SECT. 4. Of Venus.

19. Next to Mercury, in the Solar system, is Venus. This planet revolves round the sun at the mean distance of 68 millions of miles. It completes its revolution in about 7 months; and turns on its axis in little less than 24 hours. The light reflected by this planet is very brilliant, and often renders it visible to the naked eye in the day time. Its greatest elongation is about 47°. It exhibits phases similar to those of Mercury and the moon. Spots are sometimes seen on its surface the appearances of which, and its phases, are exhibited in the annexed wood-cut. Heat and light at Venus are nearly double what they are at the earth.

;

For

20. This planet is brightest, when she is about 40° distant from the sun; and then only about part of her disk is illuminated. Her brightness in this position is surprising. Her lustre far exceeds that of the moon, at the same apparent distance from the sun. though, on account of her apparent magnitude, the moon reflects more light to us than Venus does, yet this light is incomparably more dull, and has none of the life and briskness which attend the beams of Venus. This difference arises probably from the circumstance of Venus having a very dense atmosphere, while the moon has a very rare one.

21. Mercury and Venus are called interior planets, because they are nearer the sun than the earth is; while those that are farther from the sun than the earth is, are called exterior.* They exhibit some peculiarities,

* In most books on astronomy, what are here called interior planets, are styled inferior; and what are here called exterior, are there denominated superior. But why this distinction of superior and inferior was ever made, it is difficult to see. In what proper sense can the word superior be applied to Mars in comparison of the Earth or Venus? Since every natural blessing of existence is derived from the heat and light of the sun, we should suppose that planets would be

arising from their situation; but as Mercury is seldona seen, those of Venus only will be noticed. During a part of its revolution, Venus rises and sets before the sun; it is then called morning star. During another part of its revolution, it rises and sets after the sun; it is then called evening star.* (Pl. II. fig. 2.) Let S be the sun, BDEC the orbit of Venus, A the earth, AL a part of its orbit, while Venus is moving from C, (which point is called its superior conjunction) through B to D, it will appear to the inhabitants of the earth at A to be above, or eastward of the sun; it will consequently be visible after the sun has set. But while passing from D, (which point is called its inferior conjunction,) through E to C, it will appear below or westward of the sun, and will consequently set before the sun.

22. If the earth were stationary at A, it is obvious that Venus would be above the sun, and be evening star in half its orbit; and be below the sun, and be morning star in the other half. But because the earth is in motion, Venus is above and below the sun alternately, in inuch more of its orbit. For let Venus emerge above the sun at C, when the earth is at A; while it is coming through B to D, the earth passes from A to F; consequently Venus must pass from D to d, before it is seen below the sun. So while Venus moves from d to x, (half its orbit,) the earth has come to o; consequently Venus must move on from a to v before it emerges again above the sun. This effect is very much greater than is represented on the figure. For while Venus passes superior according to the degree of heat and light which they receiv ed; that is, according to their proximity to the sun. This distinction of interior and exterior is not new, though but few have adopted it; but being, (as I conceive,) much the most appropriate, I feel desirous of having it adopted.

* The Ancients called the morning star, Phosphorus; and the evening star, Hesperus. These names are now often used, especially in poetry.

from C to D, half its orbit, the earth, instead of passing through the small portion AF, has passed through nearly of her orbit; through which, and considerably more, (because the earth's motion is constant,) Venus must pass before she is seen below the sun. It is found that Venus is morning and evening star alternately, during about 290 days; a period, considerably exceeding a complete revolution of that planet in her orbit.

SECT. 5.

ART. 1. Of the Earth.

23. The planet next to Venus in the solar system, is the earth, which we inhabit. It revolves about the Isun at the mean distance of 93 millions of miles. It completes this revolution in a year, and turns on its axis in a day, or twenty-four hours. The consideration of the figure of the earth will be resumed when we come to treat of physical Astronomy; and the other phenomena relating to this planet will be continued in CHAP. II. and III.

ART. 2. Of the Moon.

24. The moon is a secondary planet, revolving round the earth in about 294 days, and is carried with the earth round the sun once a year. Its distance from the earth is about 240,000 miles. It turns on its axis in precisely the same time that it performs its revolution round the earth.

25. The most obvious fact relating to the moon, is, that her disk is constantly changing its appearance; sometimes only a semicircular edge is illuminated, while the rest is dark; and at another time, the whole surface appears resplendent. The first appearance is