3. There are four remarkable Systems connected with the Sun and Planets; viz. the Pythagorean or Copernican, the Ptolemaic, the Tychonic, and the Newtonian. The last of these is now generally received by the learned from its having been established by Sir Isaac Newton on an immovable foundation: it is also called the Solar or Planetary System. It derives the former of these names from the Sun, which is made its centre; and the latter from the word Planet by which all those bodies moving round the Sun are designated. The name Planet signifies wanderer; and inasmuch as these celestial lights never preserve for any length of time the same relative situation, they may be said to be always straying or wandering from each other.

4. All the heavenly bodies are spheres (or nearly so), but in consequence of their enlightened parts alone being seen by us, they appear by reason of their great distance as plane surfaces; and hence these apparent surfaces are sometimes called discs (from the Latin word discus) as resembling a flat round dish. A straight line passing through the centre of a circle and cutting the circumference in two parts, is called its diameter, because it measures through it (as the Greek word diaμerpet signifies): now if a circle be supposed to turn completely round on this diameter, it will form a solid figure called a Sphere. A Hemisphere is a half-sphere cut through the centre by a right line in any direction; thus we say the Northern and Southern Hemispheres, the Eastern and Western Hemispheres, the Upper and Lower Hemispheres, and so on.

5. There is no doubt about the Chaldæans and Egyptians having been the first people in the world who were acquainted with astronomy; the Greeks borrowed it from the latter people. Thales is the first Greek who is mentioned as having laid the foundations of astronomy amongst his countrymen [B. c. 600]; he was so well acquainted with the motions of the heavenly bodies that he not only explained the eclipses but actually predicted one; he taught that the earth was round, which most of his countrymen (both before and after his time) looked upon as only a plane; he likewise showed the causes of solstices and equinoxes, and divided the year into 365 days. Pythagoras was one of his disciples, and is conjectured to have been well acquainted with the annual and diurnal revolutions of the earth round the sun; his pupil Philolaus was the first who openly taught this true system of the universe, but it was lost during the reign of the Peripatetic philosophy, and was first retrieved by Copernicus. (See Plate I. fig. 2.)

The Ptolemaic System was so named from the famous geographer and

mathematician Claudius Ptolemæus, who flourished at Pelusium in Egypt during the reigns of the Roman emperors Hadrian and Antoninus Pius. He taught that the earth was at rest in the centre of the universe, and that the heavens revolved round it from East to West in twenty-four hours, carrying all the heavenly bodies, stars, and planets along with them: but as there were many difficulties in his system, he endeavoured to account for some of the motions of the heavenly bodies by the introduction of cycles and epicycles, which though they were exceedingly ingenious, were almost unintelligible. These doctrines were subsequently universally believed and maintained by the learned of all nations till they were refuted by Copernicus and Newton. (See Plate I. fig. 3.)

7. Nicholas Copernicus (born A. D. 1473 at Thorn a town of Prussia) the author of the Copernican System, unable to reconcile the confused and perplexing hypothesis of Ptolemy with his own observations, or with those notices concerning the heavenly bodies which he found scattered over the works of the earlier philosophers, set about to reform its absurdity. But so firmly had the 1400 years which had elapsed from the time of Ptolemy to his own days rooted the error, that to confute it was incurring the imputation of heresy; and for this, about a century afterwards, Galileo suffered the severest punishments. Copernicus taught that the sun occupied the centre of the universe, and that the planets moved round him in elliptical orbits proportioned to their size: this system established by the new arguments and discoveries of Galileo, Kepler, and Newton, has finally prevailed over the prejudices against the earth's motion. (See Plate I. fig. 2.)

of

8. The Tychonic System was so called from Tycho Brahe a noble Dane (born A. D. 1546), who partly revived the old system of Ptolemy concerning the earth remaining at rest whilst the other heavenly bodies moved round it. But his system differed from Ptolemy's in its allowing the monthly motion the Moon round the Earth; it also made the Sun to be the centre of the orbits of Mercury, Venus, Mars, Jupiter, and Saturn, which revolved round him in their respective years as he revolved round the Earth in a solar year he thus supposed these five planets together with the Sun to be carried round the Earth in twenty-four hours. His system was afterwards altered by some other astronomers, who allowed the diurnal motion of the Earth on its own axis but denied its annual motion round the Sun : this hypothesis, partly true and partly false, is called the Semi-Tychonic System. (See Plate 1, fig 4.)

9. THE SOLAR SYSTEM is that which was taught by Pythagoras and Philolaus, revived by Copernicus, and at length immovably established by our great countryman Sir Isaac Newton (A. D. 1687) after the most simple and uniform manner. The great principle on which the whole of this system rests, is Gravity, or that power by which all the planets are drawn to the Centre of their respective orbits: hence its name, the Centripetal force (from centrum and peto). The Centrifugal force, on the other hand (derived from centrum and fugio), is that by which all bodies, when set in motion, will move uniformly in a straight line, except they are hindered; and thus they constantly tend to fly from the centre. This centre is the

Sun, and round it revolve in regular periods those opacous bodies which derive their light from him, and are called PLANETS. The ancients appear to have been acquainted only with six of the planets, but modern astronomers have discovered five more. The nearest of these to the Sun is Mercury; then follow successively Venus, the Earth, Mars, Ceres, Pallas, Juno, Vesta, Jupiter, Saturn, and the Georgian the outmost of them all. (See Plate I. fig 1.)

