504 LECTURE XLII. 



Tlie ecliptic passes through the constellations denomiuated the signs of 

 the zodiac, between Aries, the Pleiades, the twins, and Ilegulus, to the north, 

 and Aldebaran, Spica, and Antares, to the south. Its position has varied 

 slowly in the course of many ages, so that its northmost point is now more 

 than one third of a degree more remote from the pole »tar than it was in 

 the time of Eratosthenes, who observed its place 230 years before the birth of 

 Christ. It appears from Lagrange's calculations, that the limit of its great- 

 est possible variation is about 10 or 11 degrees. The ecliptic is supposed 

 to be divided into twelve angular parts, or signs, each containing thirty 

 degrees: they are named Aries, Taurus, Gemini, Cancer, Leo, Virgo, 

 Libra, Scorpio, Sagittarius, Capricornus, Aquarius, Pisces. Those who 

 prefer the cadence of a Latin distich, in order to assist the memory, may 

 repeat them thus. 



Sunt Aries, Taurus, Gemini, Cancer, Leo, Virgo, 

 Libraque, Scorpius, Arcitcnens, Caper, Amphora, Pisces. 



The planes of the orbits of the other primary planets, excepting the three 

 minute planets lately discovered, intersect the ecliptic in small angles, and 

 the lines of intersection are called lines of the nodes. The nodes of all the 

 planets move very slowly, but not quite uniformly, from east to west, that 

 is, with respect to the fixed stars. At present the inclinations of all the 

 orbits appear to be somewhat diminishing: that of the orbit of Jupiter is less 

 by 6 minutes than it was in the time of Ptolemy. , 



The orbit of each planet is very nearly an ellipsis, one of the foci of which 

 coincides with the sun, or rather with the common centre of inertia of the 

 sun and planet. The extremities of the greater axis, where the orbit is 

 furthest from the sun and nearest to it, are called the upper and the lower 

 apsis, or the aphelion and perihelion; the mean distance being at either 

 end of the lesser axis ; and the distance of the centre of the ellipsis from the 

 sun is called the eccentricity. The slight deviations of the planets from 

 these elliptic paths are expressed by considering the apsides as moveable, 

 and this motion is direct, that is, from west towards east, in the case of 

 all the planets except Venus, of which the aphelion has a retrograde 

 motion, with respect to the fixed stars. 



The elliptic motion of the planets was first discovered by Kepler; and 



