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

Epoch, in chronology (timekeeping), a point in time, such as 302 B.C. or October 30, 1936, or 7:34 A.M.

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.


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.

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.

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

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.


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

Nova, (plural: Novae), a star that rapidly increases in brightness and then fades again.

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.

Orbit, the path of a celestial body or other object in space, governed by the gravitational attraction of other bodies.


Parallax, the apparent change in position of an object when it is viewed from two different locations.

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

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

Planetarium, is an educational device for showing the locations and movements of the planets and other objects in the universe.


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

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

Solstice: During the course of a year, the sun appears to move northward for about six months and southward for about six months.

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


Universe, in astronomy, is all matter and energy whose existence has been, will be, or could be made known to us.

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

Winter, the cold season of the year. Astronomically, in the Northern Hemisphere winter begins at the winter solstice, about December 22, and ends at the spring equinox, about March 21.

The man immortalized on the left was behind the three laws of motion and the universal law of gravitation. He was also competitive, temperamental and fascinated with alchemy. How well do you know Newton?

By Jacob Silverman


We take much for granted about our universe, like it's getting bigger. What if the universe stopped expanding and started collapsing inward with a giant crunch?

By William Harris

Whether you invented a revolutionary pair of skivvies or a way to turn cow manure into vanilla flavoring, we have an award for you. The Ig Nobels are proving that science isn't all seriousness and superstring theory.

By Robert Lamb