Jimmy McElroy, aged 11, of Lancaster, Penna., for his question:
What and how long is a sidereal day?
The word sidereal is coined from a Latin word for star, or group of stars. Sidereal time is time calculated from the stars. The stars pass over the heavens in an orderly procession. Each star rises to its fullest height and then slowly sets. Its meridian, or noon, is the highest it reaches in one day and night period. A sidereal day is the period of time from one sidereal noon to the next.
Sidereal time is somewhat different from solar time. It clocks each rotation of the earth by the distant fixed stars. Solar time is clocked by the sun and the sun varies its daily journeys. A sidereal day is four minutes shorter than a solar day. Each sidereal day in the year is exactly the same length ‑ 23 hours 56 minutes 4.0 seconds. And sidereal noon comes about four minutes earlier each day.
The job of figuring sidereal time is given to the astronomers. They spy out the paths of the stars through their telescopes. Imagine the sky to be a vast bowl turned upside down over the earth. The stars would seem like dots on the inside of a dome. The astronomers use this plan for placing the stars. It is called the celestial sphere.
Imagine a line straight up to the celestial sphere from the north pole. That line would take you to the celestial north poles The celestial south pole is directly above the earthly south pole. The celestial equator is exactly halfway between the two celestial poles. Naturally it hoops in an arch over the celestial sphere. These are some of the guide marks used for placing the stars in the heavens.
A telescope must be able to keep a star in view. And the stars move over the celestial sphere as the earth turns on its axis. So telescope is mounted on a moveable frame. Delicate clockwork and machinery keep the eye of the telescope trained on a star as it moves over the heavens.
The telescopes for telling sidereal time are even more complicated. They have machinery for spotting when a star reaches its meridian. These telescopes are mounted to move only between north and south. Certain clock stars reach their noon one after the other. Each one is carefully noted and the sidereal time is announced.
The job of sidereal time telling is done for us at the United States Naval Observatory in Washington, D.C. The correct sidereal time is broadcast from there by radio. Sidereal time is moat useful to pilots of planes and ships. It is an aid to their dead reckoning. This deduced reckoning is to figure positions and plot courses without consulting landmarks.