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.

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

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D - F
G - I
J - M
N - R
S - Z

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A BC

A

ALBEDO: The fraction of sunlight reflected by a planet or satellite.

AMMONIA (NH₃): A substance with molecules made up of a nitrogen atom and three hydrogen atoms.

APERTURE: The diameter of a telescope's primary (largest) optical surface (lens or mirror), usually expressed in inches or millimeters. Larger apertures allow the telescope to collect more light and see fainter objects.

APHELION: The point in a planet's orbit where it is farthest from the sun.

APOGEE: The point in the orbit of a natural or artificial satellite most distant from the earth. The farthest or highest point; apex.

APOAPSIS: The farthest point on an orbit from the primary body. For earth orbits, apoapsis is called apogee.

APOLLO-OBJECT: An asteroid whose orbit brings it closer to the Sun than Venus.

ARC MINUTE; ARC SECOND: These are finer divisions of a degree, which is the primary unit of measure in the sky. A degree is subdivided into minutes and seconds, much like an hour is. Thus, 60 arc minutes equals one degree and 60 arc seconds equals one minute. The size of a planet in the sky is usually given in arc seconds (Ex: Jupiter is about 38 arc seconds in diameter).

ARGON (Ar): Argon, #18 on the periodic table, is colorless and odorless, like all of the noble gases. It has an atomic mass of 39.948 amu, and its electron levels are 2-8-8. Ar was discovered by Lord Rayleigh and Sir W. Ramsay in England. It melts at 83.95 K, and boils at 87.45 K. Many people don't realize that argon makes up a full 1% of our atmosphere, while carbon dioxide is only a fraction of a percent of our atmosphere. Argon is primarily used to fill the empty space in incandescent light bulbs, and is used to create a greenish color in "neon" lights. Argon is 2.5 times as soluble in water as nitrogen, and has about the same solubility as oxygen. Argon is very inert, but it does form a hydrate, and ion molecules such as (ArKr)+, (ArXe)+, and (NeAr)+. These, however, do not have true chemical bonds. Natural argon has three stable and five radioactive isotopes. At STP (20 degrees C, 1 atm), Ar has a density of 1.784 g/L.

ASTERISM: Asterisms are sub- or supersets of constellations which build a constellation itself, or a group of stars, physically related or not. Best known is the Big Dipper as a part of the Great Bear. But there are more than just this one. Click here for a table of asterisms.

ASTEROID: A minor planet usually made up of rock and ice.

ASTEROID BELT: The region between the orbits of Mars and Jupiter where most of our solar system's asteroids are found.

ASTROLOGY: The study of the positions and aspects of heavenly bodies with a view to predicting their influence on human affairs. Has nothing to do with Modern Astronomy.

ASTRONOMICAL UNIT (AU): The average distance of the Earth from the sun.

ATMOSPHERE: The layer of gasses surrounding a planet, satellite, or star.

ATOM: A particle made up of a central nucleus surrounded by electrons.

ATOMIC NUCLEUS (plural-atomic nuclei): A tiny structure at the center of an atom, containing almost all the mass of the atom. Consists of protons and neutrons.

AXIAL TILT: The angle between the axis of rotation of an object and a line perpendicular to its plane of orbit.

AXIS: The imaginary straight line about which an object spins.

B C

BINARY STAR: A stellar system consisting of two stars orbiting about a common center of mass and often appearing as a single visual or telescope object. Also known as a double star.

BLACK HOLE: An extremely small region of space-time with a gravitational field so intense thet nothing can escape, not even light.

CARBON DIOXIDE (CO₂): A gas found in Earth's atmosphere in small amounts, unlike Mars and Venus. Probably for a long time carbon dioxide was the dominant active gas in the early Earth's atmosphere. Today CO₂ makes up only about 0.03% of the atmosphere, and the highest estimates are that it could rise to 0.09% by the year 2100 as a result of human activities. It is the gas we exhale.

CATADIOPTRIC: Any of a number of compromise telescope designs, using both a lens and mirrors. Examples are the Schmidt-Cassegrain and Maksutov-Cassegrain. Because the light path is folded twice, the telescope is very compact. These are pretty expensive. Pictures can be seen in the ads in any issue of a popular astronomy magazine: the Meade 2080 and the Celestron C-8 are examples of Schmidt-Cassegrain; the Celestron C-90 and Questar are examples of Maksutov-Cassegrain.

CENTER of GRAVITY: The point about which two bodies move as each revolves around the other. The point about which a body of mass is balanced in all directions.

CENTRIFUGAL EFFECT: The tendency for anything spinning about a center to move away from that center. (example: imagine you are on one of those fast-spinning twister rides. Once the ride starts you get pushed into the wall. This is due to centrifugal effect).

CHROMATIC ABERRATION: In refractor telescopes, which use lenses to bend the light, different wavelengths of light bend at different angles. This means that the stars you see will usually have a blue/violet ring around them, as this light is bent more than the rest of the spectrum. It is not present at all in reflectors, nor to any significant degree in catadioptrics. Different glasses and crystals (notably fluorite) are sometimes used to compensate for the aberration. Such telescopes are termed "achromatic," or "apochromatic" if the correction is is nearly perfect.

CLUSTERS: Either Open or Globular star clusters. Defined by a group of stars located in a relatively small area of space such that  each is affected by the others gravitational pull. Stars in  Open Clusters are fairly far apart while stars in a globular cluster are densely packed often containing a nucleus.

COLLIMATION: This refers to how correctly the optics are pointing towards each other. If a telescope is out of collimation, you will not get as clear an image as you should. Refractors generally have fixed optics, so you don't have to collimate them. Reflectors and catadioptrics usually have screws that you turn to collimate. (This only takes a few minutes to do- it is dead easy).

COMA: This refers to the blurring of objects at the edge of the field of view, most common in short focal ratio Newtonian telescopes (at f/10 and longer, Newtonians are very well corrected for coma).

COMET: An object moving about the sun in an eccentric orbit and composed of materials which, on warming up near the sun, form a "tail" which always points away from the sun. Comets can be compared to dirty snowballs.

CONJUNCTION: When two or more celestial objects are very close together in the sky, they are said to be in conjunction. They appear close only because of our perspective, or line of sight. In reality, they are usually very far apart in space. When one object actually crosses in front of the other, it is called an occultation.

CONSTELLATION: An apparent pattern, or grouping, of stars in a region of the sky that man has given a name. Modern astronomers maintain these ancient names and boundaries as a convenient way of mapping the regions of the sky.

CORE: The innermost portion of an astronomical body.

CRESCENT: A curved shape like the appearance of the Moon when less that half of it is lit.