It

earth 95 times. From this it follows that its density is only about that of the earth, so that it is lighter than water. revolves on its axis in about 10 h. 15 m., the inclination of the axis to the plane of its orbit being 28°. Like Jupiter it shows a belted surface, but these atmospheric belts are not so clearly marked nor so variable as those of the giant planet. A bright zone extends some distance on each side of the equator, as shown in the engraving. The spectrum of Saturn shows that the reflected sunlight has suffered some absorption in the planet's atmosphere, for there are some unexplained dark bands at the red end of the spectrum similar to those observed in the spectrum of

Jupiter. But Saturn is unique among the planets, its cloudbelted globe having a retinue of eight satellites, and being surrounded by three thin concentric rings made up of discrete bodies. Its physical condition and constitution closely resemble those of Jupiter, though the body of the planet is probably somewhat cooler.

108. Rings of Saturn.-The three rings of Saturn are three flat thin circular discs pierced in the middle and having a total breadth of about 38,000 miles. The two outer ones are bright, while the third or 'crape ring, discovered in 1850, is but feebly luminous, and its inner edge is almost 10,000 miles from the planet's disc. This inner dusky ring can be seen through at times, and must therefore be thinner or composed of more

scattered materials than the outer ones. Between the two outermost rings there is also a space of about 1,600 miles, known as Cassini's division, while in the exterior ring a division, known as Encke's division, has been noted. The rings are doubtless circular, but we see them foreshortened, so that they appear more or less oval, or else, when we are nearly in the same plane, they appear like a straight line. Thus, while the rings remain parallel to the equator of the planet, which is inclined about 28° to the plane of the ecliptic and 27° to the plane of the planet's orbit, they present to us different phases, twice appearing edgewise and twice at a maximum opening in each revolution. The apparent breadth is at present very little, and in 1891 they will appear as a line, after which they will gradually open out, so as to present a good view in 1899. At times the distinction between the dark ring and the inner edge of the neighbouring bright ring is much more marked than at others, while divisions are occasionally seen in the dark ring for a short period only. Other unexplained changes have also been noticed in the rings. It requires a good instrument to see the rings when presented edgewise to the earth, and then a slight nebulosity' is sometimes observed on each side, as shown in fig. 107.

6

FIG. 107.-Rings turned edgewise, December 26, 1861. (Wray.)

The thickness of the rings is proved to be small, probably not more than 100 miles, by their appearance as a line every fifteen years when their plane is directed towards the earth. Mathematicians have shown that the rings cannot consist of continuous sheets of either solid or liquid matter, but that they are formed of a multitude of separate independent bodies revolving in circular orbits. They are, in fact, a multitude of small satellites, like swarms of meteorites, and there is nothing improbable in supposing continual collisions among such bodies, with the production of vaporous surroundings. The spectroscope has not, however, as yet, given any decisive evidence of glowing vapours in the ring system of Saturn.

109. Satellites of Saturn. -Eight satellites are known to revolve around Saturn, all of them being exterior to the ring system. Their names, distances from Saturn, and times of revolution 1ound their primary, are set forth in the annexed table. All revolve in the same plane as the plane of the rings, except Iapetus, which is inclined to this plane about 10°.

Iapetus, the most distant, shows a remarkable variation in brightness, the western side being more than twice as bright as the eastern. This is explained by supposing that, like our moon, it continually keeps the same face towards the planet. Titan, the largest, has a diameter of about 32,000 miles. The orbit of Hyperion shows distinctly the disturbing effects produced by Titan, the largest of the satellites.

110. Urănus.—Urănus was discovered by Sir William Herschel in 1781, and its planetary character was established shortly after by Lexell. Its distance from the sun is more than 19 times that of the earth; its diameter is more than 4 times that of the earth, with a volume therefore more than 64 times that of the earth; its density is less than that of the earth; and its ellipticity amounts to about, that of the earth being only Seen in a good telescope it shows a sea-green disc, with faint bands or belts, from which a rotation-period of 10 to 12 hours has been deduced. This rotation, however, appears to take place in a direction almost perpendicular to the plane of the planet's orbit. Its spectrum shows dark bands in the green, and the line is clearly seen. Hence it has been thought to have a dense atmosphere. Uranus has four satellites, Ariel, Umbriel, Titania, and Oberon, revolving in orbits very nearly circular, and apparently in one plane; but the peculiarity of these orbits is remarkable, for the plane of the orbits of the satellites of Uranus is inclined 82° to the plane of the ecliptic, and in that plane the satellites revolve from east to west, or in a direction opposite to that of all the bodies in the solar system, except also the single known satellite of Neptune.

