Ecliptic

Ecliptic is the name given 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 name from the early observed fact, that eclipses happen only when the moon is on or near this path. A little attention about sunset or sunrise shows that the sun is constantly altering his position among the stars visible near him, leaving them every day a little farther to the west; and that this motion is not exactly east and west, 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 March 21 and September 23, the sun is exactly on the equator. 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 is 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 (solis stationes). These four points are distant from one another by a quadrant of the circle, or $90^\{\backslash circ\}$. Each quadrant is divided into three arcs of $30^\{\backslash circ\}$, and thus the whole 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 equinoctial points are not fixed, but recede yearly westwards on the ecliptic about 50 seconds, and in a century about $1^\{\backslash circ\}\; 23\text{'}$, the same constellations and signs 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 arc or sign, now stands in the second, every constellation having advanced forward $30^\{\backslash circ\}$, or a whole sign. This is due to a movement of the equator (see PRECESSION OF THE EQUINOXES). Modern astronomers, therefore, pay little attention to these constellations and signs.

Not only do the equinoctial points change, but the angle of inclination of the ecliptic to the equator, called the obliquity of the ecliptic, is also variable. It is at present nearly $23\backslash frac\{1\}\{2\}^\{\backslash circ\}$, and is diminishing at the rate of about 48 seconds in a century. The decrease, however, has a limit, the obliquity oscillating between two definite bounds. It has been calculated that it was at its greatest 2000 B.C., and was then nearly $23^\{\backslash circ\}\; 53\text{'}$. Since then it has been decreasing, and will continue to do so till about the year 6600 A.D., when it will be at its least, and about $22^\{\backslash circ\}\; 54\text{'}$. This is due to a real movement of the ecliptic.

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. Its existence 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. It is probable that the Chaldeans 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. The first known measures of this obliquity were made in the East by Techeou Kong, regent of China (1100 B.C.), and in the West by Pytheas (330 B.C.) and Eratosthenes (200 B.C.).