Of the Planets Mercury, Venus,, Mars, Jupiter, and Saturn.

T

HE Planets are diftinguished into Inferior and Superior. Mercury and Venus are called the Inferior Planets, because they are lower than the Earth, in the folar System, and nearer to the Sun; Mars, Jupiter, and Saturn, are called the Superior Planets, because they are higher than the Earth, and more remote from the Sun.

MERCURY revolves about the Sun in 87 Days 23 Hours, at the mean Distance of 32,000,000 Miles. As this Planet is almoft three times nearer the Sun than the Earth is, his Light and Heat is feven times greater than ours*; which Degree of Heat is fufficient to make Water boil,

The Quantity of Light and Heat which the feveral Planets receive from the Sun, is reciprocally as the Squares of their Distance.

N. B. No Regard is here had to Atmospheres, or other Circumftances that may be peculiar to the feveral Planets, and which may poffibly increase or diminish their Light or Heat, but merely to their Distances from the Sun.

This Planet muft therefore confift of denfer Matter than the Earth; and if it be inhabited, it must be by other fort of Creatures than any that live here.

An Eye in Mercury fees five Planets fuperior to it. Venus and the Earth, when they are in Oppofition to the Sun, shine upon Mercury with a full Orb, and afford a confiderable Light to this Planet in the Night; but the other Planets do not afford him fo much Light as they do to us.

It is not known whether Mercury revolves on his Axis, nor what is the Inclination of his Axis to the Plane of his Orbit; confequently the Length of his Days, and what Change of Seasons this Planet is fubject to, are both unknown; only the Orbit of Mercury, being the most excentric of any of the Planets, muft occafion a confiderable Alteration of his Light and Heat, in different Times of his Year,

Mercury's greatest Elongation, or apparent Distance from the Sun, is about 28 Degrees. The Inclination of his Orbit to the Plane of the Ecliptic is 6 Degrees, 54 Minutes.

There may be other Planets betwixt Mercury and the Sun; but if there are, they can never be feen by us, because of their Nearness to the Sun.

VENUS,

VENUS, the brightest of the primary Planets, and nearest to the Earth, is 7906 Miles Diameter. She revolves on her Axis in 23 Hours and in her Orbit round the Sun in 224 Days, 16 Hours, 46 Minutes at the mean Distance of 59,000,000 Miles from the Sun. Her Light and Heat is more than twice as much as ours. The Orbit of Venus being nearer the Sun than the Earth's annual Orbit, she is much nearer the Earth, viz. fix times nearer, at her inferior Conjunction, or when she is betwixt the Earth and the Sun, than at her fuperior Conjunction, viz. when the Sun is betwixt the Earth and Venus; therefore The appears much larger to us at one timet than another.

Her greatest Elongation is about 48 Degrees. The Inclination of her Orbit to the Plane of the Ecliptic is 3 Deg. 24 Min. 3 She appears with different Phafes, viz. borned and full like the Moon; and sometimes, at her inferior Conjunction, she ap

* That is, according to Caffini's Obfervations, which he made in the Years 1666 and 1667. But Sig. Francis Bianchini has advanced a new Theory of Venus, according to which the performs her Revolution on her Axis in about 24 Days and 8 Hours; and the Inclination of her Axis to the Plane of her Orbit is 15 Degrees. He grounds this Theory on Obfervations which he made on that Planet in the Years 1726 and 1727, and has published it under the Title of Hefperi et Phosphori nova Phænomena. But I do not find it has yet been confirmed by the Obfervations of other Afronomers.

- pears as a Spot paffing over the Body of

the Sun.

The different Phafes of Venus were firft discovered by the great Italian Philofopher Galileo, in the Beginning of the last Century whereby he fulfilled the famous Prophecy of Copernicus; who, when it was objected to his Hypothefis, that according to it, Venus ought to undergo the fame Changes and Phafes that the Moon does, answered, that perhaps the Aftronomers in after Ages would find, that Venus does really undergo all thefe Changes.

The inferior Planets, from the Time of their fuperior to the Time of their inferior Conjunction, are feen more easterly than the Sun, and fet after him. Then they are Evening Stars.

But from the Time of their inferior to their fuperior Conjunction, they are seen weftward of the Sun; and confequently fet in the Evening, and rife in the Morning before him. Then they are Morning Stars.

They are feen from the Earth to move much swifter in their Orbits round the Sun, at fometimes than at others; and their Motion at fometimes appears to be direct, or according to the natural Order of the Signs of the Zodiac, as from Aries to Taurus, &c. fometimes retrograde, or contrary to the Order of the Signs, as from Taurus to Aries,

&c.

&c. and fometimes they appear to be ftationary, or without any Motion at all, for fome Days together; all which is occafioned by the Earth and these Planets moving in concentric Orbits, one within another, but with different Volocities.

Let the leffer Circle about the Sun represent the Orbit of Mercury, and Fig. VI. the larger the Earth's annual Orbit, about a Quarter, or 12 Weeks of which, answers, in time, to Mercury's Year. When the Earth is at o, and Mercury at o, in his Orbit, he appears at o in the Zodiac; about a Week after at III, he appears direct; at 2 2 2 and 3 3 3 he' continues direct, though more flow in his Motion; at 4 4 4 he is flationary; at 555 he appears retrograde, as alfo at 6 66 (when he is in his inferior Conjunction ;) and at 777, till about 8 8 8, he is ftationary again; after which, at 999 and 10 10 10, he becomes direct again; and more fwiftly at II II II and 12 12 12. The Cafe of Venus is much the fame.

MARS, who looks the most red and fiery of any of the Planets, revolves on his Axis in 24 Hours, 40 Minutes, at the mean Distance of 123,000,000 Miles from the Sun. His Days and Nights are always nearly of the fame Length; because his Axis is nearly at right Angles to the Plane

of