SECTION FOURTH.

PHENOMENA OF THE HEAVENS, AS SEEN FROM DIFFERENT PARTS OF THE EARTH.

THE magnitude of the Earth is only a point when compared to the Heavens, and therefore every inhabitant upon it, let him be in any place on its surface, sees half of the Heavens. The inhabitant on the North Pole of the Earth, constantly sees the Northern Hemisphere, and having the North Pole of the Heavens directly over his head, his horizon coincides with the celestial Equator. Therefore all the Stars in the Northern Hemisphere, between the Equator and the North Pole, appear to turn round parallel to the horizon. The Equatorial Stars keep in the horizon, and all those in the Southern Hemisphere are invisible. The like phenomena are seen by an observer at the South Pole.Hence, under either pole, only one half of the Heavens is seen; for those parts which are once visible never set, and those which are once invisible never rise.

But the ecliptic or orbit, which the Sun appears to describe once a year by the annual motion of the Earth, has the half constantly above the horizon of the north pole; and the other half always below it. Therefore whilst the Sun describes the northern half of the ecliptic, he neither sets to the north pole, nor rises to the south; and whilst he describes the southern half, he neither sets to the south pole, nor rises to the north.-The same observations are true with respect to the Moon, with this difference only, that as the Sun describes the ecliptic but once a year, he is, during half that time, visible to each pole in its turn, and as long invisible.

But, as the Moon goes round the ecliptic in 27 days, 8 hours, she is only visible during 13 days and 16 hours, and as long invisible to each pole by turns.

All the planets likewise rise & set to the polar regions because their orbits are cut obliquely in halves by the horizon of the poles. When the Sun arrives at the sign Aries, which is on the twentieth of March, he is just rising to an observer on the north pole, and setting to another on the south pole.* From the Equator, he rises higher and higher in every apparent diurnal revolution, till he comes to the highest point of the ecliptic on the 21st of June, and then he is at his greatest altitude, which is 23 degrees and 28 minutes;

*It is therefore evident when the Sun is on the Equator, an observer placed at each pole, sees about one half of the Sun above the horizon, and likewise an observer at the Equator discovers both poles in the horizon.

equal to his greatest north declination, and from thence he seems gradually to descend in every apparent circumvolution till he sets at the sign Libra, on the 23d September, and then he goes to exhibit the same appearances at the south pole for the other half of the year. Hence the Sun's apparent motion round the earth is not in parallel circles, but in spirals, such as may be represented by a thread wound round a globe from one tropic to the other. If the observer be any where on the terrestrial equator, he is in the plane of the celestial equator or under the equinox, and the axis of the earth is coincident with the plain of his horizon extended to the north and south poles of the Heavens. As the earth performs her diurnal revolution on her axis from west to east, the whole Heavens seem to

turn round the contrary way. It is therefore plain that the observer at the equator has the celestial poles constantly in his horizon, and that his horizon cuts the diurnal paths of all the celestial bodies perpendicularly, and in halves. Therefore, the Sun, planets and stars rise every day, and ascend perpendicularly above the horizon for six hours, and passing over the meridian, descend in the same manner, for the six hours following, then set in the horizon and continue 12 hours below it; consequently the days and nights are equally long throughout the year. Thus we find, that to an observer at either of the poles, one half of the sky is always visible, and the other half never seen; but to an observer on the equator, the whole sky is seen every 24 hours. From the preceding observations, it is ev

ident, that as the Sun advances from the equator to the tropic of Cancer, the days continually lengthen, and the nights shorten in the northern hemisphere, and the contrary in the southern; and when the Sun descends from the equator to the tropic of Capricorn, the days continually lengthen in the southern hemisphere, and the nights shorten; and the contrary in the northern.

The earth's orbit being elliptical, and the Sun constantly keeping in its lower focus, which is one million, three hundred and seventy-seven thousand miles from the middle point of the longer axis, the earth comes twice that distance, or 2,754,000 miles nearer the Sun* at one time of the year than at another; for the Sun appearing under a larger anglet in our winter than summer, proves that the earth is nearer the Sun in winter than in summer. The Sun is about 7 days longer in the northern hemisphere than in the southern in every year; and as the earth approaches near to the Sun, its motion is accelerated, and therefore goes over an equal space in less time, and as the earth recedes from the Sun, its motion is retarded in the same ratio that it was accelerated when in the southern hemisphere, and consequently requires a longer time to pass over an equal space.

* The Sun is nearest to the Earth when he is on the tropic of Capricorn; farthest from it when he is on the tropic of Cancer.

†The nearer an object is to the eye, the larger it appears, and under the greater angle it is seen.

But here a question actually arises; why have we not the hottest weather when the earth is nearest to the Sun? In answer, it must be observed, that the eccentricity of the earth's orbit bears no greater proportion to the earth's mean distance from the Sun, than seventeen bears to a thousand, and therefore this small difference of distance cannot occasion any great difference of heat or cold.

But the principal cause of this difference is, that in winter, the same rays fall so obliquely upon us, that any given number of them is spread over a much greater portion of the earth's surface which we inhabit, and therefore each point must then have less rays than in summer. Also there comes a greater degree of cold in the long winter nights, than there can return of heat in so short days, and on both these accounts, the cold must increase. But in the summer season, the Sun's rays fall more perpendicularly upon us, aud therefore come with greater force, and in greater numbers on the same place, and by their long continuance, a much greater degree of heat is imparted by day than can fly off by night. It is for this reason that we have a greater degree of heat in the month of September, than in the month of March; the Sun being on the equator in both these months, and consequently equally distant from the earth. Those parts which are once heated, retain the heat for some time, which, with the additional heat daily imparted, makes it continue to increase, though the Sun declines towards the south, and this is the reason why we have greater heat in July