northern frigid zone, on the earth, will turn wholly in the light. The southern frigid zone will be entirely dark. Every part north of the equator, as it turns, will enjoy more light than darkness, and proportionably more, as farther distant from the equator, towards the circle of entire light. The reverse will be seen in the southern hemisphere. Here will be a near resemblance of the earth at the summer solstice. A western position of the globe, will represent the autumnal equinox, while that directly over the candle will show the winter solstice. Let the view be enlarged. Instead of the candle and globe, or a figure, let the sun and earth be contemplated, and an idea may be formed of the cause of inequality in days and nights, and of the change of seasons. Considering the great luminaries themselves, their motions, and circles, affords a more correct idea, than the examination of any diagram. Figures, though often useful, generally give a distorted view.

To those, who are not favoured with a globe, a figure may be some assistance. Plate VI. Fig. 1, represents the earth at different seasons. Let E, N, Q, S, be the earth, N the north and S the south pole. It must be apparent, from the representation, that in spring and autumn the tropics, the polar circles, and all the parallels, are one half in the light, and that, as any place revolves round the axis of the earth, in its diurnal rotation, it must have a day and night very nearly equal. But in summer, the whole Arctic regions must be in the light, and the Antarctic in darkness, the polar circles just touching the dividing line between light and darkness. The tropic of Cancer and all the parallels north of the equator, to the Arctic circle, must turn more than half in the light; the tropic of Capricorn and all the parallels south of the equator, must turn more than half in the dark. The different length of the lines in the figure must make this apparent. Compare the figure with Fig. 5 of Plate V. In winter, the scene is reversed, the southern hemisphere taking the greater proportion of light, the northern of dark

ness.

1

The difference in the modes of projection, observable in this figure, (Plate VI. Fig. 1.) was for convenience of representing the earth at different seasons. The large ellipse was not intended for the true figure of the earth's orbit, but an oblique view of the orbit.

The earth revolves round the sun from a star to the same star again in 365 days, 6h. 9m. 12s. ; from an equinox or solstice, to the same again, in 365 days, 5h. 48m. 51.6s.* The former is called the sidereal year; the latter, the tropical, equinoctial, and sometimes, solstitial year. This is usually reckoned from the first degree of Aries. From the aphelion of its orbit to the same again, the earth performs its revolution in 365 days, 6h. 14m. 2s. This is called the anomalistic year.

The earth is between 7 and 8 days longer in passing the six northern signs, than the six southern. This may be ascertained by comparing the time from the vernal equinox to the autumnal, with that from the autumnal to the vernal. By this inequality, the inhabitants of the northern hemisphere of the earth enjoy a greater share of the sun's influence, than those of the southern.

It is a circumstance not only curious, but tending to confirm the truth of revelation, that about the time of the creation, probably at the very time, the earth being in or about the aphelion, at the time of the vernal equinox, the summers were equal in the two hemispheres. But, by the aphelion moving forward in the signs 1′ 2′′ in a year, the northern gained the ascendency. The maximum was about the year 1297 of the christian era. Since which time the difference has been decreasing, and will continue to decrease till about the year 6523, when the summers will again become equal. After that, should the earth continue in its present state,

*Thus it is computed by La Place, considered a very accurate mathematician. The length of the tropical year is variously stated by authors.

This is the motion in longitude. If from this 50",118 be taken, the remainder 11,882 is the absolute motion of the aphelion in a year. By some this is computed at 11,75.

the southern inhabitants will have the superiority for more than 10,000 years; the whole revolution being 20,903 years. It may be observed, however, that this will only bring the aphelion to the first of Libra, which was in conjunction with it at the creation. A complete revolution of the aphelion requires a period of more than 110,000 years.

The earth is in the aphelion of its orbit on the first day of July, in the forepart of the present century, and more than three millions of miles farther from the sun, than on the last day of December, when it is in the perihelion. No doubt the whole earth is warmer at the time of northern winter, than at the time of northern summer. But in winter the rays fall obliquely-in high latitudes, very obliquely. A far less number, therefore, must light on a given area, than when they are more direct, as in summer. (Plate V. Fig. 5.) Add to this, that the continuance of the sun above the horizon in winter is short, compared with the long days of summer, and that the heat is proportionally less. Add farther, that three millions of miles, though a great distance, bear a small proportion, compared with 95,000,000 miles.

