A remarkable feature of the solar system is the clear separation of the eight major planets into two groups, equal in numbers, each com prising four planets with distinctive character istics. The inner four, of which our earth is one, are dense and highly compressed solid bodies. The four planets of the outer group are many times more massive than the inner ones, the least massive of all, Uranus, being seven or eight times as weighty as the four inner ones put together. As compared with the inner group, the planets of the outer group are also distinguished by being larger in a yet greater proportion than they are massive. Any one of the inner group would be a small object alongside the smallest of the outer group. A similarity of constitution also seems to mark the outer planets; it is highly probable that all are surrounded by dense atmospheres — in fact, that each consists of a comparatively small nucleus surrounded by a gaseous envelope with clouds floating in it as they do in our atmos phere.
Possible Unknown excessive motion of the perihelion of Mercury, as well as smaller, but well-defined motions in the node of Venus and the perihelion of Mars, none of which could be accounted for by the disturb ances caused by the attractions of the known bodies of the solar system, long ago led to the suspicion that there might be one, or even a ring of, undiscovered planets revolving about the sun within the orbit of Mercury. Should such planets exist they ought, from time to time, to pass between us and the sun, so as to be visible on the disc of the latter. Several ob servers, studying the sun, have supposed that they saw such objects. But there is no doubt that they were mistaken. For 20 years past the sun has been photographed almost every day, as well as constantly scanned by the telescope, in Europe and in America. No photograph has ever shown, and no really experienced observer has ever seen anything of the sort. Another way of detecting such bodies would be by scan ning the neighborhood of the sun during total eclipses. This has been done at almost every such eclipse, during the last 30 years. More over, in recent times, photography has been called to the aid of the search, especially by the Lick Observatory, in California. In one of the attempts, stars were photographed down to nearly the eighth magnitude; but nothing was ever found except known stars. We may, there fore, regard the non-existence of any visible planet between Mercury and the sun as well proved.
In addition to this, the most interesting dis covery was made by Seeliger in 1906 that the outstanding discrepancies in the motions of the planets can be fully accounted for by the dis turbing pull of the zodiacal light, provided that we ascribe to this only an almost inconceivable tenuity. It is sufficient to assume that the densest portion has but 2.52 by 10-." t;tn-s density of the sun in order that its mans may be great enough to produce the disturbances observed. Moreover, it is found that on this assumption no new disturbances are introduced into the perturbations of any of the planetary elements of a size great enough to be evident to observation.
The question whether there may be a planet beyond Neptune is not so easily settled. The probable existence of such a body has been in ferred on three grounds: (1) Outstanding dis crepancies in the computed motion of Uranus; (2) the existence of certain comets, the aphelia of whose orbits lie outside the orbit of Neptune; and (3) the inference, based on the planetesimal theory of the development of the solar system, that the orbit of the outermost planet will not be a nearly circular one. Of these, the first
furnishes the most reliable evidence and it alone can serve as a basis for predicting the planet's position and so leading to its discovery, if indeed, it has an existence. In the course of a century, accurate observations of Uranus will have accumulated throughout the entire circuit of its orbit, and by this time also the motion of Neptune will have been sufficient to indicate the position of the planet. As yet, however, an accurate indication is not possible, and careful search for the body has met with no success. It is inferred that it is from the 12th to the 13th magnitude, with an apparent diameter of perhaps 1"; as it is so far distant from the sun, its motion among the stars is certainly very slow, and all of the circumstances contribute to render its discovery extremely difficult.
The planets are all so distant from us that, to the naked eye, they appear like stars. For the most part the five nearest to us would be dassed among the brightest of the stars, Venus and Jupiter being generally brighter than any fixed star. They are also distinguished from the stars by their apparent motions, which may be seen by watching them from night to night. Their apparent situations relatively to the sun are called aspects. The latter are different ac cording to whether the orbit of the planet is inside or outside that of the earth; that is, whether it is an inferior or superior one. A glance at the arrangement of the system will show that the inferior planets, Mercury and Venus, can never be seen in the opposite direc tion from the sun, but only seem to swing back and forth on each side of the sun as they per form their revolutions around this luminary. Their apparent distance east or west from the sun at any time is called their elongation. At the greatest elongation, Venus is about 45 de grees from the sun, and Mercury at different distances, generally ranging between 20 and 30 degrees, according to its position in its very eccentric orbit. But the superior planets seem to course relatively to the sun all the way round the sky in performing their revolution. When in the opposite direction from the sun they are said to be in opposition. This is the most favorable time for observing them, because they are then nearest to the earth, and their visible hemisphere is fully illuminated. They then rise about the time of sunset, and cross the meridian about midnight.
The inferior planets nass by the sun during their apparent swinging from one side to the other. When they pass on our side of the sun they are said to be in inferior conjunction; when beyond the sun, in superior conjunction. These conjunctions take place at fairly regular intervals. The best time to see an inferior planet is in the evening, some time before in ferior conjunction; or in the early morning twilight, some time after it.
The plate shows the most important rela tions existing among the elements of the planets. The elements of the planets are given in the following tables: to the several European congresses. His most notable works are 'International Copyright in the Congress of the United States> (1886), and 'The Copyright Law in Force in the United States in 1900' (1900).