minutes from the disc of Mars, that became so faint before its occultation that it could not be seen by the naked eye, nor even with a three-feet telescope.

From a series of observations, Dr. Herschel found that the poles of Mars were distinguished by very remarkable luminous spots. These he employed to determine the situation of the axis of the planet, and its inclination to the ecliptic, &c. Their magnitude and splendour were sometimes very considerable, but subject to very great variations. The Dr. supposes that they are produced by the reflection of the sun's light from the snow near the poles; and that the variations in their size and brightness is owing to the melting of the polar ice.

Dr. H. has also determined that Mars is of a spheroidical form, somewhat similar to that of the earth, having its equatorial diameter to the polar as 103 to 98. When we reflect on the general appearance of this planet, we soon find that opportunities for making observations on its real form cannot be very frequent: for when it is near enough to be viewed to advantage, we generally see it gibbous, or more than half illuminated, and its oppositions are so seldom, and of so short duration, that in the space of two years there is not above three or four weeks for making such observations. It is therefore not at all surprising that the spheroidical form of this planet should not have been discovered before the time of Dr. Herschel.

As the orbit of Mars includes that of the earth, at the time of his opposition to the sun he is nearly five times nearer the earth than at the time of his conjunction; he therefore appears nearly five times greater at the former of those periods than at the latter. And by observing his oppositions in different parts of the heavens, or when he is in different parts of his orbit, his apparent diameter is nearly the same; hence it is inferred that the sun is nearly in the centre of the orbit of Mars, or that it does not deviate much from a circle;* the same appearance is observed when Jupiter and Saturn are in a similar situation; it is therefore concluded that the orbits of these planets are also nearly circular.

Mars appears with his disc perfectly round both at the time of opposition and conjunction. In the intermediate positions, he is found to want something of perfect rotundity, on the side turned farthest from the sun. This not only proves that Mars receives his light from the sun, but that his orbit includes that of the earth.

When Mars first emerges from the sun's rays, a few days after the conjunction, he rises some minutes before the sun, and his motion is found to be progressive, that is, he changes his place daily a little towards the east. But the earth's motion in the same direction is nearly double that of Mars, which has the effect of making that planet appear to recede from the sun towards the west, though its real motion, with respect to the fixed stars, is toward the east. This

Although the orbit of Mars does not deviate greatly from a circle, yet it is the most eccentric of all the ancient planets, except that of Mercury.

continues for nearly a year, when his angular distance from the sun is about 137°, he then appears to be stationary for a few days. After this, his motion becomes retrograde, or toward the west, and continues so till he is 180° distant from the sun, or in opposition, so as to be on the meridian at midnight. His retrograde motion is then swiftest; it afterwards becomes gradually slower, and ceases altogether when the planet has again come to be about 137° distant from the sun on the other side. The motion then becomes progressive again, and continues so till the conjunction, and beyond it, in the manner just described. The period in which all those changes take place, or the interval between one conjunction, or opposition to the next, is 780 days, which is the length of a synodical revolution of this planet.

The irregularities of Mars, in his orbit, being the most considerable of all the primary planets, Kepler fixed upon it as the first object of his investigations respecting the nature of the planetary orbits; and after extraordinary labour, he at last discovered that the orbit of this planet was elliptical; that the sun is placed in one of the foci; and that there is no point round which the angular motion is uniform.

In the pursuit of this inquiry he found the same thing of the earth's orbit; hence, by analogy, it was reasonable to think, that all the planetary orbits are elliptical, having the sun in their common focus.*

OF CERES. ?

This and the three following planets have all been discovered since the beginning of the present century, and within the space of six

years.

Ceres was discovered on the 1st of January, 1801, by Mr. Piazzi at Palermo in Sicily.

Its diameter, according to Dr. Herschel, is only 163 miles. But Shroeter makes it 1624 miles. This great difference, says Shroeter, arises from Dr. H. observing with his projection-micrometer at too great a distance from his eye, and measuring only the middle clear part of the nucleus.

Ceres moves in an orbit between Mars and Jupiter; and its mean distance from the sun is about 263,000,000 miles. The time it takes to perform its sidereal revolution round the sun in 1681 days, 12 hours, 56 minutes. The excentricity of its orbit is a little greater than that of Mercury; and its inclination to the ecliptic exceeds that of

* Logarithms not having been discovered at that time, arithmetical calculations, when pushed to great accuracy, required both great patience and great labour. In the calculation of every opposition of Mars, the work filled ten folio pages, and Kepter composed together seven oppositions of this planet, repeating each calculation ten times; so that the work for each opposition filled 100 such pages, and the whole calculation for the seven oppositions produced a large folio volume.

all the old planets very considerably. Ceres is not visible to the naked eye; but when observed by a telescope, appears of a ruddy colour, and about the size of a star of the eighth magnitude. It also seems to be surrounded by an extensive and dense atmosphere; but when examined by a telescope, which magnifies it above two hundred times, its disc may be very distinctly perceived.

OF PALLAS. f

The planet Pallas was discovered at Bremen, in Lower Saxony, on the 28th March, 1802, by Dr. Olbers. It moves in an orbit between that of Mars and Jupiter, and its mean distance from the sun is nearly the same as Ceres; but the inclination of its orbit to the ecliptic is much greater, being not less than 34°. The time it takes to perform its sidereal revolution round the sun, is about five hours more than Ceres; and it appears also to be about the same magnitude. It is less ruddy than Ceres; but is surrounded by an atmosphere similar to what surrounds that planet. It likewise undergoes similar changes, but its light exhibits greater variations.

