Imágenes de página
PDF
ePub

Let S, plate 6, fig. 3, be the sun; ABCD, the orbit of any superior planet, Mars, for instance; EFG, the earth's orbit. If the earth be at E, the suń at S, and the planet at D, the sun and the planet will be both on the same side of the earth; and, consequently, the planet will appear in conjunction with the sun. But, as the orbit of the earth is between the sun and the orbit of the superior planet, it is possible for the earth to be between the sun and the planet; and, consequently, for the planet and the sun to be on opposite sides of the earth, or the planet to be in opposition; thus, if when the earth is at E, Mars be at A, he is then in opposition to the sun.

A superior planet is in quadrature with the sun when its geocentric place is in 90° from the geocentric place of the sun. If the earth be at E, and Mars at B or C, he is in quadrature with the sun; for the line AE, EB, form a right angle; as do also the lines EA, EC.

OF THE DIRECT, STATIONARY, AND RETROGRADE MOTION OF THE SUPERIOR PLANETS.

As the earth goes round the sun in less time, and in a less orbit than any of the superior planets, it will not be amiss to suppose a superior planet to stand still in some part of its orbit, while the earth goes once round the sun in her's, and consider the appearances the planets would then have, which are these: 1. While the earth is in her most distant se

[ocr errors]
[ocr errors]

micircle, the apparent motion of the planet would be direct. 2. While the earth is in her nearest semicircle, the planet would be retrograde. 3. While the earth is near the points of contact of a line drawn from the planet, so as to be a tangent to the earth's orbit, the planet would be stationary.

To illustrate this, let ABCDEFHG, plate 7, fig. 1, be the orbit of the earth; S, the sun; PQ VO, the orbit of Mars; LMNT, an arc of the ecliptic. Let us now suppose the planet Mars to continue at l', while the earth goes round in her orbit, according to the order of the letters, ABC, &c. ABCDEFHG may be considered as so many stations, from whence an inhabitant of the earth, would view Mars, at different times of the year; and if straight lines be drawn from each of these stations, through Mars, at P, and be continued to the ecliptic, they will point out the apparent places of Mars in the heavens, at these different stations.

Thus, supposing the earth at A, the planet will be seen among the stars at L; when the earth is arrived at B, the planet will appear, at M; and in the same manner, when at C, D, and E, it will be seen among the stars at N, R, T; therefore, while the earth moves over the large part of the orbit ABCDE, the planet will have an apparent motion from L to T; and this motion is from west to east, or the same way with the earth; and the planet is said to move direct, or according to the order of the signs.

When the earth is near to A and E, the point of contact of the tangent to the earth's orbit, the planet will be stationary for a short space of time.

When the earth moves from E to H, the planet

[ocr errors]

seems to return from T to N; but while it moves mo for E from H to A, it will appear to move in a contrary direction, and thus be retrograde from N to L, where it will again be stationary: and, since the part of the orbit, which the earth describes in passing from A to E, is much greater than the part EHP, though space TL, which the planet describes in direct and retrograde motion, is the same; the direct motion from L to T must be much slower than the retrograde motion from T to L.

the

When the earth is at C, a line drawn from C, through S and P, to the ecliptic, shews that Mars is then in conjunction with the sun. But, when the earth is at H, a line drawn from H through P, and continued to the ecliptic, would terminate in a point opposite to S; therefore, in this situation, Mars would be in opposition to the sun. Thus it appears, that the motion of Mars is direct, when in conjunction; and retrograde, when in opposition.

The retrograde motions of the superior planets happen oftener, the slower their motions are; as the retrograde motions of the inferior planets happen oftener, the swifter their angular motions. Because the retrograde motions of the superior planets depend upon the motions of the earth; but those of the inferior, on their own angular motion. A superior one is retrograde once in each revolution of the earth; an inferior one in every revolution of its

own.

OTHER PHENOMENA OF THE SUPERIOR PLANETS.

The superior planets are sometimes nearer the earth than at other times; they also appear larger, or smaller, according to their different distances from us. Thus, suppose the earth to be at H, if Mars be at P, he is the whole diameter of the earth's orbit nearer to us, than if he were at V; and, consequently, his disc must appear larger at P, than it would be at V. In other places, the distances of Mars from the earth are intermediate.

The diameter of the earth's orbit bears a greater ratio to the diameter of the orbit of Mars, than it does to the diameter of the orbit of Jupiter; and a greater to that of Jupiter than of Saturn; consequently, the difference between the greatest and least apparent diameters is greater in Mars than in Jupiter; and greater in Jupiter, than in Saturn.

The superior, like the inferior planets, do not always appear in the ecliptic, their orbits being inclined also to that of the earth; one-half is therefore above the ecliptic, the other half below it; nor are they ever seen in it but when they are in their nodes.

They also move in an ellipse. They are sometimes nearer to, and sometimes further from the earth. Their apparent diameter varies according to the difference in their distance.

OF THE SECONDARY PLANETS, OR SATELLITES.

It has been already observed, that four of the primary planets, the Earth, Jupiter, Saturn, and the Georgium Sidus, are, in their revolutions round the sun, attended by secondary planets.

As the moon turns round the earth, enlightening our night, by reflecting the light she receives from the sun, so do the other satellites enlighten the planets to which they belong, and move round those planets at different periods of time, proportioned to their several distances; and as the moon keeps com pany with this earth, in its annual revolution round the sun, so do they severally accompany the planets to which they belong, in their several courses round that luminary.

I shall speak here first of the moon, which of all the heavenly bodies, excepting the sun, is the most splendid and brilliant, the inseparable companion and attendant of our earth. In mythology, she was considered as Luna; in the heavens, the radiant planet of the night; upon the earth, as the chaste Diana, and as the tremendous Hecate in hell.

OF THE MOON.

If we imagine the plane of the moon's orbit to be extended to the sphere of the heaven, it would mark therein a great circle, which may be called the moon's

« AnteriorContinuar »