DAY AND NIGHT. 



101 



nights. For an inhabitant at the equator, and one on the 

 same meridian towards the poles, would come into the light 

 at the same time, and, on the other side, would immerge into 

 darkness at the same time. And since the motion of the 

 earth is uniform, they would remain in the dark hemisphere 

 just as long as in the light ; that is, their day and night would 

 be equal; — the plane of the ecliptic would coincide with the 

 plane of the equator. But as the ecliptic and equator make 

 an angle with each other of twenty-three degrees and twenty- 

 eight nnnutes, or in other words, as the axis of the earth has 

 such an inclination to the plane of its orbit, it is manifest 

 that, except the earth be in that part of its orbit where the 

 ecliptic cuts the equator, an inhabitant at the equator and 

 one on the same meridian towards the poles, will not come 

 into the light at the same time, nor, on the other side, im- 

 merge into darkness at the same time. And since the axis 

 of the earth always preserves the same inclination, they 

 will, — except at the points w^here the two great circles inter- 

 sect each other, — remain in the dark and light hemispheres 

 different times , that is, their day and night will be unequal. 

 The points where the equator cuts the ecliptic are at the be- 

 ginning of the signs Libra and Aries. The earth is at 

 these points of its orbit, or, as it is commonly said, the sun 

 enters the sign Aries on the twentieth of March, and the 

 sign Libra on the twenty-third of September. Hence at 

 these periods, and at no others, the days and nights are equal 

 all over the world ; and on this account they are called 

 equinoxes ; the first the vernal, and the second the autum- 

 nal equinox. At these seasons, the sun rises exactly in the 

 east at six o'clock, and sets exactly in the west at six o'clock ; 

 — the light of the sun is then terminated by the north and 

 south poles, and as all parts of the earth turn round once in 

 twenty-four hours, every place must receive the rays twelve 

 hours, and be deprived of them for tho same time. But at 

 other seasons, when the rays of light are not terminated by 

 the north and south poles, but extend over the one and do 

 not reach the other, it must be manifest, from a moment's 

 inspection of the circles drawn on globes, or common maps 

 of the world, that day and night will be unequal in all places 

 except those situated on the" equator, where they will be 

 always equal. At the poles there is but one day and one 

 night in a year, each of six months. The sun can never 

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