ECLIPTIC (Lat. ee/i/dieuN. from ekiciptikos: relating to an eelipse. from ievo,lc ekleipsis, eclipse). The name gitcu to the great circle of the heavens round which the sun seems to travel from west to east in the course of a year. It took its nanie from the early observed tact that eclipses (q.v.) happen only when the sun and moon are in or near this circle. At tention about sunset or sunrise shows that the sun is constantly altering his position among the stars visible near him, leaving them every dayfarther to the west; that this motion is not exactly east and west, or parallel to the celestial equator, becomes evident by observing that the sum's height at midday is constantly altering. it was discovered very early in the history of astronomy that twice a year, about Mara 21st and September 23d. the sun is exact ly on the equator. The two points of the equator on which the sun then stands are the equinoctial points (See EQUINOXES: EQworri.u.), and are the intersections of the equator and ecliptic. Again, there are two days in the year on which the snit reaches its greatest and its least mid day elevation in the Northern Ilemisphere—the first is the 21st of June: the second, the 21st of December. On these days the sun has reached his greatest distance from the equator either way, and the points in his course where he thus seems to pause or halt in his retreat from the equator are called the solstices (soils stationes). These four points are distant from one another by a quadrant of the circle, or 90°. Each quad rant is divided into three ores of 30°, and thus the ecliptic is divided into twelve arcs of that length. called signs of the zodiac (q.v.). These arcs or signs have been named after constellations through which the ecliptic passes. As the equi noctial points are not fixed, hut recede yearly westward about 50", and in the century about 1' 2-4% the same constellations and that coincided when the division of the ecliptic took place no longer coincide. The constellation of
the Ram, for instance, which originally stood in the first are cr sign. stands in the second, every constellation having advanced forward 30°, or a whole sign. Modern astronomers therefore pay little attention to these constellations and signs. but count celestial longitudes from the existing spring equinoctial point from 0° to 360°. Not only do the points change where the ecliptic and equator cross each other. hut the angle be tween these two circles, called the obliquity of the ecliptic, is also variable. It is at present nearly 23.5°, and is diminishing at the rate of about 50" in a .century. Were it to go on di minishing. always. the ecliptic and the equator would at last coincide. The decrease, however, has a limit: the obliquity oseillates between two definite bounds, which it can never pass. It has been ealeulated that the diminution will continue for about ISO centuries. when the obli quity will only amount to 22.25°. After that it will again increase, tending toward an upper limit of about 25°. These slight alterations cannot sensibly affect the seasons.
The physical cause of this change of the ob liquity is the gravitational action of the other planets. especially Jupiter. 'Nlars, and Venus, on the mass of the earth. The fact of the change was known to astronomers in very ancient times; 11 erodotus mentions an old tradition of the Egyptians that the ecliptic had formerly been to the equator—a notion into which they were most probably led by observing for a long series of years that its obliquity was constantly diminishing. Though it was not until after the discovery of the law of gravita tion that the change of the obliquity could be explained. yet that it was changing was long believed by many astronomers. although some doubted whether the differences in the values at different times were not due to errors of obser vation. The earliest known measure of the ob liquity of the ecliptic was probably made in China.