Venus is at her greatest elongation, we have but Plate & one half of the enlightened side of her full moon toward us; and even then it may be too far distant to be seen by us. But if she have a moon, it may certainly be seen with her upon the Sun, in the year 1761; unless its orbit be considerably inclined to the ecliptic: for if it should be in conjunction or opposition at that time, we can hardly imagine that it moves so slow as to be hid by Venus all the six hours that she will appear on the Sun's disc*.

motion.

47. The EARTH is the next planet above Venus The Earth in the system. It is 82 millions of miles from the Fig. 1. Sun, and goes round him, in the circle, in 365 days 5 hours 49 minutes, from any equinox or solstice to the same again; but from any fixed star to the same again, as seen from the Sun, in 365 days 6 hours and 9 minutes: the former being the length Its diurnal of the tropical year, and the latter the length of the and annual sidereal. It travels at the rate of 58 thousand miles every hour; which motion, though 120 times swifter than that of a cannon-ball, is little more than half as swift as Mercury's motion in his orbit. The Earth's diameter is 7970 miles; and by turning round its axis every 24 hours, from west to east, it causes an apparent diurnal motion of all the heavenly bodies, from east to west. By this rapid motion of the Earth on its axis, the inhabitants about the equator are carried 1042 miles every hour, while those on the parallel of London are carried only about 580; besides the 58 thousand miles, by the annual motion above-mentioned, which is common to all places whatever.

48. The Earth's axis makes an angle of 23 de- Inclination grees with the axis of its orbit; and keeps always of its axis. the same oblique direction; inclining toward the

Both her transits are over since this was written, and no satel

lite was seen with Venus on the Sun's disc.

A proof of its being round.

Its number of square miles.

The pro

land and

sea.

same fixed star* throughout its annual course, which causes the returns of spring, summer, autumn, and winter; as will be explained at large in the tenth chapter.

49. The Earth is round like a globe; as appears, 1. By its shadow in eclipses of the Moon; which shadow is always bounded by a circular line; § 314. 2. By our seeing the masts of a ship while the hull is hid by the convexity of the water. 3. By its having been sailed round by many navigators. The hills take off no more from the roundness of the Earth in comparison, than grains of dust do from the roundness of a common globe.

50. The seas and unknown parts of the Earth (by a measurement of the best maps) contain 160 millions 522 thousand and 26 square miles; the inhabited parts 38 millions 990 thousand 569: Europe 4 millions 456 thousand and 65; Asia 10 millions 768 thousand 823; Africa 9 millions 654 thousand 807; America 14 millions 110 thousand 874. In all, 199 millions 512 thousand 595; which is the number of square miles on the whole surface of our globe.

51. Dr. LONG, in the first volume of his Astroportion of nomy, p. 168, mentions an ingenious and easy method of finding nearly what proportion the land bears to the sea; which is, to take the papers of a large terrestrial globe, and after separating the land from the sea, with a pair of scissars, to weigh them carefully in scales. This supposes the globe to be exactly delineated, and the papers all of equal thickness. The doctor made the experiment on the papers of Mr. SEN EX's seventeen inch globe; and found that the sea-papers weighed 349 grains, and the land only 124: by which it appears that almost

*This is not strictly true, as will appear when we come to treat of the recession of the equinoctial points in the heavens, § 246; which recession is equal to the deviation of the Earth's axis from its parallelism; but this is rather too small to be sensible in an age, except to those who make very nice observations,

three fourth parts of the surface our Earth between the polar circles are covered with water, and that little more than one fourth is dry land. The doctor omitted weighing all within the polar circles; because there is no certain measurement of the land within them, so as to know what proportion it bears to the sea.

Moon.

52. The Moon is not a planet, but only a satel- The lite or attendant of the Earth; going round the Earth from change to change in 29 days 12 hours and 44 minutes; and round the Sun with it every year. The Moon's diameter is 2180 miles; and her distance from the Earth's centre 240 thousand. She goes round her orbit in 27 days 7 hours 43 minutes, moving about 2290 miles every hour; and turns round her axis exactly in the time that she goes round the Earth, which is the reason of her keeping always the same side toward us, and that her day and night, taken together, is as long as our lunar month.

