Pupil. You have passed on from Spring to Autumn.
Tutor. I have so.—We will now return, and trace the earth in its orbit from spring to summer.—You have already seen that the north and south poles are both enlightened, and that the day and night are equal at the equinoxes. If the axis of the earth were perpendicular to the plane of the earth’s orbit, this would constantly be the case, and we should have no diversity of seasons: for, the sun being over the equator, the poles must be perpetually enlightened, and of course we should have equal day and night at all times of the year.
Pupil. That is plain. I suppose then that it is to the inclination of the earth’s axis we are indebted for the increase and decrease of days.
Tutor. It is occasioned by the inclination of the earth’s axis and its preserving its parallelism, which I explained to you last evening.—As the sun is now in the first point of Aries, the earth you know must be in the beginning of Libra, it being the opposite sign.—Now fix your attention on the scheme, and imagine the earth to be advancing in its orbit through Libra, Scorpio, and Sagittarius: and at the first degree of Capricorn give me your opinion of the earth’s position.
Pupil. The north pole is turned to the sun, the south pole from him, and the tropic of Cancer is opposite to him.
Tutor. How many degrees are the tropics from the equator, or, in other words, what is the inclination of the earth’s axis?
Pupil. Twenty-three degrees and a half.
Tutor. And so far are the rays of the sun cast beyond the north pole, and fall short of the south pole: so that the whole of the arctic circle is enlightened, and the antarctic circle involved in darkness.
Pupil. What conclusion am I to draw from this?
Tutor. That in the northern half of the globe it is the longest day, or summer, and in the southern half the shortest, or winter, whilst under the equator the days and nights are equal.