Part 33

Earth 6 + 4 = 10 10.0 0.0

Mars 12 + 4 = 16 15.2 0.8

Minor 24 + 4 = 28 20 to 35 planets

Jupiter 48 + 4 = 52 52.0 0.0

Saturn 96 + 4 = 100 95.4 4.6

Uranus 192 + 4 = 196 191.9 4.1

Neptune 384 + 4 = 388 300.6 87.4

It will be seen, that, before the discovery of Neptune, the agreement was so close as to indicate that this was an actual law of the distances; but the discovery of this planet completely disproved its existence.

283. The Satellite of Neptune.--Neptune is accompanied by at least one moon, whose orbit is shown in Fig. 304. The orbit of this satellite is inclined about thirty degrees to the plane of the ecliptic, and the motion of the satellite is retrograde, or from east to west.

VII. COMETS AND METEORS.

I. COMETS.

General Phenomena of Comets.

284. General Appearance of a Bright Comet.--Comets bright enough to be seen with the naked eye are composed of three parts, which run into each other by insensible gradations. These are the nucleus, the coma, and the tail.

The nucleus is the bright centre of the comet, and appears to the eye as a star or planet.

The coma is a nebulous mass surrounding the nucleus on all sides. Close to the nucleus it is almost as bright as the nucleus itself; but it gradually shades off in every direction. The nucleus and coma combined appear like a star shining through a small patch of fog; and these two together form what is called the head of the comet.

The tail is a continuation of the coma, and consists of a stream of milky light, growing wider and fainter as it recedes from the head, till the eye is unable to trace it.

The general appearance of one of the smaller of the brilliant comets is shown in Fig. 305.

285. General Appearance of a Telescopic Comet.--The great majority of comets are too faint to be visible with the naked eye, and are called telescopic comets. In these comets there seems to be a development of coma at the expense of nucleus and tail. In some cases the telescope fails to reveal any nucleus at all in one of these comets; at other times the nucleus is so faint and ill-defined as to be barely distinguishable. Fig. 306 shows a telescopic comet without any nucleus at all, and another with a slight condensation at the centre. In these comets it is generally impossible to distinguish the coma from the tail, the latter being either entirely invisible, as in Fig. 306, or else only an elongation of the coma, as shown in Fig. 307. Many comets appear simply as patches of foggy light of more or less irregular form.

286. The Development of Telescopic Comets on their Approach to the Sun.--As a rule, all comets look nearly alike when they first come within the reach of the telescope. They appear at first as little foggy patches, without any tail, and often without any visible nucleus. As they approach the sun their peculiarities are rapidly developed. Fig. 308 shows such a comet as first seen, and the gradual development of its nucleus, head, and tail, as it approaches the sun.

If the comet is only a small one, the tail developed is small; but these small appendages have a great variety of form in different comets. Fig. 309 shows the singular form into which Encke's comet was developed in 1871. Figs. 310 and 311 show other peculiar developments of telescopic comets.

287. Development of Brilliant Comets on their Approach to the Sun.--Brilliant comets, as well as telescopic comets, appear nearly alike when they come into the view of the telescope; and it is only on their approach to the sun that their distinctive features are developed. Not only do these comets, when they first come into view, resemble each other, but they also bear a close resemblance to telescopic comets.

As the comet approaches the sun, bright vaporous jets, two or three in number, are emitted from the nucleus on the side of the sun and in the direction of the sun. These jets, though directed towards the sun, are soon more or less carried backward, as if repelled by the sun. Fig. 312 shows a succession of views of these jets as they were developed in the case of Halley's comet in 1835.

The jets in this case seemed to have an oscillatory motion. At 1 and 2 they seemed to be attracted towards the sun, and in 3 to be repelled by him. In 4 and 5 they seemed to be again attracted, and in 6 to be repelled, but in a reverse direction to that in 3. In 7 they appeared to be again attracted. Bessel likened this oscillation of the jets to the vibration of a magnetic needle when presented to the pole of a magnet.

In the case of larger comets these luminous jets are surrounded by one or more envelops, which are thrown off in succession as the comet approaches the sun. The formation of these envelops was a conspicuous feature of Donati's comet of 1858. A rough view of the jets and the surrounding envelops is given in Fig. 313. Fig. 314 gives a view of the envelops without the jets.

288. The Tails of Comets.--The tails of brilliant comets are rapidly formed as the comet approaches the sun, their increase in length often being at the rate of several million miles a day. These appendages seem to be formed entirely out of the matter which is emitted from the nucleus in the luminous jets which are at first directed towards the sun. The tails of comets are, however, always directed away from the sun, as shown in Fig. 315.

It will be seen that the comet, as it approaches the sun, travels head foremost; but as it leaves the sun it goes tail foremost.

The apparent length of the tail of a comet depends partly upon its real length, partly upon the distance of the comet, and partly upon the direction of the axis of the tail with reference to the line of vision. The longer the tail, the nearer the comet; and the more nearly at right angles to the line of vision is the axis of the tail, the greater is the apparent length of the tail. In the majority of cases the tails of comets measure only a few degrees; but, in the case of many comets recorded in history, the tail has extended half way across the heavens.

The tail of a comet, when seen at all, is usually several million miles in length; and in some instances the tail is long enough to reach across the orbit of the earth, or twice as far as from the earth to the sun.