Glossary of Astronomical Terms

As with many science-related hobbies, there are a few new words you'll have to add to your vocabulary. This glossary is not meant to be an exhaustive list of techno-definitions. Rather, it is designed as an easy-to-access aid for novices to help them grapple with the terminology of astronomy.

Is there a term you don't see here? Submit it to our Staff and we will try to get you a reply.

Do you know how to pronounce the CONSTELLATIONS, STARS, or the PLANETS and their MOONS?

Go to This section of the glossary

A - C
D - F
G - I
N - R
S - Z

this page
JKL M

J K L

JOVIAN PLANETS: The giant planets: Jupiter, Saturn, Uranus, and Neptune.

JULIAN DATE: The basis of time for astrodynamics. The number of days since 12:00 on 1 January 4713 B.C. The Julian Day for 1 January 2000 at noon is 2,451,545.0.

KEPLER'S LAWS of PLANETARY MOTION: Named for Johannes Kepler who determined the following three laws of two body motion:

• First Law - the orbit of each planet is an ellipse with the sun at a focus
• Second Law - the line joining the planet to the sun sweeps out equal areas in equal times
• Third Law - the square of the a planet's period is proportional to the cube of its mean distance from the sun.

KIRKWOOD'S GAPS: Regions in the asteroid where no asteroid orbits are to be found.

LIBRATION POINTS: Unique locations in orbital mechanics (space) where the gravitational attraction of two or more celestial bodies add to zero; a neutral point in the gravitational field. Also known as Lagrangian points. When one large body (e.g., the Moon) is in orbit around another large body (e.g., Earth), there are five points in orbits around the larger body where gravitational forces balance out to enable satellites to be placed where they could not stay if the smaller of the large bodies were not present.

LIGHT BUCKET: A common slang term for a large apertures. The cure for "Aperture Fever."

LIGHT POLLUTION: The illumination of the night sky caused by artificial light sources on the ground (streetlights, billboards, etc.). Both the light and the loss of contrast make it difficult to find fainter stars and nebulae. This is becoming a bigger problem for both amateur and professional astronomers.

LIGHT-YEAR (ly): The distance that light travels (through a vacuum) in one year. 9,460,730,472,581 (commonly shortened to 9.46 x10^12) kilometers.

LITHOSPHERE: The rocky solid ball of a planet or a satellite.

M

MAGNETIC FIELD: A region in which the electromagnetic force makes itself felt.

MAGNITUDE: Astronomers use magnitudes to describe the brightness of an object in the sky. It is a non-linear, numerical scale where the smaller numbers represent brighter objects. Generally, the unaided eye can see stars down to magnitude 6 in good skies. Pluto is magnitude 14, and thus, is invisible without optical aid or photography. Venus has a magnitude of about -4 and the Sun is -27.

MAJOR AXIS: A diameter passing through the foci and center of an ellipse. (see FOCUS, ELLIPSE)

MAKSUTOV-CASSEGRAIN: See catadioptric .

MANTLE: The portion of a rocky planet surrounding the core.

MASS: The amount of matter in an object.

MAXIMUM ELONGATION: The greatest separation of a planet from the Sun in Earth's sky.

MERIDIAN: An imaginary line in the sky passing from the due south horizon, through the zenith , and on to the due north horizon. Objects are highest in the sky, and thus least affected by atmospheric distortions, when on the meridian.

METEOR: A piece of matter from space that burns up in Earth's atmosphere (never reaching the ground) causing a streak of light.

METEORITE: A meteor that is large enough to reach the Earth's surface.

METHANE (CH₄): Methane is a colorless, flammable, nontoxic gas with a sweet, oil type odor. Methane is the simplest member of the paraffin series of hydrocarbons. It is lighter than air, having a specific gravity of 0.554. It is only slightly soluble in water. It burns readily in air, forming carbon dioxide and water vapor; the flame is pale, slightly luminous, and very hot. The boiling point of methane is -162.0° C (-259.6° F) and the melting point is -182.5° C (-296.5° F). Methane in general is very stable, but mixtures of methane and air, with the methane content between 5 and 14 percent by volume, are explosive. Explosions of such mixtures have been frequent in coal mines and collieries and have been the cause of many mine disasters.

MINOR AXIS: The short diameter of an ellipse.

MINOR PLANET: see Asteroid

MOLECULE: A group of atoms held together by electromagnetic force.