8. The inclination of the orbit of Pallas to the plane of the ecliptic is much greater than that of any of the planets, being about 34° 37′8′′. The real diameter of this planet, according to Schroeter, is about 80 miles; but, according to Dr. Herschel, it is about 2099 miles.

This planet is too small to be seen by the naked eye; but when viewed in a good telescope, it appears less ruddy than Ceres; but the light of Pallas exhibits greater variations. The atmosphere of this planet, according to Schroeter, is 468 miles. Pallas was discovered March 28th, 1802, by Dr. Olbers, at Bremen, in Lower Saxony.

QUESTIONS.

How is the planet Vesta situated, and what is its relative mean distance from the sun, that of the earth being considered as 10?

What is the inclination of the orbit of Vesta to the plane of the ecliptic ?

What is the relative mean distance of Juno from the sun, that of the earth being considered as 10? What is the real diameter of Juno, according to Schroeter?

What is the relative mean distance of Ceres from the sun, that of the earth being considered as 10?

In what angle is the orbit of Ceres inclined to the plane of the ecliptic?

How is the planet Pallas situated, and what is its relative distance from the sun, that of the earth being considered as 10?

In what angle is the orbit of Pallas inclined to the plane of the ecliptic?

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 discovery 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 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.

To 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 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. Brewster, 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, about 130 and 3410 respectively.

It is, however, somewhat remarkable that the orbits of Ceres and Pallas cross each other, owing to the very great eccentricity of the orbit of Pallas; it is several millions of miles nearer the sun in its perihelion, then Ceres in the same point of its orbit. But when Pallas is in its aphelion, its distance from the sun is several millions of miles greater than that of Ceres in the same point of its orbit.

Juno is farther from the sun in its aphelion, than Ceres in the same point of its orbit; and Vesta is farther from the sun in its aphelion, than either Juno, Ceres, or Pallas, in their perihelions. The perihelion distance of Vesta is greater than that of Juno or Pallas. So that it appears Vesta may sometimes be at a greater distance from the sun, than either Juno, Ceres, or Pallas, although its mean distance is less than either of them by some millions of miles: therefore, the orbit of Vesta crosses the orbits of the other three planets.

CHAPTER X.

Of Jupiter 4, and its Satellites.

1. Jupiter, the ninth planet in order of distance from the sun, performs its sidereal revolution in 4332 days, 14 hours, 18 minutes, and 41 seconds, which is the length of its year: the rotation of this planet on its axis, from west to east, is completed in 9 hours, 55 minutes, and 49,7 seconds, which is the length of its day. The mean hourly motion of this planet in its orbit is 29,866 miles.

Jupiter is the brightest of all the planets, except Venus. He shines with a bright white light, but does not vary in apparent size and brightness like Mars.

2. The form of Jupiter, like that of the earth, is an oblate spheroid, the equatorial diameter being to the polar as 14 to 13. The mean apparent equatorial diameter of this planet is 38"; and when in opposition, it is equal to 47"; its real diameter is 91,000 miles; and its relative mean distance

from the sun is 52, that of the earth being considered as 10.

Jupiter is the largest planet in the solar system: its relative size is 1280.9; its mass 303.94; and its density .24119; the size, mass, and density of the earth being respectively considered as unity, or 1. And a body weighing 1 pound on the surface of the earth, would, if removed to the surface of Jupiter, weigh 25 pounds nearly.

3. The inclination of the orbit of Jupiter to the plane of the ecliptic is 1° 18' 47"; and the axis of this planet is so nearly perpendicular to its orbit, that it has no sensible change of season: so that in the polar regions of Jupiter, there is perpetual winter: and about his equator, perpetual summer.

The inclination of Jupiter's orbit to the ecliptic has a small diminution of about 22.6 seconds in a century; and his path, according to La Place, deviates occasionally from the ecliptic 20 42′, or 3° 36'.

4. The apparent motion of this planet is supject to inequalities similar to those of Mars; previous to, and when it is nearly 115° 12′ distant from opposition, its motion becomes retrograde, its velocity augments till the moment of opposition; then diminishes, and the motion becomes direct, when the planet, in its approach towards the sun, is only 115° 12' distant from it. The duration of this retrograde motion is about 121 days, and the arc of retrogradation is 9° 54'. But there are, according to La Place, perceptible differences in the extent and duration of the retrograde motions of Jupiter. The synodic revolution of this planet, or the time from opposition to opposition, is 398 days, 20 hours, 48 minutes, and 28 seconds.

When Jupiter is in conjunction he rises, sets, and comes to the meridian with the sun; but is never observed to

14

transit or pass over the sun's disc; when in opposition, he rises when the sun sets, sets when the sun rises, and comes to the meridian at midnight. This is a sufficient proof that Jupiter revolves round the sun in an orbit which includes that of the earth.

Jupiter, when it opposition, appears larger and more luminous than at other times, being then much nearer to the earth than a little before or after his conjunction; when the longitude of Jupiter is less than that of the sun, he will appear in the east before the sun rises, and will then be a morning star; but when his longitude is greater than that of the sun, he will appear in the west after sun-set, and will then be an evening star.

5. Jupiter, when viewed through a telescope, is observed to be surrounded by faint substances, called zones, or belts, which are not only parallel to one another, but, in general, parallel to his equator; they are, however, subject to considerable variation both in breadth and number, and are on some occasions more conspicuous than at others. Bright and dark spots are also frequently to be seen in the belts; and when a belt vanishes, the contiguous spots disappear with it. The number of belts are very variable, as sometimes only one, at others eight may be perceived.

The time of the continuance of the belts is very uncertain; they sometimes remain unchanged for three months; at others, new belts have been formed in an hour or two. In some of these belts large black spots have appeared, which moved swiftly over the disc, from the eastern to the western edge of Jupiter's disc, and returned in a short time to the same place. By observations on these, the rotation of this planet on its axis has been determined. With a telescope of a very moderate power, the disc of Jupiter is nearly as large as the moon; and though the surface be diversified by regular and parallel belts, yet it appears much smoother than that of the Moon.

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 physicalrevo