ON THE SOLAR SYSTEM. 401 



' and it is inclined in a small angle only to the plane of the earth's orbit, 

 which is called the earth's ecliptic, or simply the ecliptic. 



The ecliptic passes through the constellations denominated the signs of 



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



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



^ Mowly 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 star than it was in 



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



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



greatest possible variation is about 10 or 11 degrees. The ecliptic is sup- 



po&ji to be divided into twelve angular parts, or signs, each containing 



thh J W 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, Arcitenens, Caper, Amphora, Pisces. 



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

 [four] 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 

 he found that a right line, joining the sun and any planet, describes always 

 equal areas in equal times. The observations, on which Kepler* founded 

 these important laws, were made principally on the planet Mars. He 

 determined by calculation, upon the supposition which was then generally 

 adopted, of a motion in an eccentric circle, what must be nearly the situ- 

 ation of the planet, with respect to the sun, that is, its heliocentric place, and 

 observing its geocentric place, with respect to the earth, he was thus able to 

 construct a triangle representing the situation of the three bodies ; repeating 

 this operation in various parts of the orbit, he discovered its form ; and 

 Raving done this, the velocity of the motion in different parts of the orbit 



* See Lect. IV. and Kepler, Astronomia Nova, fol. Pragse. 1609. 



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