Marilyn Parks, age 12, Rosalia, Wash.; for her question:

How did they know where to put the time zones?

There are 24 time zones around the world because there are 24 hours in a day. They are all counted from the Prime Meridian, the line of longitude which runs from pole to pole through Greenwich, England. Nature put no such line upon the earth. So the Prime Mieridian and the time zones are where they are because the different countries agreed to put them there.

We tell time by the sun and the sun seems to make a journey around the earth every 24 hours. Actually, it is the earth and not the sun which turns around. Imagine this journey around the earth to bo a great circle. A circle has 360 degrees, so the sun travels across 15 degrees every hour. The time zones correspond roughly to an hour. So this means that they occur roughly at every 15 degrees of longitude.