ECLIPTIC

, in Astronomy, a great circle of the sphere conceived to pass through the middle of the zodiac. It is sometimes called the via solis, or sun's path, being the track which he appears to describe among the fixed stars; though more properly it is the apparent path of the earth, as viewed from the sun, and thence called the heliocentric circle of the earth. It is called the ecliptic, because all the eclipses of the sun or moon happen when the moon crosses it, or is nearly in one of those two parts of her orbit where it crosses the ecliptic, which points are called the moon's nodes.

Upon the ecliptic are marked and counted the 12 celestial signs, Aries, Taurus, Gemini, &c; and upon it is counted the longitude of the planets and stars. It is placed obliquely with respect to the equator, which it cuts in two opposite points, viz, the beginning of Aries and Libra, which are directly opposite to each other, and called the equinoxes, making the one half of the ecliptic to the north, and the other half on the south side of the equator; the two extreme points of it, to the north and south, which are opposite to each other, and at a quadrant distance from the equinoctial points both ways, are called the solstices, or solstitial points, or also the two tropics, which are at the beginning of Cancer and Capricorn, and which are at the farthest distance of any points of it from the equator, which distance is the measure of the sun's greatest declination, which is the same with the obliquity of the ecliptic, or the angle it makes with the equator.

This obliquity of the ecliptic is not permanent, but is continually diminishing, by the ecliptic approaching nearer and nearer to a parallelism with the equator, at the rate of half a second in a year nearly, or from 50″ to 55″ in 100 years, as is deduced from ancient and modern observations compared together; and as the mean obliquity of the ecliptic was 23° 28′ about the end of the year 1788, or beginning of 1789, by adding half a second for each preceding year, or subtracting the same for each following year, the mean obliquity will be found nearly for any year either before or since that period. The quantity however of this change is variously stated by different authors, from 50″ to 60″ or 70″ for each century or 100 years. Hipparchus, almost two thousand years since, observed the obliquity of the ecliptic, and found it about 23° 51′; and all succeeding astronomers, to the present time, having observed the same, have found it always less and less; being now rather under 23° 28′; a difference of about 23′ in 1950 years; which gives a medium of 70″ in 100 years. There is great reason however to think that the diminution is variable.

This diminution of the obliquity of the ecliptic to the equator, according to Mr. Long and some others, is chiefly owing to the unequal attraction of the sun and moon on the protuberant matter about the earth's equator. For if it be considered, say they, that the earth is not a perfect sphere, but an oblate spheroid, having its axis shorter than its equatorial diameter; and that the sun and moon are constantly acting obliquely upon the greater quantity of matter about the equator, drawing it, as it were, towards a nearer and nearer coincidence with the ecliptic; it will not appear strange that these actions should gradually diminish the angle between the planes of those two circles. Nor is it less probable that the mutual attractions of all the planets should have a tendency to bring their orbits to a coincidence: though this change is too small to become sensible in many ages.

It is now however well known that this change in the obliquity of the ecliptic, is wholly owing to the actions of the planets upon the earth, and especially the planets Venus and Jupiter, but chiefly the former. See La Grange's excellent paper upon this subject in the Memoirs of the French Academy for 1774; Cassini's in 1778; and La Lande's Astron. vol. 3, art. 2737. According to La Grange, who proceeds upon theory, the annual change of obliquity is variable, and has its limits: about 2000 years ago, he thinks it was after the rate of about 38″ in 100 years; that it is now, and will be for 400 years to come, 56″ per century; but 2000 years hence, 49″ per century. According to Cassini, who computes from observations of the obliquity between the years 1739 and 1778, the annual change at present is 60″ or 1′ in 100 years. But according to La Lande, the diminution is at the rate of 88″ per century; while Dr. Maskelyne makes it only 50″ in the same time.

Beside the regular diminution of the obliquity of the ecliptic, at the rate of near 50 seconds in a century, or half a second a year, which arises from a change of the ecliptic itself, it is subject to two periodical inequalities, the one produced by the unequal force of the sun in causing the precession of the equinoxes, and the other depending on the nutation of the earth's axis. See the Explanation and Use of Dr. Maskelyne's Tables and Observations, pa. vi, where we are shewn how to calculate those inequalities, and where he shews that, from his own observations, the mean obliquity of the ecliptic to the beginning of the year 1769, was 23° 28′ 9″.7.

