PCE shews the angle of its inclination; e, the pole; ed, the axis of the ecliptic, perpendicular to the plane

of the orbit.

In March, when the earth is in the first point of Libra, the sun appears in the opposite point of the ecliptic at Aries. In September, when the earth is in the first point of Aries, the sun will be in Libra. At these times, the terminator passes through the poles of the world, and divides every parallel into two equal parts (see C, fig. 1); consequently, the nocturnal and diurnal arcs, or the length of day and night, will be equal in all places over the world.

Conceive the earth to have moved from Libra to Capricorn in June, the axis, Pd, preserving its parallelism by this motion, the north pole will have gradually advanced into the enlightened hemisphere; so that the whole northern polar circle will be therein, while the southern pole is immerged in obscurity; the northern parts of the world will enjoy long days, while they are short in the southern parts. While the earth is moving from Libra through Capricorn to Aries, the north pole remains in the illuminated hemisphere, and will therefore have six months eontinual day.

But in the other half-year, while the earth is moving from Aries through Cancer to Libra, the north pole is turned from the sun, and therefore in darkness ; but the south pole is in the illuminated hemisphere. When the earth is at Cancer, the sun is at Capricorn; at this season the nights to us will as much exceed the days, as the days exceeded the

nights, when the earth was in the opposite point of her orbit.

From the foregoing explanation, it is easy to perceive that the inhabitants of the southern hemisphere have the same vicissitudes with those of the northern, though not at the same time; it being winter in one hemisphere, when it is summer in the other.

From what has been said you must have perceived that, during the course of the earth through her orbit, there are four days particularly to be remarked; these astronomers have distinguished by the names of the solstitial and equinoctial days. The solstitial days are those on which the sun appears most to the northward and southward; the equinoctial days are those on which he appears in the equator, and the days are equal to the nights.

The annual motion of the earth occasions a daily apparent change in the declination of the sun. Thus, about the 22nd of December, when the earth is in Cancer, the sun will be over the tropic of Capricorn; and, consequently, by the earth's rotation on its axis, the inhabitants of every part of this circle will successively have the sun in their zenith; or, in other words, he will be vertical to them that day

at noon.

About the 21st of March, the earth is at Libra, and the sun will then appear in Aries; a central solar ray will terminate upon the surface of the earth in the equator; and, therefore, the sun appears to be carried round the celestial equator,

and is successively vertical to those who live under that circle.

About the 21st of June, when the earth is in Capricorn, a central solar ray terminates on the surface of the earth, in the northern tropic; and, for that day, the sun appears to be carried round in the tropic of Cancer, and is vertical to those who live under that circle. About the 22nd of September, the earth is in Aries, and the sun in Libra, and the central solar ray again terminates at the equator; consequently, the sun again appears in the celestial equator, and is vertical to those that live under it.

We have seen, that as the sun moves in the ecliptic, from the vernal equinox to the tropic of Cancer, it gets to the north of the equator; or its declination towards our pole increases. Therefore, from the vernal equinox, when the days and nights are equal, till the sun comes to the tropic of Cancer, our days lengthen and our nights shorten; but when the sun comes to the tropic of Cancer, it is then in its utmost northern limit, and returns in the ecliptic to the equator again. During this return of the sun, its declination towards our pole decreases; and, consequently, the days decrease and the nights increase, till the sun is arrived in the equator again, and is in the autumnal equinoctial point, when the days and nights will again be equal. As the sun moves from thence towards the tropic of Capricorn, it gets to the south of the equator; or its declination towards the south pole increases. Therefore, at that time of the year, our days shorten and our nights lengthen,

till the sun arrives at the tropic of Capricorn; but when the sun is arrived there, it is then at its utmost southern limit, and returns in the ecliptic to the equator again. During this return, its distance from our pole lessens; and, consequently, the days will lengthen as the nights shorten, till they become equal, when the sun is come round to the vernal equinoctial point.

Our summer is nearly eight days longer than the winter. By summer is meant here the time that passes between the vernal and autumnal equinoxes; by winter, the time between the autumnal and vernal equinoxes. The ecliptic is divided into six northern and six southern signs, and intersects the equator at the first of Aries and the first of Libra. In our summer, the sun's apparent motion is through the six northern, and in winter through the six southern signs; yet the sun is 186 days, 11 hours, 51 minutes, in passing through the six first; and only 178 days, 17 hours, 58 minutes, in passing through the six last. Their difference 7 days, 17 hours, 53 minutes, is the length of time by which our summer exceeds the winter.

In plate 6, fig. 1, ABCD represents the earth's orbit; S the sun in one of its foci; when the earth is at B, the sun appears at H, in the first point of Aries; and whilst the earth moves from B through C to D, the sun appears to run through the six northern signs, from Y through to at F. When the earth is at D, the sun appears at F, in the first point of Libra; and as the earth moves from D

through A to B, the sun appears to move through the six southern signs, from through V to Aries at H.

Hence the line FH, drawn from the first point of Aries through the sun at S, to the first point of ~, divides the ecliptic into two equal parts; but the same line divides the earth's elliptical orbit into two unequal parts. The greater part BCD is that which the earth describes in the summer, while the sun appears in the northern signs. The lesser part is DAB, which the earth describes in winter, while the sun appears in the southern signs. C, the earth's aphelion, where it moves slowest, is in the greater part; A, its perihelion, is in the lesser part, where the sun moves fastest.

There are, therefore, two reasons why our summer is longer than our winter; first, because the sun continues in the northern signs, while the earth is describing the greater part of its orbit; and, secondly, because the sun's apparent motion is slower while it appears in the northern signs, than whilst it appears in the southern ones.

The sun's apparent diameter is greater in our winter than in summer; because the earth is nearer to the sun when at A in the winter, than it is when at C in the summer. The sun's apparent diameter in winter, is 32 minutes, 47 seconds; in summer, 31 minutes, 40 seconds.

But if the earth is farther from the sun in summer than in winter, it may be asked, why our winters are so much colder than our summers? To this it