Patsy Bender, age 15, of Lancaster, Penna., for her question

What are light Years and parsecs?

It would be cumbersome to write out the distances between the stars in mere miles. So the astronomers use bigger measuring units to chart the skies. One is the light year, another is the astronomical unit and another, perhaps the most usefuli is the parsec.

The light year is somewhat like a miles per hour unit. It measures distance in terms of travel time. The traveler is light   which whips across space at about 186,000 miles a second. In one year, light travels almost six million, million miles. This figure is the lit' year and latest estimates put it at 5.88 million"miles. The 'nearest stars to our sun are about 4.3 light years away.

The astronomical unit is a shorter heavenly yard stick. It is the average distance of the earth from the sun. Latest estimates set this figure at 92,907,000 miles. The earth's orbit, of course, is two astronomical units wide. The orbit of Pluto is about 39 astronomical units from the sun.

The accurate length of the astronomical unit is very important, for it is used to give us the parsec, biggest of all heavenly yardsticks. Here we have a problem in geometry, computing heavenly triangles. A s the earth swings around its orbit, we get slightly different pictures of the stars in relation to each other. The changes are so slight that they can be studied only from photographic plates.

These differences are used to measure the distance of a star. A picture is taken and the starts position noted in relation to a more distant star. Six months later another picture is taken, The earth is now half way around its orbit, two astronomical units from the first position.  The star to be measured has moved slightly    in relation to the background star.

We can now construct our heavenly triangle, taking the base line of two astronomical units. One side of the triangle points to the background star as it was when first measured; the other side as it was when measured the second time. The two lines meet and cross at the star we are trying to measure. The angle at which they meet is called the parallax and it gives us the distance of the star.

The parallax angle is measured in segments of a circle. A complete

360 circle has X19 degrees. Each degree has 60 minutes and each minute has 60 seconds. A star with a parallax of one second is one parsec away from our sun. This distance is estimated to be equal to 3.26 light years or 19,150,000 million miles. Actually, the nearest stars to our sun are 1.3 parsecs. Light leaving these stars travels some 26 million, million miles and takes more than four years to reach us.