the circle of PERPETUAL APPARITION, or the largest of all those which conftantly appear; the ftars included within it never either rife or fet, though they are at times more elevated above the horizon than at others. Towards the other pole there is another circle, opposite to this, which is the circle of PERPETUAL OCCULTATION. All the stars that are contained within this, never rise, but lie hid under the horizon, and are never feen.

AN EXPLANATION OF THE PHENOMENA WHICH ARISE FROM THE MOTION OF THE EARTH, AND OF THE INFERIOR PLANETS, MERCURY AND VENUS.

It will be neceffary in this place to define more exactly fome words which have been flightly explained before, and recall the reader's attention to fome definitions that have been already given; and it is prefumed, that these repetitions will not be an object of complaint, because they will answer the beneficial purpose of grounding the reader more firmly in the knowledge of the science, to which this effay is intended as an introduction.

When two planets are feen together in the fame fign of the zodiac, and equally advanced therein, they are faid to be in CONJUNCTION. But when they are in oppofite figns of the zodiac, they are faid to be in OPPOSITION. Thus a planet is faid to be in oppofition to the fun, when the earth is between the fun and the planet.

The elongation of a planet is it's apparent diftance from the fun. When a planet is in conjunction with the fun, it has no elongation; when in oppofition, it's elongation is 180 degrees.

The NODES OF A PLANET'S ORBIT are thofe two points where the orbit cuts the plane of the ecliptic. We before observed, that the orbits of all the planets are inclined to the plane of the ecliptic, and confequently crofs this plane. In fig. 3, plate III. A B C D is the plane of the ecliptic; EBFD is the orbit of a planet, in which the points B and D are the two nodes.

The LINE OF THE NODES is a line B D, fuppofed to be drawn through the fun from one node to the other. The LIMITS OF A PLANET'S ORBIT are two points in the middle between the two nodes. The point E is called the greatest northern limit, I the greatest fouthern limit.

The

The greatest distance of the earth, or of any planet from the fun, is called it's APHELION, or higher apfis; it's leaft diftance is called the PERIHELION, or lower apfis.

Thus in fig. 4, plate III. A is the place of the aphelion, P that of the perihelion.

The axis P A, fig. 4, of any planet's ellipfis, is called the LINE OF THE APSIDES; the extreme point of it's fhortest diameter T V are the places of it's mean distance from the fun; and S T, or S V, the line of it's mean distance.

When a planet moves according to the order of the figns, it's motion is faid to be DIRECT, or IN CONSEQUENTIA; but when it's motion is contrary to the order of the figns, it is faid to be RETROGRADE, or IN ANTECEDENTIA,

The place in the ftarry heavens that any planet. appears in, when feen from the center of the earth, is called it's GEOCENTRIC PLACE. The place where it would be seen in the celeftial sphere, by an obferver fuppofed to be in the fun, is called it's HELIOCENTRIC PLACE.

OF

OF THE MOTIONS OF THE INFERIOR PLANETS, VENUS AND MERCURY.

There are two different fituations, in which an inferior planet will appear in conjunction with the fun; one when the planet is between the fun and the earth, the other when the fun is between the earth and the planet. Let A, fig. 2, plate VII. be the earth in it's orbit, E the place of Venus in her orbit E HG, S the fun, FVPQRTD an arc in the ftarry heavens. In this fituation the fun and Venus are on the fame fide of the earth, and will appear in the same point of the heavens, fo as to be in conjunction. If the earth is at A, and Venus at G, they will also appear to be in conjunction.

If the earth is at A, the fun at S, the planet at E, nearer to the earth than the fun, it is called But if the earth is it's INFERIOR CONJUNCTION. at A, and the planet at G, farther from the earth than the fun, this is called the SUPERIOR CONJUNCTION of the planet.

If an inferior planet is at E, the earth at A, and the fun at S, the elongation is nothing, the planet being then in it's inferior conjunction. As the planet moves from E to y, it's elongation in

creases;

creases; for when it is at y, it appears in the line A y P, while the fun appears in the line ASQ; fo that PA Q will be it's elongation. When the planet is arrived at x, it appears in the line Ax V, which is a tangent to it's orbit, and then it's elongation is V AQ, which is the greatest that can be on that fide the fun; for after this, the elongation decreases. When the planet is at K, it's elongation is PAQ; when at G, it is nothing, because it is then in it's fuperior conjunction; as the planet moves on from G, it's elongation again increases; for when it comes to C, it appears in the line A CR, and it's elongation is RAQ. When the planet comes to H, a line drawn from the earth through the planet is a tangent to the orbit, and the elongation is T A Q the greatest it can have when it is on the other fide of the fun; for after this, the elongation again decreases.

Hence it is clear, that the inferior planets can never appear far from the fun, but must always accompany it in it's apparent motion through the ecliptic. When we fee either Venus or Mercury, it is either in an evening, in the west, soon after the fun has fet; or in a morning, a little before the fun rises. Venus is indeed bright enough fometimes to be seen in the day-time, but then she is