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Sidereal time ("sidereal" pronounced / s aɪ ˈ d ɪər i əl, s ə-/ sy-DEER-ee-əl, sə-) is a system of timekeeping used especially by astronomers. Using sidereal time and the celestial coordinate system, it is easy to locate the positions of celestial objects in the night sky.
Apsidal precession is considered positive when the orbit's axis rotates in the same direction as the orbital motion. An apsidal period is the time interval required for an orbit to precess through 360°, [2] which takes the Earth about 112,000 years and the Moon about 8.85 years. [3]
It differs from the sidereal period because the object's semi-major axis typically advances slowly. Also, the tropical period of Earth (a tropical year) is the interval between two alignments of its rotational axis with the Sun, also viewed as two passages of the object at a right ascension of 0 hr.
However, these variations cancel out over a year. There are also other perturbations such as Earth's wobble, but these are less than a second per year. Sidereal time is time by the stars. A sidereal rotation is the time it takes the Earth to make one revolution with rotation to the stars, approximately 23 hours 56 minutes 4 seconds.
The United States Naval Observatory states "the Equation of Time is the difference apparent solar time minus mean solar time", i.e. if the sun is ahead of the clock the sign is positive, and if the clock is ahead of the sun the sign is negative. [6] [7] The equation of time is shown in the upper graph above for a period of slightly more than a ...
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(1) τ E = 1/ν E = A/(C − A) sidereal days ≈ 307 sidereal days ≈ 0.84 sidereal years ν E = 1.19 is the normalized Euler frequency (in units of reciprocal years), C = 8.04 × 10 37 kg m 2 is the polar moment of inertia of the Earth, A is its mean equatorial moment of inertia, and C − A = 2.61 × 10 35 kg m 2 .
Sidereal astrology is not necessarily a lesser-known form of astrology, but the reason this might be your first time hearing about it is simply because it is lesser known in the West.