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

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.

By Carolyn Snare

He starred with Superman, drove the getaway car at Pluto's demise and was voted sexiest astrophysicist by People magazine. Is there anything Neil deGrasse Tyson can't do?

By Laurie L. Dove

He is famous for a phrase he never said, for wearing turtlenecks and for hosting the original 'Cosmos' TV series. As enthusiastic about the stars as he was about marijuana, Carl Sagan led a very surprising life.

By Laurie L. Dove

If you have a theory that potato chips are making you fat (with the proof being your expanding waistline), you've just used two scientific terms in a very unscientific way.

By Beth Brindle

He built President Eisenhower an indoor golf-training machine, analyzed the Zapruder film and searched for an Egyptian pyramid's treasure chamber using cosmic rays. Aren't you dying to meet this wide-ranging scientist?

By Nicholas Gerbis

The man who had some theories about relativity was also an eccentric who gleefully eschewed socks, dodged German military service and spurned social conventions.

By Nicholas Gerbis

So much about galaxies remains a mystery. We know what they're made of and that we live in one (the Milky Way), but how many galaxies are in the universe?

By Craig Freudenrich, Ph.D.

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

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 are still trying to figure out the essence of dark matter. If they do, will it lead only to greater understanding, or can we develop new technologies?

By Patrick J. Kiger

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

Space collisions are the universe's car wrecks. Only in outer space, it's stars, asteroids and even galaxies doing the smashing.

By John Fuller

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

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

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.