Imágenes de página
PDF
ePub
[ocr errors]

stars. But, if the earth is at B, at the time of either of the conjunctions; then, at the time of this conjunction, the planet will appear in the line BST, and be seen among the fixed stars at T; and the arcs of the greatest elongation will be on each side of these stars; that is, the conjunctions and elongations will happen in a different part of the heavens, when the earth is at B, from what they happen when the eartli is at A. In other respects, the foregoing phenomena will be much the same, notwithstanding the motion of the earth, only the planet will be more direct in the farthest part of the orbit, and less retrograde in the nearest.

The inferior planets always appear very near the sun; but, by the motion of the earth in its orbit, the sun appears in different parts of the heavens, in different times of the year. Therefore, the inferior planets, as they are always very near the sun, will also appear in different parts of the heavens, at different times of the year. And, consequently, their conjunctions and greatest elongations will sometimes happen when they are in one part of the heavens, and sometimes when they are in another part. Venus, seen from the earth, will appear to vibrate in an arc VT, half of which is on one side of the sun's

appa. rent place, and half on the other side.

When an inferior planet, viewed from a superior, moves apparently retrograde, the superior planet has also an apparently retrograde motion,

When a superior planet, viewed from an inferior, appears stationary, the inferior planet viewed at the same time from the superior, is also stationary.

[ocr errors]

OF THE PHASES OF VENUS.

That the planets are all opake or dark bodies; and, consequently, shine only by the light they receive from the sun, is plain, because they are not visible when they are in such parts of their orbits as are between the sun and earth; that is, when their illumi. nated side is turned from us.

The sun enlightens only half a planet at once; the illuminated hemisphere is always that which is turned towards the sun, the other hemisphere of the planet is dark. To speak with accuracy, the sun, being larger than any of the planets, will illuminate rather more than half; but this difference, on account of the great distance of the sun from any of the planets, is so small, that its light may be considered as coming to them in lines physically parallel.

Like other opake bodies, they cast a shadow behind them, which is always opposite to the sun. The line in the planet's body, which distinguishes the lucid from the obscure part, appears sometimes straight, sometimes crooked. The convex part of the curve is sometimes towards the splendid, and the concave towards that which is obscure; and vice versa, according to the situation of the

eye, spect to the planet, and of the sun which enlightens the planet.

Hence the inferior planets, going round the sun in less orbits than our earth does, will sometimes have more, sometimes less of their illuminated side

with re

towards us; and, as it is the illuminated part only which is visible to us, Mercury and Venus, will, through a good telescope, exhibit the several appearances of the moon, from a fine thin crescent to the enlightened hemisphere.

If we view Venus through a telescope, when she follows the sun's rays on the eastern side, and appears

above the horizon after sụn-set, we shall see her

appear nearly round, and but small; she is at that time beyond the sun, and presents to us an enlightened hemisphere. As she departs from the sun towards the east, she augments in her apparent size; and on viewing her through a telescope, is seen to alter her figure, abating of her apparent roundness, and appearing, successively, like the moon in the different stages of her decrease. At length, when she is at her greatest elongation, she is like the moon in her first quarter, and appears as she does; when from a full, she has decreased to a half-moon.

After this, as she approaches (in appearance) to the sun, she

appears concave in her illuminated part, as the moon when she forms a crescent; thus she continues, till she is hid entirely in the sun's rays, and presents to us her whole dark hemisphere, as the moon does in her conjunction, no part of the planet being then visible.

When she departs out of the sun's rays, on the western side, we see her in the morning just before day-break. It is in this situation that Venus is called the morning-star, as in the other she is called the evening-star. She at this time appears very

appears very beau

[ocr errors]

tiful, like a fine thin crescent: just a verge of silver light is seen on her edge. From this period, she grows more and more enlightened every day, till she is arrived at her greatest digression or elongation, when she again appears as a half-moon, or as the moon in her first quarter : from this time, if contínued to be viewed with a telescope, she is found to be more and more enlightened, though she is all the while decreasing in magnitude; and thus continues growing smaller and rounder, till she is again hid or lost in the sun's rays.

Plate 8, fig. 1, represents the orbits of Venus and the earth, with the sun in the centre of them. The planet Venus is drawn in eight different situations, with its illuminated hemisphere towards the sun. If we suppose the earth to be at T, when Venus is at A, her dark hemisphere is towards the earth, and she is therefore invisible, except the conjunction happens in her node, for then she appears like a dark spot upon the disc of the sun. When Venus is at B, a little of her enlightened side is turned towards the earth, and therefore she appears sharp-horned, when she is at C, half her enlightened hemisphere is turned towards the earth, and she appears like a half-moon; at D, more than half her enlightened hemisphere is towards us, and sheappears likethe halfmoon about three days before it is full ; at E, the whole enlightened hemisphere is towards the earth; Venus is then either behind the sun, or so very near him, that she can hardly be seen; but if she could, she would

appear round, like the full-moon. At F, she is

. like the moon three days after the full; at G, like a

half-moon again; at H, like a crescent, with the points of the horn turned the contrary way to what they were at B. All this is equally applicable to Mercury.

Plate 8, fig. 2, exhibits the different appearances of Venus, corresponding to her several situations in the foregoing figure; thus, when Venus is at A, fig. 1, she is quite dark, as at A, fig. 2; when she is at B, fig. 1, she appears as at B, fig. 2, &c.

The inferior planets do not shine brightest when they are full; thus, Venus does not appear brightest in her superior conjunction, though her illuminated hemisphere be then turned towards us.

Her splendor is more diminished, by her being at a greater distance from us; then the conspicuous part of her illuminated disc is increased. Dr. Halley has shewn that Venus is brightest when her elongation from the sun is about 40°. Mercury is in his greatest brightness, when very near his utmost elongation,

[ocr errors]

OF THE SUPERIOR PLANETS.

I have already observed, that the greatest elongation of either of the inferior planets is less than 90°, or a quarter of a circle; so that they are never far from the sun, but constantly; attend it. But the superior planets do not always accompany the sun, as I have shewn that the inferior ones do; they are, indeed, sometimes in conjunction with it, but then they are also sometimes in opposition to, or 180° from it.

« AnteriorContinuar »