10. The path described by a planet in moving round the Sun, is called its Orbit: these orbits are not circular but elliptical, neither are they parallel, for some of them cut and cross each other in various directions. Besides this motion round the sun, each planet has a daily motion round its own axis, which causes it to have its spherical shape a little flattened at the poles. This alteration in the figure of the planets is owing to the parts, which are receding from the axis, having a tendency to rise towards the equator, especially if the matter of which they consist be fluid: and therefore, unless our earth were higher at the equator than towards the poles, the sea would rise under the equator and overflow all near it. The distance of the planets from the Sun, as well as other phænomena connected with them, will be best seen from the following table.

11. TABULAR VIEW OF THE SOLAR SYSTEM.

12. All these planets are called primary, from their

revolving round the sun as their proper centre: the secondary planets are such as move round some primary planet in the same way that the latter does round the Sun, although they likewise derive all their light from the Sun. Thus the Earth, Jupiter, Saturn, and the Georgian, are each attended with secondary planets; the Georgian with six, Saturn with seven, Jupiter with four, and the Earth with one: the last mentioned secondary planet is the Moon, and hence the whole of them are sometimes called Moons, as also Satellites from their attending the primary bodies as a prince is attended by his (Satellites or) Life-guards.

13. Sol or the Sun, the great luminary of our System, whose presence constitutes day, was in the infancy of astronomy reckoned amongst the planets, but he should rather be numbered amongst the fixed stars. A spectator placed as near to a star as we are to the Sun, would see that star as large and bright a body as we see the Sun: whilst another spectator as far distant from the Sun as we are from the stars, would see him as small as we see a star, divested too of the earth and all the other planets, which are circulating round him. The revolution of the Sun on his own axis from East to West is evinced by the motion of the spots which are observed on his surface.

14. Mercury is a little bright planet, and such a close companion of the sun that it is usually lost in his splendour; it is subject to the same phases (ie appearances) as the moon. Venus, the brightest and most beautiful of all the planets, is also called Lucifer, Phosphorus, and the Morning-star, when she goes before the Sun, and Hesperus or the Evening-star, when she follows him; she is not only remarkable for her bright and white light, but for her phases varying just like those of the Moon, her illumined part being constantly turned towards the Sun, viz. towards the East when she is a Morning-star, and towards the West when she is an Evening-star. Mars has obtained its name from its fiery appearance, which is supposed to be derived from the atmosphere with which it is surrounded. The next four planets, viz. Vesta, Ceres, Pallas, and Juno, are so exceedingly dimi nutive as to be seen with some difficulty; from the remarkable phænomena connected with them, the learned have conjectured that they are the remains of one celestial body, which revolved round the sun between the orbits of Mars and Jupiter, but at last burst in pieces from some sudden convulsion. Jupiter is a bright reflulgent star, and is remarkable not only for the belts or zones with which he is marked, but for his four little satellites which constantly move round him as the Moon does round our earth. Saturn, on account of his great distance, appears to the eye with a feeble light; he has seven satellites which are constantly circulating round him, but he is more remarkable from being encompassed with a Ring, which is opacous, like the planet itself. The Georgian is at so great a distance from us that but little is known concerning its phænomena: it has six satellites which revolve round it in regular order, being subject to the same laws as those of the preceding planets. By attending to these observations the pupil may readily distinguish all the larger planets; for if after sun-set he sees one of them nearer the East than the West, it can neither be Mercury nor Venus, and

he may easily determine whether it is Mars, Jupiter, or Saturn, by the colour and brilliancy of its light; he may also distinguish Mercury from Venus by the same means.-Each of the planets is denoted in astronomical works by a certain character, which may be seen exemplified in Plate I. fig 1. 15. Besides these planets there are other Celestial Lights called Comets, or, vulgarly, Blazing Stars, which occasionally traverse our System though they do not seem to form a part of it; they appear suddenly, and, after having moved like planets in very eccentric orbits, they disappear, but return again after long periods of time. They are distinguished from the other luminaries by their being generally attended with a long train of light, which is always opposite to the sun, and becomes of a fainter lustre the farther it is from the body they are compact, fixed, and durable bodies, and their trains are composed of a very thin, slender vapour emitted by the head or nucleus of the comet ignited by the sun. When a comet moves from the sun or to the Eastward of it, it is said to be bearded, because the light precedes it in the manner of a beard; when it is to the Westward of the sun and sets after it, it is said to be tailed, because the light follows it in the form of a train or tail: but when it and the sun are diametrically opposite (the earth between them), the train is hid behind the body of the comet, excepting a small portion, which appears round it like a border of hair, and then it is called huiry, and from this last appearance the name of comet is derived. Very little is known even in modern times concerning the nature and phænomena of

comets.

16. THE FIXED STARS. But the whole of our Solar System occupies a very small portion in the infinite regions of Space. It is surrounded on all sides by an innumerable host of stars appearing to us certainly as placed in a concave sphere, but situated at such a remote distance from our system as to exceed the bounds of all calculation. As an instance of this it may be mentioned that the star called Sirius, one of the largest in the heavens, is reckoned by astronomers to be at least 27,000 times farther from us than the sun is.

17. These stars are called the Fixed Stars from their constantly retaining the same position and distance with respect to each other, and in contradistinction to the wandering stars or planets; these last shine with a steady light, and hence the fixed stars, which, owing to their immense distance, have always a twinkling appearance, may be readily distinguished from them. Each of the Fixed Stars is supposed to be a separate sun of itself, for they are all too far removed from the sun which illuminates our earth to derive any light from him; and moreover each is conjectured to be the centre of a system like our own, and to have planets circulating round it in the same harmonious and beautiful revolution.

18. The magnitudes of the fixed stars appear to us to be very different,