David Allison, age 11, of Charlotte, N.C., or his question:
What is sidereal time?
Sidereal time is the hour angle of the vernal equinox but beware of making that statement. Somebody may ask you to explain it. To an astronomer, sidereal time is simple as ABC, for he uses it in his work. We amateur star gazers may not grasp all the complex details, but we can cope with a general idea of sidereal time.
The clock and the calendar are based on the sun as it moves or seems to move against a background of fixed stars. The earth’s orbit is not perfect and our everyday solar time is not perfect. Sidereal means starry and sidereal time is based on the fixed stars. They give a more exact timepiece and a truer picture of the earth's oval and wobbly orbit.
The stars, of course, are scattered far and near across the heavens but they seem to be painted on the inside of a huge hollow ball. We call this the celestial sphere and our globe is inside it. Once a day the earth rotates, giving us a circular view of the celestial sphere. The wheeling celestial sphere is a sky map and a clock.
A map must have directions and guide lines. The celestial poles are above the earth's two poles and the celestial equator is halfway between its two poles. The celestial meridian passes through the poles and cuts the celestial equator in halves. The celestial sphere is sliced into quarters. It’s equator and meridian are divided into 24 slices called hours.
The fixed stars on the celestial sphere seem to swing around as the earth rotates. In a sidereal day, a fixed star moves around and back into position and the sidereal day is four minutes shorter than the solar day. During a year, the sun seems to pass through twelve constellations of fixed stars.
On March 21, it crosses a point on the celestial equator. This point, called the vernal equinox, is used to figure the sidereal year – which is about 20 minutes longer than the solar or calendar year.
But each day, the celestial equator swings around our globe and at one moment the vernal equinox point crosses the celestial meridian. This moment is sidereal noon. Because of the earths imperfect orbit, sidereal noon may happen at any time of day or night.
The fixed stars of the celestial sphere are the clock face for figuring sidereal time. The clock hands are the celestial equator and meridian which slice the sphere in quarters. The pivot point is the vernal equinox on the equator and sidereal time is the hour angle between this point and the celestial meridian.