OF THE PHASES OF VENUS.

That the planets are opake bodies, and shine only by the light they receive from the fun, is plain, because they are not vifible in fuch parts of their orbits as are between the fun and earth, that is, when their illuminated fide is turned from us.

The line in the planet's body, which distinguishes the lucid from the obfcure part, appears fometimes ftrait, fometimes crooked. The convex part of the curve is fometimes towards the fplendid part, and the concave fide towards that which is obfcure; and vice verfa, according to the fituation of the planet with refpect to the eye and the fun.

The inferior planets going round the fun in lefs orbits than our earth does, will fometimes have more, fometimes lefs of their illuminated fide towards us; and as it is the illuminated part only which is vifible to us, Mercury and Venus will, through a good telescope, exhibit the feveral appearances of the moon, from a fine thin crefcent to the enlightened hemifphere.

If we view Venus through a telescope, when the follows the fun's rays on the eastern fide, and appears above the horizon after fun-fet, we shall fee her appear nearly round, and but fmall; the is at that time beyond the fun, and prefents to us an enlightened hemifphere. As fhe departs from the fun towards the eaft, the augments in her apparent fize; and on viewing her through a telescope, is feen to alter her figure, abating of her apparent roundness, and appearing fucceffively like the moon, in the different ftages of her decreafe. At length, when she is at her greatest elongation, the is like the moon in her firft quarter, and appears as she does when from a full fhe has decreased to half a moon,

After

After this, as fhe approaches (in appearance) to the fun, the appears concave in her illuminated part, as the moon when the forms a crefcent; thus the continues till fhe is hid entirely in the fun's rays, and prefents to us her whole dark hemifphere, as the moon does in her conjunction, no part of the planet being then vifible.

When the departs out of the fun's rays on the western fide, we fee her in the morning, just before day-break. It is in this fituation that Venus is called the morning ftar, as in the other fhe is called the evening ftar. She at this time appears very beautiful, like a fine thin crefcent: juft a verge of filver light is feen on her edge. From this period fhe grows more and more enlightened every day, till the 19 arrived at her greateft digreffion or elongation, when the again appears as a half moon, or as the moon in her first quarter; from this time, if continued to be viewed with a telefcope, the is found to be more and more enlightened, though she is all the while decreafing in magnitude, and thus continues growing fmaller and rounder, till fhe is again hid or loft in the fun's rays.

Fig. 1, pl. 8, reprefents the orbits of Ve-nus and the earth, with the fun in the center of them. The planet Venus is drawn in eight different fituations, with it's illuminated hemifpheres towards the fun. If we fuppofe the earth to be at T, when Venus is at A, her dark hemifphere is towards the carth, and fle is therefore invifible, except the conjunction happens in her node, for then the appears like a dark spot upon the dife of the fun. When Venus is at B, a little of her enlightened fide is turned towards the carth, and therefore fhe appears fharp-horned; when the is at C, half her enlightened hemifphere is turned towards the earth, and fhe appears like an half moon; at D, more than half her enlightened hemifphere is

towards

towards us, and the appears like the moon about three days before it is full; at E, the whole enlightened hemifphere is towards the earth. Venus is then either behind the fun, or fo very near him, that the can hardly be feen; but if fhe could, fhe would appear round like the full moon. At F fhe is like the moon three days after the full; at G like a half moon again; at H like a crefcent, with the points of the horns turned the contrary way to what they were at B. All this is equally applicable to Mercury.

Fig. 2, pl. 8, exhibits the different appearances of Venus, correfponding to her feveral fituations in the foregoing figure; thus when Venus is at A, fig. 1, fhe is quite dark, as at A, fig. 2; when fhe is at B, fig. 1, fhe appears as at B, fig. 2, &c.

The inferior planets do not fhine brightest when they are full; thus Venus does not appear brightest in her fuperior conjunction, though her illuminated hemifphere be then turned towards us. Her fplendor is more diminished by her being at a greater diftance from us, than the confpicuous part of her illuminated dife is increafed. Dr. Halley has fhewn, that Venus is brightest when her elongation from the fun is about 40 degrees. Mercury is in his greatest brightnefs, when very near his utmost elongation.

OF MERCURY,

The planet Mercury refembles Venus in all the circumstances of her apparent motion, and we make fimilar inferences with refpect to the real motions. His orbit is an ellipfe, having the fun in one focus. The apfides move eastward 1 degree, 57 minutes, 20 feconds in a century; the nodes

move westward 45 feconds in a year; and areas are defcribed proportional to the times.

OF THE SUPERIOR PLANETS.

The fuperior planets exhibit phenomena confiderably different from thofe exhibited by Mercury and Venus.

They come to our meridian both at noon and midnight; when they come to our meridian at noon, and are in the ecliptic, they are never seen croffing the fun's difc.

They are always retrograde when in oppofition, and direct when in conjunction.

I have already obferved to you, that the greateft elongation of either of the inferior planets is lefs than 90 degrees, or a quarter of a circle; fo that they are never far from the fun, but conftantly attend it. But the fuperior planets do not always accompany the fun, as the inferior ones do: they are indeed fometimes in conjunction with it, but then they are alfo fometimes in oppofition to, or 180 degrees from it.

To be more particular, let S, fig. 3, pl. 6, be the fun; ABCD the orbit of any fuperior planet, Mars, for instance; EFG the carth's orbit. If the earth be at E, the fun at S, and the planet at D, the fun and the planet will be both on the fame fide of the earth; and confequently the planet will appear in conjunction with the fun. But as the orbit of the earth is between the fun and the orbit of the fuperior planet, it is poffible for the earth to be between the fun and the planet, and confequently for the planet and the fun to be on oppofite fides of the earth, or the planet to be in oppofition; thus, if when the earth is at E, Mars be at A, he is then in oppofition to the fun.

A fupe

A fuperior planet is in quadrature with the fun, when it's geocentric place is 90° from the geocentric place of the fun; thus if the earth be at E, and Mars at B or C, he is in quadrature with the fun; for the lines A E, E B, form a right angle, as do alfo the lines EA, EC.

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

As the earth goes round the fun in less time, and in a lefs orbit than any of the fuperior planets, it will not be amifs to fuppofe a fuperior planet to ftand still in fome part of it's orbit, while the earth goes once round the fun in her's, and confider the appearances the planets would then have, which are thefe: 1. While the earth is in her moft diftant femicircle, the apparent motion of the planet would be direct. 2. While the earth is in her nearest femicircle, the planet would be retrograde. 3. While the earth is near the points of contact of a line drawn from the planet, fo as to be a tangent to the earth's orbit, the planet would be fationary.

To illuftrate this, let ABCDEFGH, pl. 7. fig. 1, be the orbit of the earth, S the fun, PQQV the orbit of Mars, L M N an arc of the ecliptic. Let us fuppofe the planet Mars to continue at P, while the earth goes round in her orbit, according to the order of the letters ABC, &c. A B C D Ě FGH may be confidered as fo many ftation's, from whence an inhabitant of the earth would view Mars at different times of the year; and if ftrait lines be drawn from each of thefe ftations, through Mars at P, and continued to the ecliptic, they will point out the apparent place of Mars, at thefe different flations.

Thus fuppofing the earth at A, the planet will be feen among the ftars at L; when the earth is ar