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tance of Saturn from the sun, but cannot be readily perceived without the assistance of a telescope.

From the preceding observations, any person may easily learn to distinguish all the planets. For if after sun-set he sees a planet nearer the east than the west, he may conclude that it is neither Mercury nor Venus; and

may

determine whether it be Saturn, Jupiter, or Mars, by the colour and light; by which, also, he may distinguish between Venus and Mercury.

That the light of each planet has its peculiar tinge; and that there are certain fixed stars that have the same teints, was known to the Chaldæans. It is an observation best verified in those countries, where the air is the clearest.

Besides the motions which we observe in all the planets, their apparent magnitudes are very different at different times. Every one must have observed that Venus, though she constantly appears with great splendour, is not always of the same size: but this difference of magnitude is most conspicuous in Mars, it is remarkable in Jupiter, but less so in Saturn and Mercury.

The only phenomena visible to the unassisted sight, besides those already described, are those unexpected obscurations of the sun and moon, called eclipses, * of which we shall hereafter speak more particularly.

* To these certainly may be added comets, of which, a very remarkable one exhibited in the Autumn of 1807, his striking figure and majestic course in our northern hemisphere.--Edit.

I have now described those appearances, which are the most striking to every person who has paid the least attention to what is passing over his head. The tutor would do well in his place, first, to bring his pupil acquainted with the appearances themselves, and then explain them to him by the globe, or some other instrument. It would not be amiss, if he were now to instruct him by practical observations, and shew him, by a small quadrant, how to measure the elevation of the stars, &c. always remembering that young minds are ever active in search of impressions from external objects; and that these are more permanent than those made by words; in the former, the mind energizes, and is brought into action; in the latter, it is in a great degree passive.

ESSAY I.

PART III.

OF THE COPERNICAN OR SOLAR SYSTEM.

1

AFTER having stated what would be the appearances of the heavenly bodies, if we were placed at the centre of the system, and then given a general view of their phenomena, as seen from the earth; it will now be proper to shew how the irregularities that are discovered in one situation are to be reconciled with the harmony and order that would be visible if they were to be seen from the other; or, in other words, to shew why the motions of the planets appear to us so different from what they really are.

One of the ends for which man was formed, is to correct appearances and errors, by the investigation of truth; whoever considers him attentively, from infancy to manhood, and from manhood to old age, will find him ever busy in endeavouring to find some reality, to supply the place of the false appearances, by which he has hitherto been deceived.

It is the business of the present part of this Essay to correct the errors arising from appearances, and to point out truth by a brief detail of the principal parts of the Copernican system, which is now uni

versally received, because it rationally accounts for and accords with the phenomena of the heavens.

At the appointed time, when it pleased the Supreme Dispenser of every good gift to restore light to a bewildered world, and more particularly to manifest his wisdom in the simplicity, as well as in the grandeur of his works, he opened the glorious scene with a revival of sound astronomy;"* and raised up Copernicus to dispel the darkness in which it was then involved.

The Copernican system consists of the sun, seven primary, fourteen secondary planets, and the comets.

The seven planets, Mercury, Venus, the Earth, Mars, Jupiter, Saturn, and the Georgium Sidus, move round the sun, in orbits included one within the other, and in the order here used in mentioning their names, Mercury being that which is nearest

the sun.

The seven which revolve round the sun as their centre, are called primary planets.

The fourteen planets, which revolve round the primary ones as a centre, and are at the same time carried round the sun with them, are called secondary planets, moons, or satellites.

The Georgium Sidus is attended by two moons, Saturn by seven, Jupiter by four, and the Earth by one; all of these, excepting the last, are invisible to

* Pringle's Six Discourses to the Royal Society. + The sun is not absolutely at rest, being subject to a small degree of motion, which is considered in larger works on Astronomy.

the naked eye, on account of the smallness of their size, and the greatness of their distance from us.

Mercury and Venus, being within the Earth's orbit, are called inferior planets; but Mars, Jupiter, Saturn, and the Georgium Sidus, being without it, are called superior planets.

The orbits of all the planets are elliptical; but as the principal phenomena of the Copernican system may be satisfactorily illustrated, by considering them as circular, the latter supposition is usually adopted in giving a general idea of the disposition and motion of the heavenly bodies.

Before we enter into a description of the solar sytem, it may

be necessary to define what is meant by the aris of a planet; lest the pupil should conceive them to turn on such material axes, as are used in the machines which are contrived to represent the planetary system.

The axis of a planet is a line conceived to be drawn through its centre, and about which it is conceived to turn in the course of its revolution round the sun; the extremities of this line terminate in opposite points of the surface of the planet, and are called its poles; that which points towards the northern part of the heavens, is called the north pole; that which points towards the southern, the south pole. A ball whirled from the hand into the open air, turns round upon a line within itself, while it is moving forward; such a line as this is meant, when we speak of the axis of a planet.

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