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Science | Dictionary | 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.

How Deep Impact Works

How do scientists find details about the early days of our solar system? One way is to investigate comets. Find out how the Deep Impact spacecraft fired an impactor into Comet Tempel 1 to get some answers.

 Epoch (Chronology)

Epoch $eBp$oBk in chronology (timekeeping) a point in time such as 302 B.C. or October 30 1936 or 7:34 A.M. See more »


Equinox, During the course of a year, the sun appears to move northward for about six months and southward for about six months. The times when the sun appears to pass over the earth's equator (on or about March 21 and September 23) are called the eq ... See more »

 Evening Star

Evening Star, the name given to any of the five bright planets in the sky at sunset. It is thus not actually a star and there may be several at one time. The evening stars continually change as the planets move in relation to the sun. A planet may be ... See more »


Hour, a unit for measuring time, defined as either 1/24 of a day or 3600 seconds (60 minutes). Used alone, the word hour refers to clock time, a composite of mean solar time and atomic time. There are other types of hours, such as sidereal hours (9.8 ... See more »


Hourglass, a device for measuring time. In its usual form it consists of two cone-shaped or oval glass receptacles joined by a narrow neck. Sand or a liquid (such as water or mercury) in the uppermost section of a true hourglass will run through the ... See more »


Interferometer, an instrument that uses the interference patterns formed by waves (usually light, radio, or sound waves) to measure certain characteristics of the waves themselves or of materials that reflect, refract, or transmit the waves. Interfer ... See more »


Magnitude, in astronomy, a unit of measurement of the brightness of stars. The scale of magnitude extends from negative numbers (for example, the minus first magnitude) for very bright stars to positive numbers (for example, the fourth magnitude) for ... See more »

 Midnight Sun

Midnight Sun, a name given the sun when it can be seen at midnight during the Arctic or Antarctic summer. From March 21 to September 23, the sun is visible 24 hours a day at the North Pole. As a person here moves southward toward the midnight sun's l ... See more »