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ASTRONOMY.

Of the Solar System according to Copernicus.

S. A T the Beginning of this Discourse you promised to

I give me some Idea of Astronomy, which I should now attend to with great pleasure.

M. As to the Science of Astronomy in all its Parts, tho' it is both pleasant and useful, it may perhaps be too intricate and laborious for you to enter upon at present. I will therefore content myself with endeavouring to give you a general Notion of the Copernican System, without entering at all into the abstruse Parts of the Science,

The Earth we live upon, has been generally thought to be the Center of the Universe, and to be fixt and immoveable. Pythagoras indeed among the Ancients, taught the contrary ; but his Opinion for want of being thoroughly canvass’d by learned and ingenious Men, grow into Disrepute, and was for many Centuries totally neglected. About 250 Years ago it was again reviv'd by Copernicus, a Native of Thorn in Prulfia; and is of late, by our great Newton, establish'd on such clear and solid Principles, that it is now universally receiv'd.

This System is disposed in the following manner. The Sun is placed in the Center, from whence it never moves. But from some Observations made on its Spots, it is found

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ASTRONOMY. 313 to turn round on its own Axis, from West to East, in about 25 Days. Round about him at unequal Distances fix opaque spherical Bodies continually revolve : These are called the primary Planets. That which is nearest to the Sun is called Mercury; the next Venus ; then our Earth; the next beyond is Mars; after him Jupiter; and the most distant of all is Saturn. Saturn, Jupiter, and Mars, are called superior Planets, because their Circuits are beyond the Earth's Orbit ; Mercury and Venus are called inferior Planets, because their Circuits are within that Orbit.

Besides these, there are discover'd in this system ten other Bodies, which move about some of these primary Planets in the same manner, as they move round the Sun. These are called secondary Planets. The most conspicuous of them is the Moon, which moves round our Earth ; four move in like manner round Jupiter, and five round Saturn.

The same Planet is not always equally distant from the Sun ; but if the Distance of the Earth from the Sun be divided into ten equal Parts, the mean Distance of Saturn from the Sun will be 95 fuch Parts, of Jupiter 52, of Mars 15, of Venus 7, and of Mercury 4. Now the Distance of the Earth from the Sun is found to be about 76 Millions of English Miles. If therefore you multiply one tenth Part of avis Distance, which is about 7600000 Miles, by 95, it will give you the Distance of Saturn from the Sun, in Enlib Miles'; if by 52, it will give you the Distance of Jupiter ; if by 15, of Mars ; if by 7, of Venus ; and if by 4, of Mercury.

But from a round Calculation, the Distance of each Planet from the Sun in English Miles is about

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The Distance of the Moon from the Earth is about 30 of the Earth's Diameters, or 240 Thousand Milles. Its Proportion to the Earth in Magnitude is as 5 to 258; that is, it is more than 50 times less than the Earth. The Sun is about a Million of times bigger than the Earth.

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The Diameters of the Sun, the Earth, and each of the Planets, in English Miles, are nearly as follows;

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All these Planets, both primary and secondary, being opaque Bodies, and receiving all their Light from the Sun, as well as making their great Revolutions round him, are for these Reasons look'd upon as Dependents on him, and make up altogether what is called the Solar System

All these Planets move one Way, from West to East; and of the primary Planets, the most remote is longest in finishing its Course round the Sun. The Period of Saturn falls short only 16 Days of 29 Years and a half. The Period of Jupiter is 12 Years wanting about 50 Days. The Period of Mars is within 43 Days of 2 Years. The Revolution of the Earth is one Year. The Period of Venus is perform'd in about 224 Days and an half, and of Mercury in about 88 Days.

Such of these Bodies are revolve round their own Axis, perform that Revolution in the following times. The Sun in something more than 25 Days. Mars in one Day and 40 Minutes. The Earth in 23 Hours 56 Minutes. And Jupiter in 10 Hours.