Her

53. The Moon is an opaque globe, like the Earth, and shines only by reflecting the light of the Sun: phase therefore, while that half of her which is toward the Sun is enlightened, the other half must be dark and invisible. Hence, she disappears when she comes between us and the Sun; because her dark side is then toward us. When she is gone a little way forward, we see a little of her enlightened side; which still increases to our view, as she advances. forward, until she comes to be opposite to the Sun; and then her whole enlightened side is toward the Earth, and she appears with a round illumined orb, which we call the full moon: her dark side being then turned away from the Earth. From the full she seems to decrease gradually as she goes through the other half of her course; shewing us less and less of her enlightened side every day, till her next change or conjunction with the Sun, and then she disappears as before.

shines not

by her

A proof 54. This continual change of the Moon's phases that she demonstrates that she shines not by any light of her own; for if she did, being globular, we should alown light. ways see her with a round full orb like the Sun. Her orbit is represented in the scheme by the little circle m, upon the Earth's orbit. It is indeed drawn fifty times too large in proportion to the Earth's; and yet is almost to small too be seen in the diagram.

Fig. I.

One half

of her al

55. The Moon has scarce any difference of seaways en- sons; her axis being almost perpendicular to the lightened. ecliptic. What is very singular, one half of her has no darkness at all; the Earth constantly affording it a strong light in the Sun's absence; while the other half has a fortnight's darkness, and a fortnight's light by turns.

Our Earth

is her moon.

56. Our Earth is a moon to the Moon; waxing and waneing regularly, but appearing thirteen times as big, and affording her thirteen times as much light, as she does to us. When she changes to us, the Earth appears full to her; and when she is in her first quarter to us, the Earth is in its third quarter to her; and vice versa.

57. But from one half of the Moon, the Earth is never seen at all. From the middle of the other half, it is always seen over head; turning round almost thirty times as quick as the Moon does. From the circle which limits our view of the Moon, only one half of the Earth's side next her is seen; the other half being hid below the horizon of all places on that circle. To her, the Earth seems to be the largest body in the universe: appearing thirteen times as large as she does to us.

58. The Moon has no atmosphere of any visible density surrounding her, as we have: for if she had, we could never see her edge so well defined A proof as it appears; but there would be a sort of mist of the or haziness around her, which would make the having no stars look fainter, when they are seen through it. atmos. But observation proves, that the stars which disap. phere,

Moon's

pear behind the Moon, retain their full lustre until they seem to touch her very edge, and then they vanish in a moment. This has been often observed by astronomers, but particularly by CASSINI of the star in the breast of Virgo, which appears single and round to the bare eye; but through a refracting telescope of 16 feet, appears to be two stars so near together, that the distance between them seems to be but equal to one of their apparent diameters. The moon was observed to pass over them on the 21st of April 1720, N. S. and as her dark edge drew near to them, it caused no change whatever in their colour or situation. At 25 min. 14 sec. past 12 at night, the most westerly of these stars was hid by the dark edge of the Moon; and in 30 seconds afterward, the most easterly star was hid: each of them disappearing behind the Moon in an instant, without any preceding diminution of magnitude or brightness; which by no means could have been the case if there were an atmosphere round the Moon: for then one of the stars falling obliquely into it before the other, ought, by refraction, to have suffered some change in its colour, or in its distance from the other star, which was not yet entered into the atmos phere. But no such alteration could be perceived; though the observation was made with the utmost attention to that particular; and was very proper to have made such a discovery. The faint light which has been seen all round the Moon, in total eclipses of the Sun, has been observed, during the time of darkness, to have its centre coincident with the centre of the Sun; and was therefore much more likely to arise from the atmosphere of the Sun, than from that of the Moon; for if it had been owing to the latter, its centre would have gone along with the Moon's.*

It has been lately ascertained by Mr. Schroeter, that the Moon is indeed furnished with an atmosphere, similar to that of the Earth, and of proportional density: the former being about one 29th part the density of the latter.