Science Dictionary

Do you know what a meteor is, or what scientists mean when they are talking about cryogenics? Our collection of science terms explains the meaning of some of the most common scientific ideas.

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Sundial, a device used to tell time by the sun. Nearly every sundial has two basic parts: a pointer, called a style or gnomon, which casts a shadow; and a numbered dial on which the shadow falls to show the hour.


Year, a unit of time. It is based on the time it takes the earth to make one revolution around the sun.

Nebular Hypothesis

Nebular Hypothesis, an explanation of how the solar system was formed, proposed by Pierre Simon de Laplace in 1796.

Planetesimal Hypothesis

Planetesimal hypothesis is a theory of the origin of the solar system. It was proposed by Forrest R.

Midnight Sun

Midnight Sun, a name given the sun when it can be seen at midnight during the Arctic or Antarctic summer.


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.

Eudoxus of Cnidus

Eudoxus of Cnidus (NY duhs or kuh NY duhs) (400 B.C.?-350 B.C.?) was a Greek astronomer who made important contributions to the field of geometry.


Sextant, a navigation instrument used to measure angles, particularly the altitudes of the sun and stars above the horizon.

Nasir al-Din Al-Tusi

Al-Tusi, Nasir al-Din (1201-1274) was one of the greatest scholars of his time and one of the most influential figures in Islamic intellectual history.


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.

Perihelion and Aphelion

Perihelion and Aphelion, The perihelion is the point on the orbit of a planet or comet that is closest to the sun.


Chronometer, a timepiece that is exceptionally accurate. Traditionally, the term refers to the marine chronometer, a rugged mechanical instrument used at sea to keep time for navigational purposes.


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.


Perturbation, in astronomy, is a disturbance in the orbit or motion of a heavenly body.

Double Star

Double Star, a pair of closely-spaced stars that to the unaided eye usually appear as a single star.


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

Van Allen Radiation Belts

Van Allen Radiation Belts, two zones encircling the earth in which there are relatively large numbers of high-energy (fast-moving) charged particles.


Day, in astronomy, the average length of time between successive noons. Noon is defined as the instant when the sun is highest in the sky.


Bolometer, an instrument used to measure infrared, or heat, radiation. The bolometer is essentially a very sensitive thermometer.

Aberration of Light

Aberration of Light is a phenomenon in which a star or other celestial body, as viewed from the earth, appears to be slightly displaced from its true position.


Twilight is the light diffused over the sky from sunset to darkness and from darkness to sunrise.


Astrophysics, the application of the theories and techniques of modern physics to astronomy.

Andromeda Galaxy

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.


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 dimmer ones.


Transit, in astronomy, is the passage of one celestial body across the disc (face) of a larger, more distant body, or across the observer's meridian.