ECLIPTIC, in astronomy, 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 constellations on the ecliptic give their name to the 12 signs of the zodiac (q.v.), each of which covers 30 degrees. The Greeks observed that eclipses of the sun and moon took place near this circle; whence they called it the ecliptic. The sun does not always rise to the same height in the merid ian, but seems to revolve round the earth in a spiral, and is in the equator twice a year, about 21 March and 23 September. The points of the equator in which the sun is on these days are at the intersection of the equator with the ecliptic. On 21 June the sun reaches its great est height in the heavens, and 21 December it descends the lowest. The position of the equi noxes on the ecliptic is changing about 1 degree 24 minutes per century. (See EQUINOX). The ecliptic has two poles, 90 degrees from it, which appear to revolve about the north and south celestial poles, respectively, every 24 hours. What appears to be the path of the sun, however, is in reality the path of the earth. The planets and the moon revolve in different planes; but these are inclined at only a very small angle to the plane of the ecliptic; hence these bodies can be but a small distance from the ecliptic. The plane of the ecliptic is very
important in theoretical astronomy, because the courses of all the other planets are pro jected upon it and reckoned by it. By the obliquity of the ecliptic we understand its in clination to the equator, or the angles formed by the planes of these two great circles. This angle is measured by the arc of a third great circle, drawn so as to intersect the two others perpendicularly, in the points at which they are farthest apart. The ancients endeavored to measure the obliquity of the ecliptic. Accord ing to Pliny, it was first determined by Anaxi mander. The most celebrated measurement of this obliquity in ancient times was made by Pytheas, at Marseilles. I-Te found it, 350 s.c., to be 23 degrees 49 minutes 23 seconds. A hundred years later, according to Ptolemy, Eratosthenes found it to be 23 degrees 51 min utes 13 seconds. In 1900 the obliquity was 23 degrees 27 minutes 8 seconds; it is diminishing at the rate of nearly half a second a year and is expected to reach a minimum value of 22 degrees 15 minutes in about 15,000 years. It will increase thereafter to about 25 degrees.
See DAY; NUTATION.