Now, as the earth is an opake spherical body, at a great distance from the sun, one-half of it will always be illuminated thereby, while the other half will remain in darkness.

The circle which distinguishes or divides the illuminated face of the earth from the dark side, and is the boundary between light and darkness, is generally called the terminator. A line drawn from the centre of the sun to the centre of the earth, is perpendicular to the plane of this circle.

It is plain that, when any given place on the globe first gets into the enlightened hemisphere, the sun is just risen to that part; when it gets half-way, or to its greatest distance from the terminator, it is then noon; and when it leaves the enlightened hemisphere, it is then sun-set.

Here it will be necessary to premise a few considerations: First, that on account of the immense distance of the sun from the earth, the rays which proceed from it may be considered as parallel to each other. Secondly, that only one-half of a globe can be illuminated by parallel rays; and, therefore, only one-half of the earth will be enlightened by the sun at one time.

These considerations will be rendered more forcible, by an attentive survey of fig. 1, plate 5 ; in which

S

represents the sun, from whom we suppose parallel rays to flow in all directions. At A, B, C, are represented three different positions of the globe of the earth, the bright part being that which is illuminated by the rays proceeding from the sun; the shaded

F

part, the portion of the globe which is in darkness; of course, the line TT is the terminator, or boundary of light and darkness.

In the globe at C, the poles coincide with the terminator.

In the globe at A, the north pole N is in the enlightened portion, and the south pole in the dark hemisphere; while, in the opposite globe, at B, the southern pole S is in the illuminated part, and the north pole in obscurity.

It is evident, that it is day in any given place on the globe, so long as that place continues in the enlightened hemisphere; but when, by the diurnal rotation of the earth on its axis, it is carried into the dark hemisphere, it becomes night to that place.

The length of the day and the night depend, therefore, on the position of the terminator, with respect to the axis of the earth.

If the poles of the earth be situated in the terminator, as at C, every parallel will be divided into two equal parts; and as the uniform motion of the earth causes any given place to describe equal parts of its parallel in equal times, the day and the night would be equal on every parallel of latitude; that is, all over the globe, except at the poles, where the sun would neither rise nor set, but continue in the ho rizon.

But if, as at A and B, the axis be not in the plane of the terminator, the terminator will divide the equator into two equal parts, but all the circles parallel to it into unequal parts; those circles that are

situated toward the enlightened pole, will have a greater part of their circumference in the enlightened than in the dark hemisphere; while similar parallels towards the other pole, will have the greater part of their circumference in the dark hemisphere. Whence it follows, that the first-mentioned parallels will enjoy longer days than nights: and the contrary will happen to the latter, where the days will be the shortest, and the nights the longest; while, at the equator, the days will always be of the same length.

Having shewn, that the vicissitudes in the length of the days and nights are occasioned by the position of the terminator with respect to the axis of the earth, I have now only to explain what occasions those various positions; which is the more important, as on these depend the diversity in the seasons of the year.

OF THE SEASONS OF THE YEAR.

In considering this subject, you will find further proofs of that DIVINE WISDOM which pervades all the works of GOD; and see that no other conformation of the system could have given such commodious distributions of light and heat, or imparted fertility and pleasure, to so great a part of the revolving globe.

The changes in the position of the terminator are occasioned-1. By the inclination of the earth's axis to the plane of the ecliptic, or orbit in which it

moves.-2. Because, through the whole of its annual course, the axis of the earth preserves its position, or continues parallel to itself; that is, if a line be conceived as drawn parallel to the axis, while the earth is in any one point of its orbit, the axis will, in every other position of the earth, be parallel to the said line.

It must be evident to you, that the parallelism of the axis must occasion considerable differences. By a bare inspection of the globes, A, B, plate 5, fig. 1, you will see that, when the earth is in one position of its orbit, the north pole will be turned towards the sun; but, in the opposite part, will be turned from him. But the absence of the sun's light produces a proportionable degree of cold; hence the seasons are, in the northern and southern parts of the globe, distinctly marked by different degrees of heat and cold. It is this annual turning of the poles towards the sun, that occasions the very long days in the northern and southern parts. It is owing to the same cause, that the sun seems to rise higher in the heavens during summer than in winter; and this alternate sinking and rising is perceptible over the whole globe.

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If the axis of the earth were perpendicular to the plane of its orbit, the equator and the orbit (or ecliptic), would coincide; and as the sun is always in the plane of the ecliptic, it would, in this case, be always over the equator, as at C, fig. 1, and the two poles would be in the terminator; and there would be no diversity in the days and nights, and but one

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season of the year; but, as this is not the case, we may fairly infer, that the axis of the earth is not perpendicular to the plane of its orbit.

But if the earth's axis be inclined to the plane of the ecliptic, when the earth is in the situation represented at A, plate 5, fig. 1, the pole N will be towards the sun, and the pole S will be turned from it; but just the contrary will happen, when the earth, by going half round the sun, has arrived at B, the opposite point in its orbit. Hence the sun will not always be in the equator, but at one time of the year it will appear nearer to one of the poles, and at the opposite season it will appear nearer to the other. Here, then, you find a cause for the change of the seasons; for, when the sun leaves the equator and approaches to one of the poles, it will be summer on that side of the equator; and when the sun departs from thence, and approaches to the other pole, it will be winter.

These ideas may be strengthened, and a clearer notion obtained of the effect produced by the inclination of the earth's axis, by considering fig. 2, plate 5, in which the ellipsis is supposed to represent the earth's orbit, the eye somewhat elevated above the plane thereof. The earth is here represented in the first point of each of the twelve signs of the ecliptic, as marked in the figure, with the twelve corresponding months annexed; P, the north pole of the globe; Pm, its axis, round which the earth performs its diurnal revolution from west to east; this axis is exhibited as parallel to itself in every part of the orbit;