its mean distance being about 35,400,000 miles. Its diameter is 3000 miles, and it completes its orbital revolution in 88 days.

Venus, the next member of the system, is sometimes our morning and sometimes our evening star. Its magnitude is almost exactly the same as that of the earth. It revolves round the sun in 225 days.

The earth is the third planet from the sun in the order of distance; the radius of its orbit being about 92,000,000 miles. It is attended by one satellite,—the moon,—the diameter of which is 2160 miles.

Mars is the first planet exterior to the earth's orbit. It is considerably smaller than the earth, and has no satellite. It revolves round the sun in 687 days.

The Asteroids.—Since the commencement of the present century a remarkable zone of telescopic planets has been discovered immediately exterior to the orbit of Mars. These bodies are extremely small; some of them probably containing less matter than the largest mountains on the earth's surface. 131 members of the group are known at present, and the number is annually increasing.

Jupiter, the first planet exterior to the asteroids, is nearly 500,000,000 miles from the sun, and revolves round it in a little less than 12 years. This planet is 86,000 miles in diameter, and contains more than twice as much matter as all the other planets, primary and secondary, put together. Jupiter is attended by four moons or satellites.

Saturn is the sixth of the principal planets in the order of distance. Its orbit is about 400,000,000 miles beyond that of Jupiter. This planet is attended by eight satellites, and is surrounded by three broad flat rings. Saturn is 73,000 miles in diameter, and its mass or quantity of matter is more than that of all the other planets except Jupiter.

Uranus is at double the distance of Saturn, or nineteen times that of the earth. Its diameter is about 34,000 miles, and its period of revolution 84 years. It is attended by at least four satellites.

Neptune is the most remote known member of the system; its distance being 2,800,000,000 miles. It is somewhat larger than Uranus; has certainly one satellite, and probably several more. Its period is about 165 years. A cannon-ball flying outward from the sun at the uniform velocity of 500 miles per hour would not reach the orbit of Neptune in less than 639 years.

These planets all move round the sun in the same direction,—from west to east. Their motions are nearly circular, and also nearly in the same plane. Their orbits, except that of Neptune, are represented in the frontispiece. It is proper to remark, however, that all representations of the solar system by maps and planetariums must give an exceedingly erroneous view either of the magnitudes or distances of its various members. If the earth, for instance, be denoted by a ball half an inch in diameter, the diameter of the sun, according to the same scale (16,000 miles to the inch), will be between four and five feet; that of the earth's orbit, about 1000 feet; while that of Neptune's orbit will be nearly six miles. To give an accurate representation of the solar system at a single view is therefore plainly impracticable.

The Zodiacal Light.—This term was first applied by Dominic Cassini, in 1683, to a faint nebulous aurora, somewhat resembling the milky way, apparently of a conical or lenticular form, having its base toward the sun and its axis nearly in the direction of the ecliptic. The most favorable time for observing it is when its axis is most nearly perpendicular to the horizon. This, in our latitudes, occurs in March, for the evening, and in October, for the morning. The angular distance of its vertex from the sun is frequently seventy or eighty degrees, while sometimes, though rarely (except within the tropics), it exceeds even one hundred degrees. It was noticed in the latter part of the 16th century by Tycho Brahe. The first accurate description of the phenomenon was given, however, by Cassini. This astronomer supposed the appearance to be produced by the blended light of innumerable bodies too small to be separately observed,—a theory still very generally accepted. In other words, the zodiacal light is probably a swarm of infinitesimal planets; the greater part of the cluster being interior to Mercury's orbit.

The distances between the different members of our planetary system, vast as they may seem, sink into insignificance when compared with the intervals which separate us from the so-called fixed stars. Alpha Centauri, the nearest of those twinkling luminaries, is 7000 times more distant than Neptune from the sun. Even light itself, which moves 185,000 miles in a second, is more than three years in traversing the mighty interval.

CHAPTER II.
COMETS.

The term comet—which signifies literally a hairy star—may be applied to all bodies that revolve about the sun in very eccentric orbits. The sudden appearance, vast dimensions, and extraordinary aspect of these celestial wanderers, together with their rapid and continually varying motions, have never failed to excite the attention and wonder of all observers. Nor is it surprising that in former times, when the nature of their orbits was wholly unknown, they should have been looked upon as omens of impending evil, or messengers of an angry Deity. Even now, although modern science has reduced their motions to the domain of law, determined approximately their orbits, and assigned in a number of instances their periods, the interest awakened by their appearance is in some respects still unabated.

The special points of dissimilarity between planets and comets are the following:—The former are dense, and, so far as we know, solid bodies; the latter are many thousand times rarer than the earth's atmosphere. The planets all move from west to east; many comets revolve in the opposite direction. The planetary orbits are but slightly inclined to the plane of the ecliptic; those of comets may have any inclination whatever. The planets are observed in all parts of their orbits; comets, only in those parts nearest the sun.

The larger comets are attended by a tail, or train of varying dimensions, extending generally in a direction opposite to that of the sun. The more condensed part, from which the tail proceeds, is called the nucleus; and the nebulous envelope immediately surrounding the nucleus is sometimes termed the coma. These different parts are seen in Fig. 2, which represents the great comet of 1811.

Zeno, Democritus, and other Greek philosophers held that comets were produced by the collection of several stars into clusters. Aristotle taught that they were formed by exhalations, which, rising from the earth's surface, ignited in the upper regions of the atmosphere. This hypothesis, through the great influence of its author, was generally received for almost two thousand years. Juster views, however, were entertained by the celebrated Seneca, who maintained that comets ought to be ranked among the permanent works of nature, and that their disappearance was not an extinction, but simply a passing beyond the reach of our vision. The observations of Tycho Brahe first established the fact that comets move through the planetary spaces far beyond the limits of our atmosphere. The illustrious Dane, however, supposed them