CONVERSATION XXXII.

Of the Seasons.

Tutor. Let us now take a view of the earth in its annual course round the sun, considering its axis as inclined 234 degrees to a line perpendicular to its orbit, and keeping, through its whole journey, a direction parallel to itself; and you will find, that according as the earth is in different parts of its orbit, the rays of the sun are presented perpendicularly to the equator, and to every point of the globe, within 23 degrees of it both north and south.

This figure (Plate vi. Fig. 9.) represents the earth in four different parts of its orbit, or as it is situated with respect to the sun in the months of March, June, September, and December.

Charles. The earth's orbit is not made circular in the figure.

Tutor. No: But the orbit itself is nearly circular, but we are, however, supposed to view it from the side B D, and therefore, though almost a circle, it appears to be a long ellipsis. All circles appear elliptical in an oblique view, as is evident, by looking obliquely at the rim of a basin, at some distance from you. For the true

figure of a circle can only be seen, when the eye is directly over its centre. You observe that the sun is not in the centre.

James. I do; and it appears nearer to the earth in the winter, than in the summer.

Tutor. We are indeed more than three millions of miles nearer to the sun in December than we are in June.

Charles. Is this possible? and yet our winter is so much colder than the summer.

Tutor. Notwithstanding this, it is a wellknown fact. For it is ascertained, that our summer, that is, the time that passes between the vernal and autumnal equinoxes, is nearly eight days longer than our winter, or the time between the autumnal and vernal equinoxes. Consequently the motion of the earth is slower in the former case than in the latter, and therefore, as we shall see, it must be at a greater distance from the sun. Again, the sun's apparent diameter is greater in our winter than in summer, but the apparent diameter of any object increases in proportion as our distance from the object is diminished, and therefore we conclude, that we are nearer the sun in winter than in summer. The sun's apparent diameter in winter is 32'. .47"; in summer 31'. .40".

James. But if the earth is farther from the sun in summer than in winter, why are our winters so much colder than our summers?

Tutor. Because first in the summer, the sun rises to a much greater height above our horizon, and therefore its rays coming more perpendicularly, more of them, as we showed you yesterday, must fall upon the surface of the earth, and come also with greater force, which is the principal cause of our great summer's heat. Secondly, in the summer, the days are very long, and the nights short; therefore the earth and air are heated by the sun in the day, more than they are cooled in the night.

James. Why have we not, then, the greatest heat at the time when the days are longest?

Tutor. The hottest season of the year is certainly a month or two after this, which may be thus accounted for. A body once heated does not grow cold again instantaneously, but gradually; now, as long as more heat comes from the sun in the day, than is lost in the night, the heat of the earth and air will be daily increasing, and this must evidently be the case for some weeks after the longest day, both on account of the number of rays which falls on a given space, and also from the perpendicular direction of those rays.

James. Will you now explain to us in what manner the seasons are produced?

Tutor. By referring to the figure (Plate v1. Fig. 9.) you will observe, that in the month of June, the north-pole of the earth inclines to

wards the sun, and consequently brings all the northern parts of the globe more into light, than at any other time in the year.

Charles. Then to the people in those parts it is summer.

Tutor. It is but in December, when the earth is in the opposite part of its orbit, the northpole declines from the sun, which occasions the northern places to be more in the dark than in the light; and the reverse at the southern places.

James. Is it then summer to the inhabitants of the southern hemisphere ?

Tutor. Yes, it is; and winter to us. In the months of March and September, the axis of the earth does not incline to, nor decline from, the sun, but is perpendicular to a line drawn from its centre. And then the poles are in the boundary of light and darkness, and the sun being directly vertical to, or over the equator, makes equal day and night at all places. Now trace the annual motion of the earth in its orbit for yourself, as it is represented in the figure.

Charles. I will, sir: about the 20th of March the earth is in Libra, and consequently to its inhabitants the sun will appear in Aries, and be vertical in the equator.

Tutor. And then the equator and all its parallels are equally divided between the light and dark.

Charles. Consequently the days and nights are

equal all over the world. As the earth pursues its journey from March to June, its northern hemisphere comes more into light, and on the 21st of that month, the sun is vertical to the tropic of Cancer.

Tutor. And you then observe that all the circles parallel to the equator are unequally divided; those in the northern half have their greater parts in the light, and those in the southern have their larger parts in darkness.

Charles. Yes; and of course it is summer to the inhabitants of the northern hemisphere, and winter to the southern.

I now trace it to September, where I find the sun vertical again to the equator, and of course, the days and nights are again equal. And following the earth in its journey to December, or when it has arrived at Cancer, the sun appears in Capricorn; and it is vertical to that part of the earth called the tropic of Capricorn, and now the southern pole is enlightened, and all the circles on that hemisphere have their larger parts in light, and, of course, it is summer to those parts, and winter to us in the northern hemisphere.

Tutor. Can you, James, now tell me why the days lengthen and shorten from the equator to the polar circles every year?

James. I will try to explain myself on the subject. Because the sun in March is vertical to the equator, and from that time to the 21st of