ECLIPTIC

This article appears in Volume V08, Page 895 of the Encyclopedia Britannica.

Encyclopedia Britannica - Main :: ECG-EMS

ECLIPTIC , in astronomy. The plane of the ecliptic is that plane in or near which the centre of gravity of the earth and moon revolves round the sun. The ecliptic itself is the

great

GREAT

circle in which this plane meets the

celestial

CELESTIAL, OR STELLAR

sphere. It is also defined, but not with absolute rigour, as the apparent path described by the sun around the

celestial

CELESTIAL, OR STELLAR

sphere as the earth performs its annual revolution. Owing to the action of the moon on the earth, as it performs its monthly revolution in an orbit slightly inclined to the ecliptic, the centre of the earth itself deviates from the plane of the ecliptic in a period equal to that of the nodal revolution of the moon. The deviation is extremely slight, its maximum amount ranging between o5" and o6". Owing to the action of the planets, especially

Venus

VENUS

and 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 secular variation
amounting, during our time, to nearly 47" per century. The rate of this motion is slowly diminishing.
The obliquity of the ecliptic is the angle which its plane makes with that of the

equator

EQUATOR (
Late
- LATE
Lat. aequator, from aequare, to make equal)

. 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

laws

LAWS

of motion of
the ecliptic and

equator

EQUATOR (
Late
- LATE
Lat. aequator, from aequare, to make equal)

are stated in the article PRECESSION OF THE EQUINOXES.
Attempts have been made by Laplace and his successors to 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 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 epoch. The

formula

for the obliquity of the ecliptic, as derived from the

laws

LAWS

of motion of it and of the equator, may be developed in a

series

SERIES (a Latin word from serere, to join)

proceeding according to the ascending powers of the time as follows: we put T, the time from 1900, reckoned in solar centuries as a unit. Then,
Obliquity=23 27' 31.68"46.837" T0.0085" T2-f-oooI7" T3. From this expression is derived the value of the obliquity at various epochs given in the following table. The left-hand portion of this table gives the values for intervals of 500 years from 2000 B.C. to A.D.2500 as computed from modern data. For

dates

DATES

more than three or four centuries before or after 185o the result is necessarily uncertain by one or more tenths of a 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. N.)

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