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CHAP. III.: Of our Solar System. - John Locke, The Works, vol. 2 An Essay concerning Human Understanding Part 2 and Other Writings 
The Works of John Locke in Nine Volumes, (London: Rivington, 1824 12th ed.). Vol. 2.
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Of our Solar System.
Our solar system consists of the sun, and the planets and comets moving about it.
The planets are bodies, which appear to us like stars; not that they are luminous bodies, that is, have light in themselves; but they shine by reflecting the light of the sun.
They are called planets from a Greek word, which signifies wandering; because they change their places, and do not always keep the same distance with one another, nor with the fixt stars, as the fixt stars do.
The planets are either primary, or secondary.
There are six primary planets, viz. Mercury, Venus, the Earth, Mars, Jupiter, and Saturn.
All these move round the sun, which is, as it were, the centre of their motions.
The secondary planets move round about other planets. Besides the moon, which moves about the earth; four moons move about Jupiter, and five about Saturn, which are called their satellites.
The middle distances of the primary planets from the sun are as follows:
The orbits of the planets, and their respective distances from the sun, and from one another, together with the orbit of a comet, may be seen in the figure of the solar system hereunto annexed.
The periodical times of each planet’s revolution about the sun are as follows:
The planets move round about the sun from west to east in the zodiac; or, to speak plainer, are always found amongst some of the stars of those constellations, which make the twelve signs of the zodiac.
The motion of the planets about the sun is not perfectly circular, but rather elliptical.
The reason of their motions in curve lines, is the attraction of the sun, or their gravitations towards the sun, (call it which you please); and an oblique or side-long impulse or motion.
These two motions or tendencies, the one always endeavouring to carry them in a straight line from the circle they move in, and the other endeavouring to draw them in a straight line to the sun, makes that curve line they revolve in.
The motion of the comets about the sun is in a very long slender oval: whereof one of the focuses is the centre of the sun, and the other very much beyond the sphere of Saturn.
The moon moves about the earth, as the earth doth about the sun. So that it hath the centre of its motion in the earth; as the earth hath the centre of its revolution in the sun, about which it moves.
The moon makes its synodical motion about the earth, in 29 days, 12 hours, and about 44 minutes.
It is full moon, when, the earth being between the sun and the moon, we see all the enlightened part of the moon: new moon, when, the moon being between us and the sun, its enlightened part is turned from us; and half moon, when the moon being in the quadratures, as the astronomers call it, we see but half the enlightened part.
An eclipse of the moon is, when the earth, being between the sun and the moon, hinders the light of the sun from falling upon, and being reflected by, the moon. If the light of the sun is kept off from the whole body of the moon, it is a total eclipse; if from a part only, it is a partial one.
An eclipse of the sun is, when the moon, being between the sun and the earth, hinders the light of the sun from coming to us. If the moon hides from us the whole body of the sun, it is a total eclipse; if not, a partial one.
Our solar system is distant from the fixt stars 20,000,000,000 semi-diameters of the earth; or, as Mr. Huygens expresses the distance, in his Cosmotheorosa : the fixt stars are so remote from the earth, that, if a cannon-bullet should come from one of the fixt stars with as swift a motion as it hath when it is shot out of the mouth of a cannon, it would be 700,000 years in coming to the earth.
This vast distance so much abates the attraction to those remote bodies, that its operation upon those of our system is not at all sensible, nor would draw away or hinder the return of any of our solar comets; though some of them should go so far from the sun, as not to make the revolution about it in less than 1000 years.
It is more suitable to the wisdom, power, and greatness of God, to think that the fixt stars are all of them suns, with systems of inhabitable planets moving about them, to whose inhabitants he displays the marks of his goodness as well as to us; rather than to imagine that those very remote bodies, so little useful to us, were made only for our sake.
[a ]Christiani Huygenii ΚΟΣΜΟΘΕΩΡΟΣ, sive de terris cœlestibus earumque ornatu, conjecturæ, &c. p. m. 137.