direction. See also: asteroid.

conjunction: When a planet is in line with the Sun and the Earth. Inferior conjunctions only apply to Mercury and Venus as they pass between the Earth and the Sun. All planets have superior conjunctions on the far side of the Sun. See also: opposition.

constellation: A collection of stars visually nearby each other as seen from the Earth, which appear to form a shape or picture in the sky. For example, Crux (the Southern Cross), or Scorpius (the scorpion). See also: zodiac.

culmination: The highest point reached by any given object in the sky as seen from a given point on the Earth, during the Earth's daily rotation.

Dec
declination: The equivalent of latitude except projected into the sky. The declination of any given direction is equivalent to the latitude directly underneath it on the Earth. Declinations south of the celestial equator are usually given as negative values for convenience. See also: epoch, right ascension.

eccentricity: A value describing just how un-circular any given orbit is. (Most orbits are not circles, but ellipses.)

ecliptic: The path which the Sun and planets appear to follow through the sky. Since all the known planets lie on roughly the same plane when orbiting the Sun, they appear to follow roughly the same path - the ecliptic. Because of the tilt of the Earth's axis, this line differs from the celestial equator by about 23.5 degrees. See also: celestial pole, meridian, zodiac.

epoch: A precise moment in time which is specified alongside right ascension and declination to pinpoint an exact point in the sky. Because the Earth's axis "wobbles" over a period of 26,000 years, Dec and RA coordinates are invalid on their own.

equator: The imaginary plane through the Earth which divides the northern and southern hemispheres. See also: celestial equator.

exit pupil: The small image which is projected onto the lens in an eyepiece, and seen by the eye of someone who looks through an optical device.

eyepiece: The lens which fits on a telescope or binoculars, and allows someone to focus and examine the image given by the main lens or mirror.

fireball: A really bright meteor.

first point of aries: The position marking the zero degree point of right ascension. The First Point of Aries is the position of the Sun at the exact moment of the Sun crosses the equator in a northerly direction, the Spring Equinox.

focal length: The distance between the eyepiece and the objective lens within an optical device.

globular cluster: Very old and dense clusters of stars (up to 15 billion years) dating back to the formation of the Galaxy. Usually contain only low density stars, as the larger stars will have evolved and died by now. See also: cluster, open cluster.

horizon: Line at which the Earth and sky appear to meet. See also: zenith.

inferior conjunction: See: conjunction.

kelvin: A measurement of temperature which is derived from the the celsius scale, but begins at absolute zero: -273 degrees celsius. The increments of the scale are identical to the celsius scale. Because nothing can be colder than absolute zero, the kelvin scale does not need negative values.

light year: Distance measurement. One light year is the distance travelled by light in one year. Light travels at 299,792,458 m/s. One light year equals approximately 9.416 x 10¹² km. See also: astronomical unit, parsec.

magnitude: Measurement of brightness. The lower the magnitude value of an object, the brighter that object is. Any given value is about 2.5 times dimmer than that value minus one. Magnitude can be negative, and often is with bright objects. Some stars are negative but most bright stars are at most 1st magnitude.

meridian: An imaginary line drawn in the sky which joins the zenith with the north and south celestial poles. See also: celestial equator, ecliptic.

meteor: Particles (usually small) as they burn up in the atmosphere, usually leaving a brief trail of light behind them as seen from the ground. See also: fireball, meteorite, meteoroid, radiant.

meteorite: The remains of a meteor after it has hit the ground, assuming it doesn't burn up first. See also: meteoroid.

meteroid: See: asteroid. See also: meteor, meteorite.

minor planet: See: asteroid.

nebula: Cloud of gas and dust usually left by a dead star. Such clouds are where most stars form. They are primarily located by reflecting light from stars, emitting light from young stars which lie within, or by blocking the view of background stars behind them. See also: open cluster.

occultation: When one object passes behind another object. The Moon regularly occults background stars. See also: transit.

open cluster: Cluster of young stars which formed from the same nebula. These clusters usually disintegrate as the stars move their separate ways. See also: cluster, globular cluster.

opposition: The point in a planet's orbit when it on the direct opposite side of the Earth from the Sun. Mercury and Venus both orbit inside the Earth's orbit, so do not have a state of opposition. See also: conjunction.

orbit: ¹When two or more objects move around a common centre of gravity. ²The elliptical (occasionally circular) path followed by an object as it moves around a centre of gravity.

parallax: The way in which an object appears to move against background objects when seen from two different locations. See also: parsec.

parsec: The distance at which a given object appears to shift one arcsecond against the background of stars when seen from two points distanced by the diameter of the Earth's orbit. This distance is approximately 3.0857 x 10¹³ km. See also: astronomical unit, parallax, light year.

perigee: The point in the Moon's orbit which is closest to the Earth. See also: apogee, aphelion, perihelion.

perihelion: The point in the Earth's orbit which is closest to the Sun. See also: aphelion, apogee, perigee.

planet: Object revolving in an approximately circular orbit around a star. See also: asteroid.

radiant: The point in the sky from which a meteor shower appears to originate. See also: meteor.

RA
right ascension: The equivalent of longitude except projected into the sky, expressed in hours, minutes, and seconds. The zero point of RA is the first point of aries. See also: declination, epoch.

star: ¹ Celestial body appearing as luminous point in the night sky. ² An object with enough individual mass to naturally sustain the nuclear reaction from hydrogen to helium. See also: constellation, planet.

superior conjunction: See: conjunction.

transit: When one object passes in front of another object. Mercury and Venus regularly transit the Sun. See also: occultation.

zenith: The point directly above a given point on the Earth. ie. Go outside, look directly up, and you are looking at the zenith. See also: horizon.

zodiac: The name given to the twelve constellations sitting on the ecliptic. The concept of the zodiac is functionally more important to astrology than to astronomy, but still referred to occasionally.