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THE VICISSITUDES OF THE SEASONS, AND
THE DIVISIONS OF TIME.

THE Constant inclination of the axis of the earth to the plane of the ecliptic, and its parallelism, produce the vicissitudes of the seasons. That part of the earth which has the sun at mid-day as near to its zenith as possible, consistently with its latitude, enjoys the pleasant season of summer; and it is winter when the sun is as far distant as possible from its zenith. As, therefore, the track of the ecliptic lies between the two tropics, it is evident that our midsummer must be on the day of the summer solstice, or that on which the sun is vertical, or in the zenith, to all that part of the globe which is under the tropic of Cancer, and our midwinter or winter solstice must be on that day on which that part which lies under the tropic of Capricorn has the sun vertical, or in the

VOL II.

B

zenith, and that period is midsummer to all that part of the southern hemisphere which lies beyond that tropic. Those places also which are situated under the equator will evidently have the sun in their zenith twice a year, at the time of the equi

noxes.

In order that this vicissitude of seasons should take place, that is, that the solar rays should be vertical at the tropic of Cancer at the time of our summer solstice, at that of Capricorn at our winter solstice, and also at the equator at the time of the equinoxes, it is necessary that the axis of the earth should have an inclination to the plane of the ecliptic, and that this axis should preserve its parallelism (as to sense) during the whole of the earth's annual revolution.

To explain this more clearly by a diagram, let S (Pl. I. fig. 1) be the sun; T and t two points diametrically opposite in the annual orbit of the earth; T the point where it is found towards the 21st of June; t the point where it is found towards the 21st of December; EF, or e f, the equator; It the ecliptic or the orbit of the earth; GH, or gh, the tropic of Cancer at the distance of 23 degrees from the equator; IK, or i k, the tropic of Capricorn at the same distance from the equator on the opposite side; P A, or pa, the axis of the earth; P, or p, the north pole; A, or a, the south pole. Now the equator of the earth being inclined at an angle of 231⁄2 degrees to the ecliptic, the sun is bright so many degrees nearer

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