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Astronomy Terms

Astronomy terms are used to describe the various phenomena in space. In this section you can learn what every astronomy term means and how it helps us to better understand the cosmos.


Occultation, in astronomy, the passing of the moon or some other object of the solar system in front of a planet, star, or other celestial body, hiding it from view.

Andromeda Galaxy, a spiral galaxy that is larger than the Milky Way (the galaxy to which Earth belongs) but similar to it in structure, and the closest to ours. (A galaxy is a large concentration of stars, dust, gas, and other material.) It is also k ... See more »

Eros, a small asteroid (minor planet) discovered in 1898. It was the first asteroid known to come closer to the earth than the planet Mars. Eros passes within 14,000,000 miles (22,500,000 km) of the earth's orbit every 1.8 years. Usually, however, th ... See more »

Astrogeology, the science that applies the principles of geology to the study of solid bodies of the solar system other than the earth. Using the techniques of geophysics, geochemistry, and other fields of geology, scientists can learn about the comp ... See more »

Transit, in astronomy, the passage of one celestial body across the disc (face) of a larger, more distant body, or across the observer's meridian. (A meridian is an imaginary line that extends north and south through the point in the sky directly abo ... See more »

Astrophysics, the application of the theories and techniques of modern physics to astronomy. Astrophysics includes the study of the sun and other stars, planets, comets, nebulae, and the universe as a whole. It is a major branch of astronomy, coverin ... See more »

Autumn, or Fall, the season of the year that follows summer and comes before winter. By astronomical calculations, autumn in the Northern Hemisphere begins about September 23, at the autumnal equinox, when the days and nights are of equal length. It ... See more »

Azimuth, the horizontal direction of an object, measured clockwise in degrees, minutes, and seconds of arc from true north or south along the theoretical horizon. (The theoretical horizon is the horizon as it would appear if the earth were perfectly ... See more »

Bolometer, an instrument used to measure infrared, or heat, radiation. The bolometer is essentially a very sensitive thermometer. It can be used with a spectroscope to measure the ability of certain chemical compounds to absorb various wavelengths of ... See more »

Brown Dwarf, a celestial object more massive than a planet but less massive than a star. Such an object is very dim and difficult to detect. The first reliable observations of brown dwarfs were made in the mid-1990's, long after the existence of such ... See more »

Chronology, the science of measuring time. Chronology divides time into regular divisions or periods, and assigns events their proper place and sequence by giving them dates. Some striking event or change is chosen as the starting point in measuring ... See more »