Michael Woods, age 12, of Kingston, Ontario, Canada, for his question:

Is the day really 24 hours long?

From midnight to midnight, a perfect clock ticks off 1,440 minutes, which is exactly 24 hours. This neat and orderly day is based on our whirling world, which is not quite so neat and orderly. What's more, it is not easy to figure how fast it rotates on its axis and moves around its orbit. These motions must be measured from the changing positions of the sun and the stars. And according to the stars, the day is about four minutes shorter than 24 hours.

We expect the earth to rotate around its axis once in 24 hours. Well, sometimes it does but mostly it doesn't. We expect it to complete 365 daily spins in one yearly revolution around its orbit. Actually it takes about 365 days    hence its rotations never fit neatly into one revolution. What's more, our whirling world tends to speed up and slow down as it goes.

We try to base our time system on the motions of the earth, but its erratic irregularities make this very difficult. For example, its orbit is not a perfect circle. In December we are about three million miles closer to the sun than we are in July. It so happens that orbiting bodies are ruled by tricky cosmic traffic laws. For example, the earth must move a little faster in December, when it is closer to the sun. These and other variations change the lengths of the days.

The earth's daily rotation must be measured by a line from the center of our globe to some object out in space. As we spin around, this object out in space changes its position in the sky. The solar day measures ro¬tation by the changing positions of the sun.

This figuring is not perfect, mainly because of our lopsided orbit.

For example, in December the solar day is about 24 hours and 30 seconds. In September it is about 23 hours, 59 minutes and 39 seconds. These slight variations throughout the year are computed to give us the average 24 hours on which we base our clocks. Because it is based on the positions of the sun, it is called the mean solar day.

Sidereal time measures the earth's rotation by the stars. This is more precise because the fixed stars are so far away that our distance from them varies hardly at all throughout the year. The sidereal day is measured by a line from the center of the earth to a selected star.

At midday, the selected star crosses the meridian line, directly over¬head. The next midday it crosses this precise line about four minutes earlier. The time period is about four minutes shorter than the 24 hour solar day. Actually, the sidereal day is 23 hours, 56 minutes and 4.09054 seconds.

Obviously we cannot base a worldwide time system on the periods of daylight and darkness, because they change with the seasons and vary from place to place. We must figure the earth's rotation from clocks in the sky. We set our earth clocks by the 24 hour solar day, measured from the positions of the sun. Sidereal tine is computed from positions of the stars    and the sidereal day is almost four minutes shorter than 24 hours.