ECLIPTIC is the name given to the great circle of the heavens round which the sun seems to travel from w. to e., in the course of a year. It took its name from the early observed fact, that eclipses happen only when both bodies are in or near this path. A little attention about sunset or sunrise shows that the sun is constantly altering his posi tion among the stars visible near him, leaving them every day a little further to the w. ; and that this motion is not exactly e. and w., or parallel to the equator, becomes also evident by observing that the sun's height at mid-day is constantly altering. It is further observed that, twice a year, about Mar. 21 and Sept. 23, the sun is exactly on the equa tor. The two points of the equator on which the sun then stands are the equinoctial points, and are the intersections of the equator and ecliptic. Again, there are two days in the year on which the sun reaches his greatest and his least mid-day elevation: 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 iu his course where he thus seems to pause or halt in his retreat from the equator are called the solstices (soils stations). These four points are distant from one another by a quad rant of the circle, or 90°. Each quadrant is divided into three arcs of 30°, and thus the whole ecliptic is divided into 12 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 equinoctial points are not fixed, but reced6 yearly westwards about 50 seconds, and in a century about 1° 23', the same constellations and signs that coiucided when the division of the ecliptic took place, no longer coincide. The constellation of the rain, for instance, which originally stood in the first arc or sign, now 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 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, but the angle of their inclination, called the obliquity of the ecliptic, is also variable. It is
at present nearly 23°, and is diminishing at the rate of about 50 seconds in a century. Were it to go ou diminishing always, the ecliptic and the equator would at last coincide, and the earth would then have an everlasting spring. The decrease, however, has a limit; the obliquity oscillates between two definite bounds, which it can never pass. It has been calculated that it was at its greatest and was then nearly 23' 53'. Since then, it has been decreasing, and will continue to do so till about the year 6,600 A.D., when it will be at its least, and about 22° 54'. These slight alterations cannot sen sibly affect the seasons.
The physical cause of this change of the obliquity is the action of the other planets, especially Jupiter, Mars, and Venus, on the mass of the earth. The fact of the change was known to astronomers in very ancient times; Herodotus mentions an old tradition of the Egyptians, that the ecliptic had formerly been perpendicular 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. There can be little doubt that the Chal deans arrived at the epoch of 403,000 years before the entry of Alexander into Babylon, to which they proudly referred for their first astronomical observations, by computing the time when the ecliptic was perpendicular to the equator, on the supposition of its obliquity diminishing 1' in 100 years. Though it was not till after the discovery of the law of gravitation that the change on the obliquity could be explained, yet that it was changing was believed in by many astronomers, although some doubted whether the differences in the values at different times were not due to errors of observation. The earliest known measure of the obliquity of the ecliptic was made by Tcheou Kong, the regent of China. Among the western nations, the earliest measurements were made by Pytheas and by Eratosthenes.