horns are turned towards the east when it is a morning star, and towards the west when it is an evening star. Some astronomers have thought they perceived a satellite moving round Venus; but, as succeeding observers have not been able to verify their observations, they are supposed to have originated in error. In observing the transit of Venus, Mr. Dunn, and other gentlemen, saw a penumbra which took place about five seconds before the contact, preceding the egress of the planet; and from thence they concluded, that it had an atmosphere of about 50 geographical miles in height.

to us.

We are told, that when Copernicus first published his account of the solar system, it was objected to him that it could not be true; because, if it was, the inferior planets must have different phases, according to their different situation with respect to the sun and earth; whereas they always appear round The answer said to be made by him is, that they appear round to the eye by reason of their distance; but, if we could have a nearer, or more distinct view of them, we should see in them the same phases as we do in the moon. The invention of telescopes is said to have verified this prediction of Copernicus. But it is neither probable that a defender of the Ptolemaic system should make such an objection, or Copernicus such an answer; since, in the Ptolemaic, as well as in the Copernican system, the shape of these planets ought to change just as the moon dees; consequently, the mere change of shape in the inferior planets is an argument which,

in the common way of urging it, proves nothing at all as to the truth or falsehood of the Copernican system. If, besides the changes of shape made in the inferior planets, we consider the situation of the planets with respect to the sun, when these changes happen, this, indeed, will shew us that the Ptolemaic system is false *; as will be seen in a subsequent part of these Essays.

Venus is sometimes seen passing over the disc of the sun, as a round dark

appearances,

spot. These which are called transits, happen very seldom; though there have been two within these few years, the one in June 1761, the other in June 1769; the next will be in the year 1874.

OF THE EARTH.

The next planet that comes before us is the Earth that we inhabit: small as it really is, when compared to some of the other planets, it is to us of the highest importance; we wish only to attain knowledge of others, that we may find out their relation to this, and from thence learn our connection with the universe at large. But, when viewed with an eye to eternity, its value to us is heightened in a manner that exceeds expression, and surpasses all the powers of the human mind. He alone can form some idea of it, who, in the regions of celestial bliss,

* Rutherford's System of Natural Philosophy, vol. ii. p. 781.

is become a partaker of the length and breadth, the depth and height, of divine love.

The orbit of the Earth is placed between those of Venus and Mars. The diameter of the Earth is 7970 miles; its distance from the sun is 96 millions of miles, and goes round him in a year, or 365 days, 6 hours, 9 minutes; moving at the rate of 68,856 miles per hour. Its apparent diameter, as seen from the sun, is about 21 seconds.

It turns round its axis from west to east in 24 hours, which occasions the apparent diurnal motion of the sun, and all the heavenly bodies round it, from east to west in the same time; it is of course the cause of their rising and setting, of day and night.

The axis of the Earth is inclined 23 degrees to the plane of its orbit, and keeps in a direction parallel to itself throughout its annual course, which causes the returns of spring and summer, autumn and winter. Thus his diurnal motion gives us the grateful vicissitude of night and day, and his annual motion the regular succession of seasons.

OF THE MOON. (

Next to the sun, the Moon is the most splendid and shining globe in the heavens, the satellite, or inseparable companion of the earth. By dissipating, in some measure, the darkness and horrors of the night, subdividing the year into months, and regulating the flux and reflux of the sea, she not only

becomes a pleasing, but a welcome object; an object affording much for speculation to the contemplative mind, of real use to the navigator, the traveller, and the husbandman. The Hebrews, the Greeks, the Romans, and, in general, all the ancients, used to assemble at the time of new moon, to discharge the duties of piety and gratitude for its manifold uses.

That the moon appears so much larger than the other planets, is owing to her vicinity to us; for, to a spectator in the sun she would be scarcely visible, without the assistance of a telescope. Her distance is but small from us, when compared with that of the other heavenly bodies; for among these, the least absolute distance, when put down in numbers, will appear great, and the smallest magnitude immense.

The Moon is 2161 miles in diameter; her bulk is about 3 of the earth's; her distance from the centre of the earth 240,000 miles; she goes round her orbit in 27 days, 7 hours, 43 minutes, moving at the rate of 2299 miles per hour. The time in going round the earth, reckoning from change to change, is 29 days, 12 hours, 44 minutes. Her apparent diameter, at a mean distance from the earth, is 31′16′′; but as viewed from the sun, at a mean distance about 6",

Her orbit is inclined to the ecliptic, in an angle of five degrees, 18 minutes, cutting it in two points, which are diametrically opposite to each other; these points are called her nodes. Her nodes have a motion westward, or contrary to the order of the signs, making a complete revolution in about 19 years; in which

time each node returns to that point of the ecliptic whence it before receded.

If the moon were a body possessing native light, we should not perceive any diversity of appearance; but, as she shines entirely by light received from the sun, and reflected by her surface, it follows, that, according to the situation of the beholder with respect to the illuminated part, he will see more or less of her reflected beams; for only one-half of a globe can be enlightened at once.

Hence, while she is making her revolution round the heavens, she undergoes great changes in her appearance. She is sometimes in our meridian at midnight, and, therefore, in that part of the heavens which is opposite to the sun: in this situation she appears as a complete circle, and it is said to be full moon. As she moves eastward, she becomes deficient on the west side, and in about 7 days comes to the meridian at about six in the morning, having the appearance of a semicircle, with the convex side turned towards the sun; in this state, her appearance is called the half moon. Moving on still eastward, she becomes more deficient on the west, and has the form of a crescent, with the convex side turned towards the sun; this crescent becomes continually more slender, till about fourteen days after the full moon, she is so near the sun that she cannot be seen, on account of his great splendor. About four days after this disappearance, she is seen in the evening a little to the eastward of the sun, in the form of a fine crescent, with the convex side turned from the sun;