Page:Astronomy for Everybody.djvu/330

Astronomers avoid all confusion from this cause by the use of sidereal time, that is star-time, or time measured by the stars. As already explained, a sidereal day is the interval between two successive pasages of a star over the meridian, and is three minutes fifty-six seconds less than our ordinary day. It is divided into twenty-four sidereal hours, and each hour into sidereal minutes and seconds. A sidereal clock gains three minutes fifty-six seconds daily on an ordinary clock and thus shows the same time at the same position of the stars the year around.

One who wishes to keep the run of the stars will find it very convenient to have some idea of sidereal time. This may be had by the following rule: Double the number of the month; the product will be the sidereal time at six o'clock in the evening. At seven o'clock it will be one hour later, and at eight it will be two hours later, and so on.

Suppose, for example, that one looks at the sky in November at nine o'clock in the evening. This is the eleventh month; multiplying by two gives twenty-two, adding three gives twenty-five, from which we drop twenty-four, giving one hour as the sidereal time. The time thus obtained will not often be more than an hour in error, except during the first week or ten days of the month, when it may be an hour or more too great. It may then be diminished by one hour.

Applying the same rule in January we have five hours as the sidereal time at nine in the evening. But early in the month the sidereal time at nine in the evening will be four hours instead of five.