Page:EB1911 - Volume 21.djvu/748

 one which absorbs the same fraction of the thermal radiation falling upon it whatever the wave length of this radiation—exposed to the sun’s radiation in void space tends to assume a definite temperature, called the normal temperature, the degree of which depends upon the distance of the body from the sun. This is a result of Kirchhoff’s laws of radiation.

2. An atmosphere surrounding such a body, if at rest, will tend to assume a state of thermal equilibrium, in which the temperature will be the same at all heights.

3. If the atmosphere is kept in constant motion by an interchange between its higher and lower portions, the tendency is towards adiabatic equilibrium, in which the temperature diminishes at constant rate with the height, until it may approach the absolute zero. The rate of diminution depends upon the intensity of gravity and the physical constants of the gases composing the atmosphere.

4. In the actual case of a planet surrounded by an atmosphere and exposed to the sun’s radiation, the actual rate of diminution of temperature with height above the surface of the planet lies between the extreme limits just defined, the rate varying widely with the conditions The general tendency will be towards a condition in which the temperature at the base of the atmosphere is higher than the normal, while in the upper regions it is lower. The temperature of the surface of the planet on which the atmosphere rests is determined partly by the sun’s radiation and partly by the temperature of the air. What we should generally expect in the absence of any selective absorption by the air is that the temperature of the lower air would be higher than that of the material surface on which it rests. But this condition might be reversed by the effect of such absorption in either the air or the material of the planet.

It would follow from these laws that the temperature of the superior planets diminishes rapidly with distance from the sun, and must therefore be far below that of the earth, unless they are surrounded by atmospheres of such height and density as to be practically opaque to the rays of heat, or unless they have no solid crust.

The resemblance of the spectra of Mars, Jupiter and Saturn to that of the sun leads to the conclusion that the atmospheres of these planets are transparent down to the reflecting surface of the body. The temperature of these surfaces must therefore be determined by Kirchhoff’s law, unless they resemble the sun in being entirely liquid or gaseous, or in having only solid nuclei surrounded by liquid matter in a condition of continual movement. Something of this sort has been suspected in the case of Jupiter, which has several points of resemblance to the sun. The planets Uranus and Neptune which, but for their atmospheres, would approximate to the absolute zero in temperature, may be prevented from doing so by the dense atmosphere which the spectroscope shows around them.

A very elaborate investigation of the probable mean temperatures of the surfaces of the several planets has been made by J. H. Poynting, ''Phil. Trans.'' (vol. 202, 1904).

Tables of Planetary Elements and Constants.

Table I. gives the elements determining the motions of each major planet, and Table II. certain numbers pertaining to its physical condition. For explanation of terms used see. The elements are given for the epoch 1900, Jan. 0, Greenwich mean time, except the mean longitudes, which are for 1910, Jan 0.

In interpreting or using the numbers it must be remembered that only the mean distances and mean daily motions can be regarded as well determined and invariable quantities. The other elements are subject to a secular variation, and all vary more or less from the action of the planets. In Table II. the reciprocal of the mass is given, the mass of the sun being unity. Some of these and other quantities are extremely uncertain. This is especially the case with the mass of Mercury, which the astronomical tables put at 1/6,000,000 that of the sun, while G. W. Hill has computed from an estimate of the probable density of the planet that it is probably less than 1/11,000,000. In the table we assume the round number 1/10,000,000. The volumes are derived from micrometric measures of the diameters, which are more or less uncertain. From these and the mass follows the density of each planet. From this again is derived the intensity of gravity at the surface; this is also frequently uncertain. Finally the normal temperature is that which a black or neutrally coloured body would assume when every part of it is equally exposed to the sun’s rays by a rapid revolution. As has already been intimated, the actual temperature may also depend upon the interior heat of the planet, which is an unknown quantity.

PLANETS, MINOR. The minor planets, commonly known as asteroids or planetoids, form a remarkable group of small planetary bodies, of which all the known members but three move between the orbits of Mars and Jupiter. Until recently they were all supposed to be contained within the region just I mentioned; but the discovery of one, which at perihelion comes far within the orbit of Mars, and of two others, which at aphelion pass outside the orbit of Jupiter, shows that no well-defined limit can be set to the zone containing them. Before the existence of this group was known, the apparent vacancy in the region occupied by it, as indicated by the arrangement of the planets according to Bode's law, had excited remark and led to the belief that a planet would eventually be found there. Towards the