Tim Fay, age 12, of San Bruno, California, for his question:

Why is the day divided into 24 hours?

When we switch to the decimal system, tae are not likely to streamline our clocks in multiples of ten. Even if we could improve upon our 24 hour day, the changes involved would create too much confusion. Besides, the system has worked successfully for thousands of years. It was invented by the scholars o£ ancient Babylon, who needed math to pursue their studies of astronomy. They coped with circles and angles to measure sky distance    and time, of course, is related to the motions of heavenly bodies.

The Babylonians invented a number system based on six and multiples of six. They used it to section the circle into 360 equal degrees, from which they measured angles. In a day and night period, the sun completes a great circle around the globe. In one hour, it travels 15 degrees of this circle    and there are 24 of these hourly sections in the complete circle.