Marie Groux, aged 13, of Philadelphia, Penna., for her question:

How is longitude related to time?

The sun, as everybody knows, reaches high noon once every day. It travels from east to west over the sky. So high noon for people living east of you comes earlier. Those living west of you get their high noon later than you do. In Philadelphia, noon comes almost five hours after it comes to London, England.

Longitudes are those imaginary lines running from pole to pole. There are 360 degrees of longitude evenly spaced around the globe. At the equator they are just over 69 miles apart, As the globe narrows towards the poles they become closer together. At Philadelphia they are about 53 miles apart,

Now the sun passes over 360 degrees of longitude in every 24 hour period. It crosses 15 degrees of longitude every hour. For many years sailors used the longitude which passes through Greenwich, England for finding longitude. A special clock was carried on the ship. It was set to keep time according to high noon in Greenwich.

Now the sailor had instruments for checking high noon wherever he happens to be. Suppose his high noon arrived when Greenwich time said 2 P.M. The sailor would know that he was 30 degrees west of Greenwich. If Greenwich time said 10 A.M. when his high noon arrived, then he would be 30 degrees east of Greenwich. So, a sailor could figure out his longitude from the time of day.