The Moon revolves about her Axis in the same time that she makes her Course round the Earth, which is a Month ; so that her Inhabitants have but one Day throughout the Year. It is very probable, that Mercury and Saturn also revolve round their own Axes, as all Parts of their þurfaces cannot otherwise receive the Light and Heat of the Sun, which in all Probability are as neceilary and convenient to them, as we find them to be to the Earth. The Certainty of this Revolution in the other Planets is proved by the Appearance and Disappearance of certain Spots on their Surfaces, which rising first on one side or Edge of the Planet's Disk, move by Degrees to the Middle, and so on till they reach the opposite Edge, where they set and disappear : And after they have þeen hid for about the fame Space of Time that they were

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as they.and of Saave hitherto or Revoltely determinPath of this

visible, they again appear to rise in or near the same Place as they did at first. Now by reason of Mercury's Nearness to the Sun, and of Saturn's great Distance from him, no Observations of this kind have hitherto been made on them, and therefore their diurnal Motion, or Revolution round their own Axis, tho' probable, is not yet absolutely determined.

As the Ecliptic Line is the Orbit or annual Path of the Earth, so each Planet has its proper Orbit, whose Plane differs some few Degrees from the Plane of the Orbit of the Earth ; and to a Spectator's Eye placed in the Center, would intersect or cut the Earth's Orbit at two opposite Points or Nodes. To represent this more plainly to your Imagination, suppose, says Mr. Watts, as many Hoops as there are Planets, thrust through with several strait Wires, and thereby join'd in different Places to the Hoop that represents the Plane of the Ecliptic, i.e. the Earth's Orbit; and then let those Hoops be turn’d more or less obliquely from the Plane of the Ecliptic: For all the several Orbits or Paths of the Flanets do not cross or intersect the Ecliptic in the same Point, nor at the same Angles; but their Nodes or Intersections of the Ecliptic are in different Parts of the Ecliptic, and also make different Angles with it. · Each of the primary Planets moves round the Sun in a Line which forms an Ellipsis, which I will here show you how to describe.

Fix upon any Plane two Pins, as at A and B. To these tie a String ACB somewhat longer than their Distance

from

more than viz. the Efecondary Platin fomewhes Di

from one another. Then apply a third Pin D in the Double of the Thread, so as to hold it strain'd, and in that manner carrying this Pin about, the Point of it will describe an Ellipsis. If through the Points AB the strait Line EABF be drawn, and terminated at the Points E and F, this is the longest Line that can be drawn within the Figure, and is called the greater Axis of the Ellipfis. The Line GH, drawn perpendicular to this Axis EF, so as to pafs thro' the middle of it, is called the lesser Axis. The two Points A and B are called Focus's. Now each primary Planet moves round the Sun in a Line of this kind, the Place of the Sun being in one of the Focus's. Suppose A to be the Place of the Sun, then E is the Point wherein the Planet will be nearest to the Sun, and at F it will be the most remote. The Point E is call'd the Perihelion of the Planet, and F the Aphelion. In G and H the Planet is said to be in its middle or mean Distance, because the Distance AG or AH is truly the middle between AE the least, and AF the greatest Distance.

Of the fix primary Planets, it hath not been observ'd that more than three are attended with Secondaries, Moons, or Satellites, viz. the Earth, Jupiter, and Saturn.

The Moon is a secondary Planet to the Earth, and performs her Revolution round it, in somewhat less than 28 Days, at about thirty Diameters of the Earth's Distance from it; and in the Space of a Year is carry'd along with the Earth round the Sun.

* Yupiter has four Satellites attending him. The first or innermost of which performs its Revolution in about i Day 18 Hours, at a Distance from the Center of that Planet equal to about 5 Semi-diameters of Jupiter's Body, The next Satellite revolves round Yupiter in about 13 Days 13 Hours, at the Distance from Jupiter of about 9 of that Planet's Semi-diameters. The third performs its Period nearly in 7 Days 3 Hours, at the Distance of about 14 Semi-diameters. The fourth, which is the outermost, makes its Period in about 16 Days 16 Hours, at a Distance of about 25 } Semi-diameters.

Saturn has five Satellites attending him, which perform their Periods round him as follows. The innermost is distant about 4 of Saturn's Semi-diameters, and revolves round him in about 1 Day 21 Hours. The next is distant about 5 Semi-diameters, and makes its Period in 2 Days 17 Hours.

The third is about 8 Semi-diameters distant, and performs its Revolution in near 4 Days 1 Hours. The fourth is near 18 Semi-diameters distant, and moves round Saturn in about

15

• Vide Pumberton's View of Newton's Philosophy,

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