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Because of the orbital motion of the Earth, a solar day is longer than a sidereal day. As illustrated in Fig. J-4, the fixed stars are sufficiently far away that lines connecting one of them to the Earth are essentially parallel. Because the Earth's orbital perico is approximately 360 days, angle A is approximately I deg. A sidereal day is defintu as one complete rotation of the Earth, 360 deg, relative to the stars. The Earth has to rotate 360+A deg to complete a solar day. The ratio of the rrran solar day to the mean sidereal day is 1.00273 79093; the mean sidereal day equa's 23 hours, 56 min, 4.09054 sec of mean solar time and the mean solar day equals 24 hours, 3 min, 55.55536 sec of mean sidereal time [U.S. Naval Observatory, 1973]. Note that the "76 days" in the above example indicates the excess number of sidereal days, one for each year, that had occurred since the beginning of the century.

Sidereal time and mean solar time are affected proportionally by variations in the Earth's rotation. Although the irregular fluctuations in the Earth's rotation cannot be predicted, the general deceleration can be seen in Fig. J-5. The lengths of

the two types of day in se are given approximately by:

Sidereal day = 86164.09055 + 0.0015 T Mean solar = 86400 + 0.0015 7" where T is in Julian centuries from 1900.0 [Allen, 1973]. These terms are the dashed line in Fig. J-5..

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