ECLIPTIC. 



291 



!<?ar one of its nodes, having a small degree of lati- 

 tude, it will only pass over a part of the sun's disc, 

 or produce a partial eclipse. In a total eclipse of 

 tiie sun, the shadow or umbra of the moon falls upon 

 that part of the earth where the eclipse is seen, and 

 a spectator, placed in the shadow, will not see any 

 part of the sun, because the moon will intercept all 

 the rays of light coming directly from the sun. In 

 a partial eclipse of the sun, a penumbra, or imperfect 

 shadow of the moon, falls upon that part of the 

 earth where the partial eclipse is seen. Were the 

 orbit of the earth and that of the moon both in the 

 same plane, there would be an eclipse of the sun 

 every new moon, and an eclipse of the moon every 

 full moon. But the orbit of the moon makes an 

 angle of five degrees and a quarter with the plane 

 of the earth's orbit, and crosses it in two points, 

 called the nodes. Astronomers have calculated, that 

 if the moon be less than 17 21' from either node, 

 at the time of new moon, the sun may be eclipsed ; or 

 if less than 11 34' from either node, at the full 

 moon, the moon may be eclipsed ; at all other times 

 there can be no eclipse, for the shadow of the moon 

 will fall either above or below the earth at the time 

 of new moon ; and the shadow of the earth will fall 

 either above or below the moon, at the time of full 

 moon. An eclipse of the sun begins on the western 

 side of his disc, and ends on the eastern ; and an 

 eclipse of the moon begins on the eastern side of her 

 disc, and ends on the western. The average num- 

 ber of eclipses in a year is four, two of the sun, and 

 two of the moon ; and as the sun and moon are as 

 long below the horizon of any particular place as 

 they are above it, the average number of visible 

 eclipses in a year is two, one of the sun and one 

 of the moon. See Astronomy. 



ECLIPTIC ; the sun's path ; the great circle of 

 the celestial sphere, in which the sun appears to de- 

 scribe 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 stare 

 in the neighbourhood 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 

 iu 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 stationet). At these 

 points, the sun has attained its greatest distance from 

 the equator. These four points, the equinoctial and 

 solstitial points, are distant from one another a quar- 

 ter of a circle, or 90 degrees. Each of these qua- 

 drants, 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 eclip- 

 tic passes, and which extend each 30 degrees. The 

 constellations, or 12 celestial signs, succeed one 

 another in the following order, from the vernal 

 equinox, reckoned towards the east : 



f Aries, March 20. ^ Libra, September 23. 



8 Taurus, April 20. TT\ Scorpio, October 23. 



II Gemini, May 21. f Sagittarius, Nov. 22. 



So Cancer, June 21. Vj Capricornus, Dec. 21. 



1, Leo, July 22. ~r Aquarius, January 19. 



ll Virgo, August 23. \ Pisces, February 18. 



The days of the month annexed show when the sun, 

 in its annual revolution, enters each of the signs of the 

 zodiac. The thirty degrees in every sign are divided 

 into minutes and seconds, not reckoned separately, 

 but after the signs. An arc of the ecliptic, for ex- 

 ample, of 97 15' 27'', reckoned from Aries, east- 

 ward, is called three signs, 7 15' 27'' long, or, what 

 is the same thing, it terminates in 7 15' 27" of Can- 

 cer. In this way the longitude of the stars is given. 

 The ecliptic, like all circles, has two poles, which 

 move about the poles of the earth every twenty-four 

 hours, and in this manner describe the polar circles. 

 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 eclip- 

 tic is very important in theoretical astronomy, be- 

 cause 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 inclination 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. These points of in- 

 tersection are ninety degrees distant from those points 

 at which the equator and ecliptic intersect each other, 

 i. e. the solstitial points. The ancients endeavoured 

 to measure the obliquity of the ecliptic. According 

 to Pliny, it was first determined by Anaximander ; 

 according to Gassendi, it had been ascertained by 

 Thales. The most celebrated measurement of this 

 obliquity in ancient times was made by Pytheas, at 

 Marseilles. He found it, 350 B. C., to be 23 49' 

 23''. A hundred years later, according to Ptolemy, 

 Eratosthenes found it to be 23 51' 20". Various 

 measurements have subsequently taken place, even 

 down to our own time ; and it is remarkable that 

 almost every measurement makes the angle less than 

 those which preceded it. Among the modern esti- 

 mates are that of Cassini, 23 28' 35" ; of La Caille, 

 23 28' 19" ; of Bradley, 23 28' 18' ; and of Mayer, 

 23 28' 16" : the observations of Delambre, Maske- 

 lyne, Piazzi, Bessel, and others, give this important 

 astronomical element, for the year 1800, at 23 27' 

 >&'. In respect to the decrease of the inclination of 

 the ecliptic, the most celebrated astronomers of our 

 time, as Lalande, adopted the opinion that this de- 

 crease continues uninterruptedly. Louville deter- 

 mined the annual decrease to be I', La Caille 44", 

 and Lalande 33". Several philosophers of modern 

 times concluded, from these observations, that the 

 equator and the ecliptic were formerly in the same 

 plane ; that the shock of a comet, or some mighty 

 revolution on the earth, gave the axis of our planet 

 this inclination, and that, for thousands of years, the 

 axis has been returning to its original position, which 

 it will reach after 1 90,000 years. Laplace, on the con 

 trary, in his Mecanique Celeste, showed that this will 

 never take place, but that the decrease of the angle 

 between the planes of the equator and the ecliptic 

 depends merely upon a periodical effect, arising from 

 the action of the other planets ; that, 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 obser- 

 vations 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 de- 

 crease alternately for nine years, during which time 

 the greatest difference amounts to 18'' : of this 

 more is said under the article Nutation o/ the 