To find the Obliquity of the Ecliptic, or the greatest declination of the sun: about the time of the summer solstice observe very carefully the sun's zenith distance for several days together; then the difference between this distance and the latitude of the place, will be the obliquity sought, when the sun and equator are both on one side of the place of observation; but their sum | will be the obliquity when they are on different sides of it. Or, it may be found by observing the meridian altitude, or zenith distance, of the sun's centre, on the days of the summer and winter solstice; then the difference of the two will be the distance between the tropics, the half of which will be the obliquity sought.

By the same method too, the declination of the sun from the equator for any other day may be found; and thus a table of his declination for every day in the year might be constructed. Thus also the declination of the stars might be found.

The observations of astronomers of all ages, on the obliquity of the ecliptic, have been collected together; and although some of them may not be quite accurate, yet they sufficiently shew the gradual and continual decrease of the obliquity from the times of the earliest observations down to the present time. The chief of those observations may be seen in the foregoing table; where the first column contains the name of the observer, or author, the 2d the year before or after Christ, and the 3d the obliquity of the ecliptic for that time.

See Ptolom. Alm. lib. 1, cap. 10; Ri<*>cioli Ahr. vol. 1, lib. 3, cap. 27; Flamsteed Proleg. Hist. Cœl. vol. 3; Philos. Trans. number 163; ib. vol. 63, pt. 1; Long's Astron. vol. 1, cap. 16; Memoirs of the Acad. an. 1716, 1734, 1762, 1767, 1774, 1778; Acta Erud. Lipsiæ 1719; Naut. Alm. 1779; Maskelyne's Observ. Explan. pa. vi; &c.

According to an ancient cradition of the Egyptians, mentioned by Herodotus, the ecliptic had formerly been perpendicular to the equator: they were led into this notion by observing, for a long series of years, that the obliquity was continually diminishing; or, which amounts to the same thing, that the ecliptic was continually approaching to the equator. From thence they took occasion to suspect that those two circles, in the beginning, had been as far off each other as possible, that is, perpendicular to each other. Diodorus Siculus relates, that the Chaldeans reckoned 403,000 years from their first observations to the time of Alexander's entering Babylon. This enormous account may have some foundation, on the supposition that the Chaldeans built on the diminution of the obliquity of the ecliptic at the rate of a minute in 100 years. M. de Louville, taking the obliquity such as it must have been at the time of Alexander's entrance into Babylon, and going back to the time when the ecliptic, at that rate, must have been perpendicular to the equator, actually finds 402,942 Egyptian, or Chaldean years; which is only 58 years short of that epocha. Indeed there is no way of accounting for the fabulous antiquity of the Egyptians, Chaldeans, &c, so probable, as from the supposition of long periods of very slow celestial motions, a small part of which they had observed, and from which they calculated the beginning of the period, making the world and their own nation to commence together. Or perhaps they sometimes counted months or days for years.

Should the diminution always continue at the rate it has lately done, viz at 50″ or 56″ a century, it would take 96,960 years, from the year 1788, to bring the ecliptic exactly to coincide with the equator.

, in Geography, a great circle on the terrestrial globe, in the plane of, or directly under, the celestial ecliptic.

, Eclipticus, something belonging to the ecliptic, or to eclipses; as ecliptic conjunction, opposition, &c.

Ecliptic Bounds, or Limits, are the greatest distances from the nodes at which the sun or moon can be eclipsed; namely, near 18 degrees for the sun, and 12 degrees for the moon.

Ecliptic Digits, digiti ecliptici. See Digits.

Poles of the Ecliptic, are the two opposite points | of the sphere which are each everywhere equally distant from the ecliptic quite around, or 90° distant from it. The distance of the poles of the ecliptic from the poles of the equator, or of the world, is always equal to the varying distance of the obliquity of the ecliptic, and at the beginning of the year 1789 it was just 23° 28′.

Reduction to the Ecliptic. See Reduction.