OF JUNO. *

The planet Juno was discovered by Mr. Harding at the observatory of Lilianthal, near Bremen, on the evening of the 1st of September, 1804, while he was making a catalogue of all the stars which were near the orbits of Ceres and Pallas. Its diameter, according to Shroeter, is 1425 miles; and its mean distance from the sun is about 253,000,000 miles. The time it takes to perform its sidereal revolution round the sun is nearly 1590 days. The orbit in which it performs its revolution round the sun is situated between that of Mars and Jupiter, and is so eliptical that its greatest distance from the sun is nearly double its least distance. This great excentricity has a remarkable effect on the motion of the planet in its orbit: for it moves through that half of its orbit which is nearest the sun nearly in half the time that it moves through the other. The inclination of the orbit to the ecliptic is about 13o.

Juno is of a reddish colour, and free from that whitish light which surrounds Pallas.

OF VESTA.

This planet was discovered by Dr. Olbers, of Bremen, on the 29th March, 1807. Its diameter is stated at 238 miles; but Shroeter makes it much greater. The orbit of Pallas is situated between the orbits of Mars and Jupiter; but nearer the former than any of the other new planets. The time it takes to perform its sidereal revolu

tion round the sun is 1335 days, 4 hours, 54 minutes. Its mean distance from the sun is about 225,000,000 miles.

In a clear evening this planet may be seen by the naked eye, like a star of the sixth magnitude, of a dusky colour, similar in appearance to Uranus,—its light being more intense, pure, and white, than any of the other three new planets.

It appears rather extraordinary that the orbits of the four new planets, just described, should all be nearly at the same distance from the sun, and in a part of the heavens, where it was conjectured some planet might perform its revolution round the sun, although no astronomer had ever been so fortunate as to discover it. What led to this supposition was the great distance between the orbits of Mars and Jupiter, a thing so unlike the regular order in which the orbits of the planets between the sun and Mars were disposed. Accordingly, upon the discovery of Ceres, the harmony and regularity of the system seemed to be established; but the subsequent discovery of Pallas and Juno, seemed again to overturn these speculations. This new difficulty suggested to Dr. Olbers, what may perhaps be considered a very romantic idea, namely, that the three recently discovered planets might be fragments of a planet, which had been burst asunder by some internal convulsion. This opinion seemed to receive considerable support from a comparison of their magnitudes with that of all the other planets: from the circumstance of their orbits being nearly at equal distances from the sun; and from the very singular fact, that all their orbits cross one another in two opposite points in the heavens.

The support which this hypothesis derived from the last of these circumstances is peculiarly strong and conclusive; for it can be demonstrated, that if a planet, in motion, be rent asunder by any internal force, however different the inclinations of the orbits of the fragments may be, they must all meet again in two opposite points. Prosecuting this idea, Dr. Olbers every year examined the small stars that were near these points in the heavens, and was so fortunate as to discover a fourth fragment, or the last discovered planet Vesta. Dr. Brewster, of Edinburgh, has suggested another view of the subject, which seems to give additional support to the theory of Olbers. If a planet, says Dr. B., be rent asunder by any explosive force, the form of the orbits assumed by the fragments, and their inclination to the ecliptic, or to the orbit of the original planet, will depend upon the size of the fragments, or the weight of their respective masses: the larger masses will deviate least from the original path, while the smaller fragments being thrown off with greater velocity, will revolve in orbits more eccentric, and more inclined to the ecliptic. Now this is precisely what happens. Ceres and Vesta are found to be the largest, and their orbits have nearly the same inclination to the ecliptic as some of the old planets; while the orbits of the smaller ones, Juno and Pallas, are inclined to the ecliptic, 21° and 34° respectively.

As to these four bodies, so very unlike the other primary planets, Dr. Herschel has given the name of Asteroids. La Lande and some

others have named each of them after the person who discovered it; but this produces some degree of confusion, because Dr. Olbers discovered both Pallas and Vesta.

OF JUPITER,

Beyond the sphere of Mars, in distant skies,
Revolves the mighty magnitude of Jove,
With kingly state, the rival of the Sun.
About him round, four planetary moons,

On earth, with wonder, all night long beheld,
Moon above moon, his fair attendants dance.

Jupiter performs his revolution round the sun in an orbit, which includes all the planets yet described. His mean distance from the sun is about 490 millions of miles; and the time he takes to complete a sidereal revolution round that luminary is 4332 days, 14 hours, 27 minutes. The motion on his axis is extremely rapid, being performed in the short space of 9 hours, 56 minutes; but his hourly motion in his orbit is only 25,000 miles.

Jupiter is the largest of all the planets, his diameter being 89,170 English miles.

Next Jove, prodigious planet of the skies!
His orb presents of huge amazing size.

BROWN.

Jupiter is also the brightest of all the planets, except Venus, which, on some occasions, exceeds him in splendour. The character by which he is represented is 2.

Jupiter is surrounded by faint substances, called zones, or belts, which lie parallel to each other on his surface. They are, however, subject to considerable variation both in breadth and number and are on some occasions more conspicuous than at others. They are not only parallel to each other, but, in general, parallel to the equator of Jupiter. Bright and dark spots are also frequently to be seen in the belts; and when a belt vanishes, these spots vanish with it. The broken ends of some belts have often been observed to revolve in the same time with the spots; only those near the equator of the planet more quickly than those nearer the poles. This is, perhaps, on account of the greater heat of the sun near the equator than the poles; the equator being parallel to the belts and course of the spots. On some occasions the spots have been observed to change their forms gradually, and sometimes with very unequal velocities.

Astronomers are very different in their opinions respecting the cause of these appearances. Some consider them as the effect of changes in the atmosphere that surround Jupiter; while others regard them as indications of great physical revolutions on the surface of that planet. The first of these hypotheses appears to explain the variations in the form and magnitude of the belts; but it by no means accounts for their parallelism, nor for the permanence of some of the