111. Neptune.-Neptune, the most distant planet, was discovered at Berlin in 1846. The history of this discovery furnishes a striking example of the marvellous results achieved by mathematical astronomy. The

motions of Uranus had for some time made evident the existence of a more distant planet, as the perturbing influence of such a body would alone account for the effects observed. Two distinguished mathematicians, Leverrier in France and Adams in England, therefore in 1844-5 entered upon a laborious investigation and calculated the approximate place of the unknown planet. While Professor Challis was employing the results of the calculation of Adams in searching for the missing planet, Dr. Galle, aided by a more recent star-map of that part of the heavens, found it close to the place indicated by Leverrier. Neptune's mean distance from the sun is nearly thirty times that of the earth; its density not quite that of the earth; and its mass 17 times that of the earth. At so great a distance no distinct markings on its surface can be discerned and hence the time of its axial rotation is unknown. The light and heat received from the sun, diminishing with the square of the distance, can only be part of what the earth receives, while the sun itself, seen from Neptune, would only appear as large as Venus when nearest to us. Neptune has one satellite, probably as large as our moon, and this satellite moves at a distance of 223,000 miles in a period of 5 d. 21 h. 3 m. Like the satellites of Uranus, it moves backwards, i.e. in a reverse direction to that of the planetary motions. The spectrum of Neptune closely resembles that of Uranus.

112. The Zodiacal Light.-This is a faint broad beam of light extending from the sun both ways along the ecliptic or zodiac. After sunset in spring, it may often be seen about latitude 40° N. as a faint cone of light proceeding from the place where the sun has set, and reaching 70° or 80° eastward. In the tropics it has been seen to stretch right across the sky. Its cause is not certainly known. Its spectrum is a faint continuous one. The general idea is that it consists of a ring of meteoric or nebulous matter surrounding the sun in the plane of the ecliptic, and that its light is only reflected sunlight. (For illustration see p. 312).

CHAPTER VII.

THE MOON-ITS DIMENSIONS-ORBIT-ROTATIOŃ – PHASES-PHYSICAL CONDITION-ECLIPSES.

113. Dimensions of the Moon.-The moon is a satellite of the earth and accompanies it in the journey round the sun. By methods to be afterwards explained, it has been found to be a body having a mean distance from the earth of 238,840 miles. Its mean apparent diameter is 31' 7", from which a real diameter of 2,163 miles is obtained. This is equal to 0.273 of the earth's diameter, and as the surfaces of globes are proportional to the squares of the diameters, and their volumes proportional to the cubes, we find that the surface of the moon is a little more than that of the earth, and its volume or bulk almost exactly the earth's volume. Its mass or weight, however, is

only about

that of the earth, so that its density will only be

0613 of the earth's density, that is 3'4 compared with water. On its surface the force of gravity would only be one-sixth the force of gravity on the earth's surface, so that the same force could hurl a body six times higher there than on the earth.

114. Revolution of the Moon.-Observation of the situations of the moon on successive nights shows that it daily moves eastward about 13° among the stars, and consequently rises about fifty minutes later each succeeding night. It takes about 27 days to make a complete circuit of the stars; in other words, the moon revolves round the earth in this time. In reality both the moon and the earth revolve round their common centre of gravity in this period; but so greatly does the earth's mass exceed that of the moon, that this point is about 1,200 miles within the surface of the earth on the line joining their centres. For most purposes we can regard the moon as revolving round the earth in an elliptic orbit. When at its greatest distance the moon is said to be in apogee, and when at its least distance in perigee, the difference between these two distances being over 26,000 miles. On tracing the moon's path among the stars, its orbit is found to be inclined to the ecliptic at a mean angle of 5°8′, and the points at which it cuts the ecliptic are called the moon's nodes.

115. Different kinds of month.-The period of the moon's revolution is the month; but there will be different sorts of months according to the standard by which we judge the revolution. The sidereal month is the time occupied by the moon in passing from one star to the same star again. This is equal to 27.32166 days. The moon's average daily motion among the stars will therefore be equal to 360° divided by this number of days, which is 13° 11' nearly. The synodic month is the time occupied by the moon in passing from the sun round to the sun again, i.e. the interval from new moon to new moon again. It is longer than the sidereal month, because the sun is also advancing, though at a slower rate, eastward among the stars, and the moon requires a longer time to overtake it. It varies slightly in length on account of the varying speed of the earth in its orbit (or the apparent varying speed of the sun in its path along the ecliptic). The mean value of the synodic month is 29.53 days nearly, and this is the length of the ordinary lunar month. For purposes of civil life calendar months are employed. These consist of a certain number of whole days28, 30, or 31, as the case may be. Of such months there are twelve in the year, but in this time there are thirteen lunar months.

116. Phases of the Moon.-The different forms or appearances presented by the visible disc of the moon during a