74 SCENERY OF THE HEAVENS. 



have a constant motion westward, at a very small yearly rate, but which effects 

 considerable alterations in a series of ages. To give a familiar illustration of the 

 fact, we may suppose three roads to extend round the earth : one, running due east 

 and west, representing the equator or equinoctial; another, 

 proceeding above and below the former, crossing it at two 

 opposite points, representing the ecliptic ; a third, crossing the 

 first at right angles, at the points of intersection with the 

 second, representing the prime meridian, or initial point of 

 longitude. We may now suppose a carriage to be started along 

 the ecliptic from the common point of intersection, returning, 

 after a complete circuit of the earth, not to the place from 

 whence it started, but passing the equinoctial about a hundred 

 rods to the west of it. The repetition of this at each circuit, 



the more westerly intersection of the roads, will illustrate the annual retrograde 

 movement of the equinoctial points, termed on account of its effect in accelerating 

 the time of the equinoxes, their precession, or going forward. This movement, first 

 observed by Hipparchus, causes a progressive increase of the longitude of the stars, 

 and has been clearly shown to be a necessary consequence of the rotation of the earth, 

 combined with its elliptical figure, and the unequal attraction of the sun and moon on its 

 polar and equatorial regions. The mass of matter about the earth's equator being greater 

 than at the poles, the former is more powerfully acted on by the law of attraction than 

 the latter, which produces a slow reeling motion of the axis of the earth from east to west, 

 and the retrocession westward of the equinoctial points. The retrogradation is at the 

 rate of about 50 \" in a year, or 1 in 71^ years, so that in a period of 25,000 years the 

 equinoxes will accomplish a complete revolution along the circle of the ecliptic. While 

 the plane of the earth's equator, or the equinoctial, thus experiences a constant displacement 

 from the action of the sun and moon, it is a remarkable and well ascertained fact that the 

 plane of the earth's orbit, or the ecliptic, is subject to a slow annual displacement, which 

 diminishes its obliquity, an effect due to the baiting which our globe endures from the 

 other planets, chiefly from the attacks of Jupiter and Venus. In consequence of this, 

 the tropics are slowly and steadily approaching the equinoctial, so that the sun does not 

 now come so far north of the equator in summer, nor decline so far south in winter, by 

 nearly a degree, as he must have done at the creation. The obliquity of the ecliptic, or 

 the angle which the plane of the earth's orbit makes with that of the equator, has been 

 observed with care in different ages, and the following results obtained. 



Date. Obliquity. 



Eratosthenes 230 B.C. 23 51' 20" 



Ptolemy - 140 A.D. 23 48' 45" 



Ulugh Beigh - 1463 23 30' 17" 



Cassini - 1655 23 29' 15" 



Flamstead - < 1690 23 29' 00" 



Bradley - "/. 1750 23 28' 18" 



Maskelyne V 1800 23 27' 56"'5 



Airy . * '. 1840 23 27' 36"-5 



Thus, in the interval of two thousand years, the obliquity of the ecliptic has decreased 

 by only 23' 43"'5. There is little ground, therefore, for the apprehension of the seasons 

 being annihilated owing to the ecliptic coinciding with the equator, and equalising the 

 length of our days and nights. The event is far away in the womb of the future, even 

 supposing the diminution to go on without check. But the theory of gravitation tells us 



