Astronomy. [Lecture 23. 



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 win- 

 ter 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 to- 

 wards the 21st of December; EF, or e f, the 

 equator ; 1 1 the ecliptic or the orbit of the earth ; 

 G H, or g 7t, the tropic of Cancer at the distance 

 of 23f degrees from the equator ; I K, or i k, the 

 tropic of Capricorn at the same distance from the 

 equator on the opposite side ; P A, or p a, 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 23f degrees to the 

 ecliptic, the sun is bright so many degrees nearer 



