Bill Lucas, age 13, of Henryetta,  for his question:

Why do they use a light year?

We measure rainfall in inches and fractions of inches. We measure the depth of a swimming pool in feet and we measure cloth in yards, But we would not measure the distance around the equator in yards, in feet or in inches. The figure would be too large. So for larger distances we use a larger measuring unit, the mile.

But a mile, or even a million miles is no distance at all in the vast oceans of space. We are almost 93 million miles from the sun and the next nearest star is about 26 million, million miles away. We need a still greater measuring unit to estimate distances in the heavens. One of these units is the light year, the speed which light travels in one year. Other heavenly measuring rods are the parsec and the astronomical unit.