The seasons in the southern hemisphere are the reverse of those in the northern. Spring and summer in the north are coincident with autumn and winter in the south; autumn and winter in the north, with spring and summer in the south.

By the transit of Venus over the sun's disk in 1761 and 1769, the mean distance of the earth from the sun was found to be about 95,000,000 miles; of course the diameter of the orbit must be 190,000,000 miles. This gives a circumference of 596,902,000 miles; nearly equal to the elliptical orbit. As the earth moves this immense distance in a year, it must travel at a mean rate of 68,091 miles an hour, and 1,634,184 miles every day. At this inconceivable velocity, 140 times greater than that of a cannon ball, are all the inhabitants of the earth carried. Even this is increased on a part of each day by the revolution of the earth on its axis.

That such a velocity should be imperceptible, may shock the credulity of those who are unaccustomed to the contemplation of such objects. But we are to consider, that every object around, even the atmosphere, is carried with us; so that there is nothing, by which we can compare our motion, except the heavenly bodies. By observations on these, such motion is now rendered past doubt. Its being imperceptible is not wonderful to those who have sailed in a ship or boat on still water. There a person, having obtained the motion of the vessel, feels no inconvenience from its swiftness, and is nearly insensible of movement, but from surrounding objects, till he strikes a shore, or other obstruction. The motion of the earth in its orbit is far more uniform and even, than any movement on the stillest water.

The motion of the earth's axis round the pores of the ecliptic causes the difference between the sidereal and tropical year. For by this motion the equinoxes are annually carried backward 50.118" of a degree from east to west, contrary to the order of the signs. Thus in every year they meet the sun 20 minutes, 20.4 seconds, the difference between the tropical and sidereal year, before the earth arrives at the point of the heavens, whence it started at the commencement of the year.

From this precession of the equinoxes," and with them all the signs of the ecliptic, it follows, that those stars, which in the infancy of astronomy were in Aries, are now in Taurus; those of Taurus, in Gemini. Hence likewise it is, that the stars, which rose or set at any particular season of the year, in the times of Hesiod, Eudoxus, Virgil, or Pliny, by no means answer at this time to their descriptions."

The constellations on our celestial globes, placed 30° from the signs, to which they originally belonged, show the motion of the equinoxes for 2154 years. The signs make a complete revolution in 25,858 years. Hence, in something more than 12,000 years, the star, which is called the north pole, will be about 47° from the pole of the earth, and

when on the meridian will be in the zenith of some parts of New-England.

How should the contemplation of these celestial motions, and long periods, lead us to improve the short fleeting moments of time assigned to us; and bring us to admire and adore the wisdom and power of Him, who formed and still governs all with infinite ease; with whom 66 a thousand years are as one day!"

SECTION VII.-OF THE MOON.

The earth has one satellite, the moon. This constant attendant is distant from the earth 240,000 miles,* and revolves round it from a point in the ecliptic to the same again, in 27 days, 7 hours, 43 minutes, and 5 seconds; from a star to the same again, in 27 days, 7 hours, 43 minutes, 12 seconds. It performs a mean lunation from the sun to the sun again, in 29 days, 12 hours, 44 minutes, 3 seconds. This is called her synodical revolution. The moon always exhibits the same surface to the earth. Hence, it must revolve round its axis in the same time, that it performs a revolution; or we must suppose, what is very improbable, that the different sides of the moon present the same prospect. Astronomers seem agreed in the former opinion. If it be correct, it is highly probable, that the side of the moon nearest to the earth is composed of matter more dense than the opposite, and that its rotation on its axis is caused by the powerful attraction of the earth.

Let E, (Plate V. Fig. 2.) be the earth, A, B, C, D, the moon in different parts of her orbit; a a mountain on the side of the moon next to the earth. As the moon passes in her orbit from A to B, 90°, it is manifest,

*This is taken for the mean distance.

De la Lande makes the sidereal revolution 27d. 7h. 43′ 11.5259′′.