Dominic Ciarniello, age 10, of Youngstown, Ohio, for his question:

When did they start using the light year unit

This unit is the distance that light travels in one earth year. It is based on the speed of light    and this was very difficult to measure. In the 1930s, the uniform speed of light was finally verified to be about 186,000 miles per second. For a long time, scientists had suspected that light velocity could be used as a basis for an astronomical measure. They began using it with confidence in the 1930s and during World War II, the light year was accepted as a universal standard.

In the 1700s and 1800s, celestial distances were measured by the A.U., the Astronomical Unit. This is 93 million miles    the distance between the earth and the sun. The light year measures distance in terms of travel time    and equals 63,290 A.U. Nowadays, the parsec unit of measure is used to measure star distances by triangulation. One parsec equals 3.26 light years and one megaparsec equals a million parsecs.