Like HowStuffWorks on Facebook!

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.


Cosmogony, the study of the origin and development of the universe as a whole and of the individual bodies that compose it.

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, 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 »

Millennium, in a common Christian belief, the period of 1,000 years before the Last Judgment, during which time Christ and the saints will reign on earth. The word sometimes refers to an ideal society in which there is perfect peace and justice. The ... See more »

Minute, a unit for measuring both time and space. As a unit of time, a minute is 60 seconds, or 1/60 of an hour. As a unit of space, a minute (often called a minute of arc) is 1/60 of a degree. (A degree is 1/360 of the circumference of a circle.) A ... See more »

Month, a unit of time. The calendar month is approximately 1/12 of the calendar year. According to the Gregorian calendar, which is used in everyday life throughout most of the world, the various months do not have an equal number of days but contain ... See more »

Nebular Hypothesis, an explanation of how the solar system was formed, proposed by Pierre Simon de Laplace in 1796. Laplace said that the material from which the solar system was formed was once a slowly rotating cloud, or nebula, of extremely hot ga ... See more »