CHAMBERS'S INFORMATION FOR THE PEOPLE. 



it Two astronomers, M. Le Verrier, in Paris, 

 and Mr J. C. Adams of Cambridge, independently 

 of one another, calculated where this disturbing 

 cause must be situated. The results of Le Verrier 

 were first made public ; and Dr Galle of Berlin 

 detected the new planet at the first search, Sep- 

 tember 23, 1846, within two diameters of the 

 moon's disk from the place assigned for it by both 

 astronomers. 



UNIFORMITIES OR LAWS IN THE SOLAR SYSTEM. 



These details of the movements, magnitudes, 

 distances, &c. of the separate members of the 

 solar system, are apt at first sight to appear a 

 mass of unconnected facts, without any discern- 

 ible plan ; but more attentively considered, they 

 display in several respects order and law. 



The first important uniformity to be noticed 

 among the planets is, that their orbits lie all nearly 

 in one plane. If we take the plane of the earth's 

 orbit, which is the same as that of the ecliptic, as a 

 standard of reference, and suppose it represented 

 by a ring held horizontally with the sun in its 

 centre, then the other orbits will be represented by 

 other rings, two within, and the others without, held 

 so as also to have the sun in the centre, and (with 

 the exception of a few of the planetoids) never rising 

 above or sinking below the level of the earth's ring 

 more than a very few degrees. In consequence of 

 this arrangement, the motions of the planets, as 

 seen from the earth, are confined to a narrow zone 

 of the heavens, extending 9 on each side of the 

 ecliptic. This circular belt, called the Zodiac, was 

 from the earliest times divided into twelve equal 

 parts, called signs, containing, of course, 30 each. 

 These signs received each a particular name, from 

 the groups of stars or constellations in them having 

 a fancied resemblance to certain figures, chiefly 

 of animals. The names of the signs, with the 

 symbols by which they are usually represented, 

 are as follows : 



cp Aries, the Ram. sQ: Libra, the Balance. 



y Taurus, the Bull. 1H, Scorpio, the Scorpion. 



TJ Gemini, the Twins, f Sagittarius, the Archer. 



Q Cancer, the Crab. Vj Capricomus, the Goat 



Ji Leo, the Lion. %& Aquarius, the Water-bearer. 



njj Virgo, the Virgin. X Pisces, the Fishes. 



This division of the zodiac affords a ready 

 means of pointing out the place of a planet, or of 

 the sun or moon, at any particular time, by telling 

 in what sign it is. 



Besides moving nearly in one plane, the planets 

 move all in one direction namely, from west to 

 east. The same law prevails among the satellites, 

 with the remarkable exception of those of Uranus, 

 which have a retrograde movement, or from east 

 to west. 



But the grand uniformities of the solar system 

 are those discovered by the celebrated astronomer 

 Kepler, and known as ' Kepler's Laws.' Kepler 

 merely deduced them as matters of fact from the 

 observations of himself and others ; it remained 

 for Newton to discover their cause. One of these 

 laws respects the exact form of the paths in which 

 the planets move. The ancients, from some 

 imaginary perfection which they attributed to the 

 circle, had taken it for granted that all celestial 

 movements must be circular ; Kepler made the 

 important discovery that this is an error, and that 

 the planetary orbits are ellipses. 



The ellipse is an important figure in astronomy ; 

 for not only are the planetary orbits ellipses, but 

 the planets themselves have an elliptical shape, as 

 we saw when speaking of the form of the earth. 

 There is a ready practical way of describing an 

 ellipse, which at the same time gives a good 

 sensible notion of its nature. If the ends of a 

 thread are fastened by 

 pins to two points S 

 and s, at a distance 

 apart less than the 

 length of the thread, A ' 

 and if the point of a 

 pencil P, is put into the 

 loop of the thread, and 

 moved round so as to 

 keep it stretched, the pencil will trace an ellipse. 

 The two points S and s are called the foci of the 

 ellipse ; AB is the major or transverse axis, C the 

 centre ; and SC, the distance of the focus from the 

 centre, is the eccentricity. With the same length 

 of thread, a variety of ellipses may be described, 

 by altering the distance between the points S 

 and s. The nearer they are brought to each other, 

 the rounder does the figure become ; and when 

 they come together, it forms a perfect circle. In 

 the planetary orbits, the sun is always in the focus 

 S, and therefore the planet is at different distances 

 from the sun in different parts of its orbit. When 

 it is at A, the nearest point, it is said to be in 

 perihelion; and when at B, in aphelion. SE is the 

 mean distance, and is equal to AC. The planetary 

 orbits differ very little from circles, or have very 

 little eccentricity. 



Another of Kepler's laws connects the change 

 of a planet's distance from the sun with the speed 

 of its motion : when the distance increases, the 

 speed diminishes / and at the least distance we 

 find the greatest speed. The exact nature of the 

 relation is expressed by saying that the areas 

 swept over by the line joining the sun and planet, 

 which line is called the Radius Vector, are equal 

 in equal times. This is known as the law of 

 Equal Areas. 



The two laws already noticed refer to the 

 motions of a single planet ; the third law shews a 

 relation between the motions of all the planets, or 

 all the bodies that revolve round the same centre. 

 It is, that the squares of the periodic times of any 

 two planets are to each other as the cubes of their 

 mean distances from the sunj that is, if there be 

 two planets, and one farther off than the other, 

 the near planet will perform its revolution quicker 

 than the other, in the proportion above expressed. 

 The same three laws apply to the revolutions of 

 satellites about their primaries. 



DIURNAL AND ANNUAL MOTIONS OF THE EARTH. 



The earth, like all the other planets, has two 

 motions : it whirls round its axis once a day ; and 

 while doing so, it is all the while travelling bodily 

 through space in a wide circuit round the sun, 

 which it accomplishes in a year. The first is 

 called a motion of rotation, the second of transla- 

 tion. The two combined give rise to the vicissi- 

 tudes of day and night, and of the seasons. 



The circumference of the earth being 25,000 

 miles, any spot at the equator, in order to go 

 round in twenty-four hours, must move upwards 

 of loco miles an hour. This velocity decreases 



