ECLIPTIC

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ECLIPTIC ; the sun's path; the great circle of the celestial sphere, in which the sun appears to describe his annual course from west to east. The Greeks observed that eclipses of the sun and moon took place near this circle; whence they called it the ecliptic, from eclipses. By a little attention, we shall see that the sun does not always rise to the same height in the meridian, but seems to revolve round the earth in a spiral (see Day). We likewise observe every day, at its rising and setting, new stars in the neighborhood of the sun. It will also be seen, that the sun is in the equator twice a year; about March 22 and September 22. The points of the equator, at which the sun is stationary on these days, are at the intersection of the equator with the ecliptic. June 21, the sun reaches its greatest height in the heavens; and December 21, it descends the lowest. Because the sun appears to turn back at these points, they are called the tropics; and the times at which the turning appears to commence are called solstices (solstitia, solis stationes). At these points, the sun has attained its greatest distance from the equator. These four poin,ts, the equinoctial and solstitial points, are distant from one another a quarter of a circle, or 90 degrees. Each of these quadrants, or quarters of a circle, is divided into 3 equal arcs of 30 degrees ; thus the whole ecliptic is divided into 12 equal arcs or signs: these receive their names from certain constellations through which the ecliptic passes, and which extend each 30 degrees. The constellations, or 12 celestial signs, succeed one another in the following order, from the vernal squinox, reckoned towards the east: °f> Aries, March 20.y Taurus, April 20.EL Gemini, May 21. <EB Cancer, June 21.Si Leo, July 22.n^ Virgo, August 23.£± Libra, September 23.v\ Scorpio, October 23.f Sagittarius, November 22.V? Capricorn us, December 21.z? Aquarius, January 19.X Pisces, February 18. The days of the month annexed show when the sun, in its annual revolution, enters pnrh ni" thft simis of thfi Tindinr.. inclined at only a very s plane of the ecliptic; he can be but a small di echptic. The plane of tl important in theoretical cause the courses of all are projected upon it, an By the obliquity of the e stand its inclination to th angles formed by the pk great circles. This angt the arc of a third great ci to intersect the two other* in the points at which apart. These points ol 90 degrees distant from which the equator and each other, i. e. the solsti ancients endeavored to ] quity of the ecliptic. Ac it was first determined I according to Gassendi, il tained by Thales. The measurement of this oh times was made by Pyth He found it, 350 B. C, t A hundred years later, a emy, Eratosthenes founc 20". Various measureir quently taken place, e^ own time; and it is rer most every measurement less than those which prec the modern estimates are 23° 28'35"; ofLaCailk Bradlev, 23° 28' 18"; ai 28' 16": the observatio Maskelyne, Piazzi, Besse this important astronom the year 1800, at 23° 27'i the decrease of the inclin tic, the most celebrated ai time, as Lalande, adoptei this decrease continues Louville determined the tn ho i/ T.n r,"u\^ AA'f > after a certain time, it will increase again, and that the limits of variation are narrow and fixed. A very long space of time will be required to make satisfactory observations respecting this fact. The inclination of the ecliptic, or, which is the same thing, the inclination of the axis of the earth towards the ecliptic, is subject to another change, which makes it increase and decrease alternately for nine years, during which time the greatest difference .amounts to 18": of this more is said under the article Nutation of the Earth's Axis. (See Astronomy, Degree, Equinoctial, Day &c.)