Science Dictionary

The kilometer (km) is a standard unit of measurement used in the metric system. Derived from the French "kilomètre," the unit is used to measure length and distance along a straight path.

By Mitch Ryan

In any scientific research, there are typically two variables of interest: independent variables and dependent variables. In forming the backbone of scientific experiments, they help scientists understand relationships, predict outcomes and, in general, make sense of the factors that they're investigating.

By Marie Look

Physicist and cosmologist Stephen Hawking was a fierce spirit who symbolized the foibles and complexities inherent in human nature. Learn more about him by taking our quiz.

By Nathan Chandler

The world often seems chaotic and events appear to occur randomly, but what's the difference between chaos and randomness?

By Mitchell Newberry

You've heard of the big bang, of course, but do you have any idea as to what was happening during that massive flurry of activity billions of years ago?

By Robert Lamb

So much of our cosmological history starts with the much-discussed Big Bang, but what led up to that cataclysmic moment? And did time even exist back then?

By Robert Lamb & Patrick J. Kiger

Scientists announced the discovery of the largest known planet in the universe. TrES-4 has a density similar to balsa wood, and some say this gas giant could float on water. Learn why this planet is so puzzling and how planet hunters make amazing discoveries like these.

By Jacob Silverman

While routinely scanning the stars, NASA scientists came across something they didn't expect to see: a vast area of space empty of stars, planets and matter.

By Jacob Silverman

Nebulae are collections of dust and gases scattered across the galaxy. They're the sites where stars are born and what's left behind after they die.

By William Harris

The big bang theory is well-known, but there are many misconceptions about it. Like what? Let's start with this one: There was no bang.

By Jonathan Strickland

Payne-Gaposchkin, Cecilia Helena (1900-1979) was a British-born astronomer who became an authority on variable stars (stars that change in brightness) and the structure of the Milky Way Galaxy.

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.

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.

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.

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

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.

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

The nebular theory, also known as nebular hypothesis, presents one explanation of how the solar system was formed, proposed by Pierre Simon de Laplace in 1796.

By Yara Simón

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.

Solar wind is a continuous stream of mostly hydrogen and helium that flows outward from the sun in all directions. It does everything from disrupt GPS signals to create the aurora borealis.

By Mark Mancini

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

By Yara Simón

Frequency has to do with wave speed and wavelength is a measurement of a wave's span. Learn how frequency and wavelength of light are related in this article.

By HowStuffWorks.com Contributors

Every day, astronomers unravel a little more of the universe's inner workings, but the jury is still out on 95 percent of its contents.

By Robert Lamb

Berg, Paul (1926-), an American biochemist and molecular biologist, has been at the forefront of genetic engineering, both as an inventor of a pioneering procedure and as an advocate concerned about the risks of genetic research.