Page:EB1911 - Volume 21.djvu/745

Rh The phases of a superior planet are less strongly marked, because the lines from the planet to the earth and sun never increase to a right angle. The result is that although the apparent dlsk of Mars is sometimes gibbous in a very marked degree, it is always more than half illuminated. In the case of the other superior planets, from Jupiter outward, no variation in phase is perceptible even to telescopic vision. The entire disk always seems fully illuminated.

The most favourable time for viewing an inferior planet is near that of greatest brilliancy. As it recedes further from the earth, although a continually increasing proportion of its disk is illuminated by the sun, this advantage is neutralized by the diminution in its size produced by the increasing distance. When a superior planet is in opposition to the sun it rises at sunset and is visible all night. This is also the time when nearest the earth, and therefore when the circumstances are most favourable for observation.

The greater the distance of a planet from the sun the less is the speed with which it moves in its orbit. The orbit being larger, the time of its revolution is greater in a yet larger degree. An approximation to the general laws of speed in different planets is that the linear speed is inversely proportional to the square root of the mean distance. From this follows Kepler's third law, that the squares of the times of revolution are proportional to the cubes of the mean distances.

It was once supposed that the planets were surrounded by comparatively dense atmospheres. The question whether such the case, and, if so, what is the physical constitution of the atmospheres, is a difficult one, on which little light is thrown except by the spectroscope. If any of these bodies is surrounded by a transparent atmosphere like that of the earth, the light which reaches us from it will have passed twice through this atmosphere. If the latter were materially different in its constitution from that of the earth, that fact would be made known by the spectrum showing absorption lines or bands different from those found in the solar spectrum as we observe it. If, however, the planetary atmosphere had the same composition as ours we should see only an intensification of the atmospheric lines, which might be imperceptible were the atmosphere rare.

Actual observation has thus far shown no well marked deviation in the spectra of any of the inner group of planets, Mercury, Venus and Mars, from the solar spectrum as we see it. It follows that any atmospheres these planets may have must, if transparent, be rare. The evidence in the cases of Venus and Mars is given in the articles on these planets. Taking the outer group of planets, it is found that the spectrum of Jupiter shows one or more very faint shaded bands not found in that of the sun. In Saturn these bands become more marked, and in Uranus and Neptune many more are seen. The spectra in question have been observed both optically and photographically by several observers, among whom Huggins, Vogel and Lowell have been most successful. It may be sa1d, in a general way, that seven or eight well marked dark bands, as well as some fainter ones are observable in the spectra of the two outer planets. The general conclusion from this is that these planets are surrounded by deep and dense atmospheres, semi-transparent, of a constitution which is probably very different from that of the earth's atmosphere. But it has not, up to the present time, been found practicable to determine the chemical constitution of these appendages, except that hydrogen seems to be an important constituent. (See Plate.)

Intimately associated with this subject is the question of the conditions necessary to the permanence of an atmosphere round a planet. Dr Johnstone Stoney investigated these conditions, taking as the basis of his work the kinetic theory of gases (Trans. Roy. Dubl. Soc. vi. 305). On this theory every molecule of a gaseous spheres mass is completely disconnected from every other and is in rapid motion, its velocity, which may amount to one or more thousand feet per second, depending on the temperature and on the atomic weight of the gas. At any temperature the velocities of individual molecules may now and then increase without any well-defined limit. If at the boundary of an atmosphere the velocity should exceed a certain limit fixed bythemass and force of gravity of the planet, molecules might fly away through space as independent bodies. The absence of hydrogen from the atmosphere of the earth, and of an atmosphere from the moon, may be thus explained. If the fundamental hypotheses of Dr Stoney's investigations are correct and complete, it would follow that neither the satellites and minor planets of the solar system nor Mercury can have any atmosphere. If the separate molecules thus flying away moved according to the laws which would govern an ordinary body, they would, after leaving their respective planets, move round the sun in independent orbits. The possibility is thus suggested that the matter producing the zodiacal light may be an agglomeration of gaseous molecules moving round the sun; but several questions respecting the intimate constitution of matter will have to be settled before any definite conclusions on this point can be reached. It is not to be assumed that a molecule would move through the ether without resistance as the minutest known body does, and there is probably a radical difference between the minutest particle of meteoric matter and the molecule of a gas. The relations of identity or difference between such finely-divided matter as smoke and atmospheric haze and a true gas have yet to be fully established, and until this is done a definite and satisfactory theory of the subject does not seem possible.

Since the radiation of heat by a planet is, with our present instruments, scarcely capable of detection and measurement, the temperature of these bodies can be estimated only from general physical laws. The laws governing the radiation of heat have been so developed during recent years that it is now possible to state at least the general principle on which a conclusion as to the temperature of a planet may be reached. At the same time our knowledge of the conditions which prevail on other planets is so limited, especially as regards their atmospheres, that only more or less probable estimates of the temperature of their surfaces can even now be made. Summarily stated, some of the physical principles are these:—

A neutrally coloured body—understanding by that term