Julie Boran, age 16, of Johnson City, Tenn., for her question:

HOW IS THE SPEED OF LIGHT RECORDED?

Light travels 186,282 miles per second. In kilometers, the speed is 229,792. Scientists call the speed of light a fundamental constant of nature. That is, its speed is the same whether the observers who measure it are moving or not.

In 1926, an American physicist named Albert Michelson made one of the first precise measurements of the velocity of light. He used a rapidly rotating mirror that reflected a beam of light to a distant reflector. The returning beam was then reflected back to the observer by the rotating mirror.

Michelson adjusted the speed of the mirror until the mirror turned to the correct angle during the time the light traveled to the reflector and back. The speed of the mirror indicated the velocity of the light.

Michelson actually used several mirrors on a drum so that the angle the drum had to turn while the light traveled out and back was small.