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.company 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.

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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

apparent orbit; because the moon appears to the inhabitants of the earth to move in that circle, through the twelve signs of the zodiac, in a periodical month. This position is illustrated by the following figure; let EFGHI, plate 9, fig. 3, be the orbit of the earth; S the sun; abcd the orbit of the moon, when the earth is at E: let ABCD be a great circle in the sphere of the heaven, in the same plane with the moon's orbit.

The moon, by going round her orbit according to the order of letters, appears to an inhabitant of the earth to go round in the great circle ABCD, according to the order of those letters: for, when the moon is at a, seen from the earth at E, she appears at A; when the moon is got to b, she appears at B; when to c, she will appear at C; when arrived at d, she will appear at D. It is true, when the moon is at b, the visual line drawn from E, through the moon, terminates in L; as it does in M, when the moon is at d; but the lines LM, and DB, being parallel, and not farther distant from each other than the distance of the earth's orbit, are, as to sense, coincident; their distance measured in the sphere of the heaven being insensible: for the same reason, though the earth moves from E to F, in the time that the moon goes round her orbit, so that, at the end of a periodical month, the moon will be at a, and is seen from the earth at F, in the line FN; the moon will, notwithstanding, appear at A, the lines FN and EA being parallel, and, as to sense, coincident: in like in whatever part of her orbit the earth is, as at H or I, the moon, by going round in her orbit,

manner,

will appear to an inhabitant of the earth to go round in the great circle ABCD.

The plane of the moon's orbit extended to the heaven's, cuts the ecliptic in two opposite points.

The two points where the moon's apparent orbit thus cuts the ecliptic, are called the moon's nodes.

The point where the moon appears to cross the ecliptic, as she goes into north latitude, is called the moon's ascending node, of which this is the character; the point where the moon goes into south latitude is her descending node, and is marked thus ; the moon's ascending node is often called the dragon's head; her descending node the dragon's tail.

The line of the moon's node is a line drawn from one node to the other.

The extremities of the line of the nodes are not always directed towards the same points of the ecliptic, but continually shift their places from east to west; or, contrary to the order of the signs, per ́forming an entire revolution about the earth, in the space of something less than nineteen years.

The moon appears in the ecliptic only when she is in one of her nodes; in all other parts of her orbit she is either in north or south latitude, sometimes nearer to, sometimes further removed from the ecliptic, according as she happens to be more or less distant from the nodes.'

When the place, in which the moon appears to an inhabitant of the earth, is the same with the sun's place, she is said to be in conjunction. When the

moon's place is opposite to the sun's place, she is said to be in opposition. When she is a quarter of a circle distant from the sun, she is said to be in quadrature. Both the conjunction and the opposition of the moon are termed syzigies.

The common lunar month, or the time that passes between any new-moon and the next that follows, is called a synodical month, or a lunation. This month contains 29 days, 12 hours, 44 minutes, 3 seconds.

A periodical month is the time the moon takes up to describe her orbit: or, in other words, the time in which the moon performs one entire revolution about the earth, from any point in the zodiac to the same again; and contains 27 days, 7 hours, 43 minutes.

If the earth had no revolution round the sun, or i the sun had no apparent motion in the ecliptic, the

periodical and synodical month would be the same; but as this is not the case, the moon takes up a longer time to pass from one conjunction to the next, than to describe its whole orbit; or the time between one new-moon and the next, is longer than the moon's periodical time.

The moon revolves round the earth from west to east, and the sun apparently revolves round the earth the same way. Now at the new-moon, or when the sun and moon are in conjunction, they both set out from the same place, to move the same way round * the earth; but the moon moves much faster than the "sun, and, consequently, will overtake it; and when the moon does overtake it, it will be a new-moon