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ECLIPTIC
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ECLIPTIC , in See also:astronomy. The See also:plane of the ecliptic is that plane in or near which the centre of gravity of the See also:earth and See also:moon revolves See also:round the See also:sun. The ecliptic itself is the See also:great circle in which this plane meets the See also:celestial See also:sphere. It is also defined, but not with See also:absolute rigour, as the apparent path described by the sun around the celestial sphere as the earth performs its See also:annual revolution. Owing to the See also:action of the moon on the earth, as it performs its monthly revolution in an See also:orbit slightly inclined to the ecliptic, the centre of the earth itself deviates from the plane of the ecliptic in a See also:period equal to that of the nodal revolution of the moon. The deviation is extremely slight, its maximum amount ranging between o•5" and o•6". Owing to the action of the See also:planets, especially See also:Venus and See also:Jupiter, on the earth, the centre of gravity of the earth and moon deviates by a yet minuter amount, generally one or two tenths of a second, from the plane of the ecliptic proper. Owing to the action of the planets, the position of the ecliptic is subject to a slow See also:secular variation amounting, during our See also:time, to nearly 47" per See also:century. The See also:rate of this See also:motion is slowly diminishing. The obliquity of the ecliptic is the See also:angle which its plane makes with that of the See also:equator. Its mean value is now about 23 27'. The motion off, the ecliptic produces a secular variation in the obliquity. which is now diminishing by an amount nearly equal to the entire motion of the ecliptic itself. The See also:laws of motion of the ecliptic and equator are stated in the See also:article PRECESSION OF THE EQUINOXES. Attempts have been made by See also:Laplace and his successors to See also:fix certain limits within which the obliquity of the ecliptic shall always be confined. The results thus derived are, however, based on imperfect formulae. When the problem is considered in a rigorous See also:form, it is found that no absolute limits can be set. It can, however, be shown that the obliquity cannot vary more than two or three degrees within a million of years of our See also:epoch. The See also:formula for the obliquity of the ecliptic, as derived from the laws of motion of it and of the equator, may be See also:developed in a See also:series proceeding according to the ascending See also:powers of the time as follows: we put T, the time from 1900, reckoned in See also:solar centuries as a unit. Then, Obliquity=23° 27' 31.68"—46.837" T—0.0085" T2-f-o•ooI7" T3. From this expression is derived the value of the obliquity at various epochs given in the following table. The See also:left-See also:hand portion of this table gives the values for intervals of 500 years from 2000 B.C. to A.D.2500 as computed from See also:modern data. For See also:dates more than three or four centuries before or after 185o the result is necessarily uncertain by one or more tenths of a See also:minute, and is therefore only given to o• I'. 28' 41.91" 28 18.51 27 55.10 27 31.68 27 8.26 26 44.84 26 21.41 25 57.99 25 34.56 (S. Additional information and CommentsThere are no comments yet for this article.
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