ASTRONOMY. 



[THE SEASONS. 



I..- .-. 



N -.'.. 



\ .:.. 



trmry direction to the 1st of Deoember. From June 



to December they de- 

 scribe ourvod lines, with 

 the concavity turned to 

 the north. 



XiMiIACAL LlOHT. - 



In the evenings of 

 March and April, a 

 cone of faint light is oc- 

 oMionally seen immedi- 

 ately after twilight in the western horizon, [xmiting in the 

 dinvti.'ii, and sometimes reaching to the Pleiades, from 



s ' : - 



tooth. 



, 



the quarter in which the xun sots, and nearly along the 

 ecliptic. The breadth at the base is from 20 to 30, and 

 it* height occasionally fifty degrees. This is knowu as 

 the Zodiacal Light, a name which was given to it by 

 CassinL It is very transparent, since the faintest stars 

 can be perceived through it, equally as well as any other 

 portion of the sky, although the glare of its bright light 

 ; to extinguish very faint objects. Its nature is 

 altogether unknown. It was formerly considered to be 

 the atmosphere of the sun ; but it has been proved by 

 Laplace that this could not extend to such a distance as 

 the zodiacal light has been ob- 

 nerved to pass. An atmosphere 

 to the sun could only extend to 

 such a distance as the orbit of a 

 planet could be situated, whose 

 period of revolution would be 

 equal to that of the sun on its 

 axis, or 25 days, which would be 

 placed considerably within the 

 orbit of Mercury. Neither would 

 the form of such au atmosphere 



agree with that observed with zodiacal light, aa the equa- 

 torial and polar axes could not be beyond the ratio of 

 three to two. Its appearance in the heavens may, how- 

 ever, be explained in this manner, although the cause is 

 unknown ; and it would seem as if a lenticular envelope 

 surrounded the sun, a portion of which we see above 

 the horizon. (See Fig. 29). 



nomena of the seasons. Let S be the sun ; T, T, T", ,(.-<., 

 the different positions of the earth in its orbit . I 1 

 axis of rotation invariably parallel to itself ; T A the line 

 of the equinoxes ; then at the vernal and autumnal 

 equinoxes the aun is in the earth's equator, and the days 

 and nights are of equal length, the BUD illuminating at 

 once one-half of the convex hemisphere. When the sun 

 is at the winter solstice, the south pole of the earth is 

 turned towards the sun, and the equator is above his 

 path by an angle of 23 27'. In our latitude, the cold 

 nurt hern hemisphere, the nights are longer than the days. 

 At the summer solstice, the north pole is turned towards 

 the sun, which is above the equator by its greatest incli- 

 nation. In this latitude the days are longer than the 

 nights. By an attentive examination of the figure, it 

 will be seen that contrary appearances will take place in 

 the southern hemisphere. Thus, if we consider the earth 

 in its diurnal rotation at the summer solstice, the south 

 pole will be constantly in darkness. At the venial and 

 autumnal equinoxes, the sun will shine on both poles of 

 the earth. 



Had the equator coincided with the ecliptic, the days 

 and nights would be always equal ; and had the inclination 

 of the ecliptic to the equator been greater than at present, 

 there would have been a corresponding difference in the 

 seasons. Providence has wisely ordained that thia in- 

 clination cannot exceed certain limits ; the seasons will 

 not, on this account, be sensibly different in occurrence 

 and temperature. 



THB EAKTII'S EQUATOR. In all the preceding investi- 

 gations, we have supposed the pole of the earth's equator 

 to remain fixed, but this is not the case ; and, although 

 it retains the samo inclination to the ecliptic, it des'Tilie*, 

 in the course of a lapse of years, a complete revolution 

 around the pole of the ecliptic. A very simple experi- 

 ment will show, in the spinning top, that a motion of 

 rotation around an axis may exist, without at the same 

 time affecting its inclination, the axis of which will 

 describe any figure. This phenomenon is termed the 

 "precession of the equinoxes," and was discovered 

 by Hipparchus, the astronomer, to whom we aro iu- 



Fig. SO. 

 Vernal Equinox- 



Winter 



Solstice. 



I \ 



Autumnal Equinox. 



Ow THI SEASONS. If we admit the annual revolution 

 of the earth around the un, its diurnal rotation on its 

 axis, and the inclination of this axis to the ecliptic, we 

 can readily account for the seasons, and the derivation of 

 day and night The annual revolution of the earth in its 

 orbit is forced upon us by its extreme simplicity ; we 

 cannot imagine that the sun would revolve around a body 

 1,400,000 times smaller, as the earth is. We thus rank 

 the earth as a planet, obeying all the laws of the other 

 planeU, describing an elliptic orbit around the sun, in the 

 MIDO manner as Mercury, Venus, Mars, <bc. Another 

 proof of the revolution of the earth around the sun is by 

 the well-known existence of a phenomenon, termed tho 

 "Aberration of Light," arising from the velocity of light 

 emanating from a sUr, combined with the orbital motions 

 of the earth. The amount of this angular displacement 

 nsbeen determined with great accuracy by astronomers. 



The above diagram (Pig. 30) will elucidate the phe- 



debted for one of our most ancient catalogues of stars. 

 Kin. 31. He found, by comparing 



the longitudes of his cata- 

 logue with those of some 

 ancient catalogues, that 

 whilst the latitudes of the 

 stars were not changed, their 

 longitudes wore increase.] in 

 the proportion, as he con- 

 sidered, of 1 in 72 years. 

 The physical cause of the 

 precession of the c'|iii. 

 was partly explained liy Sir 

 Isaac Newton, and afterwards more fully by t.he < 

 D'Alembert, and by La Place, in his IfefJumiyi 

 on the theory of gravitation. It results from the ellect 

 of the attraction of the sun and moon on the excess of 

 matter at the earth's equator, wlu'ch produces a slow 



