# 1911 Encyclopædia Britannica/Planet

See also Planet on Wikipedia; and our 1911 Encyclopædia Britannica disclaimer.
​PLANET (Gr. πλανήτης, a wanderer), in the ancient astronomy, one of seven heavenly bodies characterized by being in motion relative to the fixed stars, which last appeared immovable upon the celestial sphere. As thus defined the planets were the sun, the moon, Mercury, Venus, Mars, Jupiter and Saturn. In modern astronomy since Copernicus, the term is applied to any opaque body moving around the sun. Taken in its widest sense it applies to the satellites which are sometimes termed secondary planets. Each of these moves around a planet larger than itself, which it accompanies in its revolution round the sun. A planet not revolving round another is termed a primary planet.
The primary planets are classified as major and minor. The former are eight in number and, with the sun, form the principal members of the solar system, under which head their arrangement is described. The earth on which we live is the third in the order of the major planets from the sun. With respect to the positions of their orbits relative to the earth, the other planets are distinguished as inferior and superior. The former, only two in number, comprise Mercury and Venus, which revolve between the earth and the sun. The superior planets are those whose orbits are outside that of the earth. The synodic revolution of an inferior planet is the time in which it performs a revolution relative to the line joining the earth and the sun. This is greater than its actual time of revolution. The phases or appearances presented by such a planet depend upon its configuration with respect to the earth and sun, and therefore go through their complete periods in a synodic revolution. At superior conjunction the illuminated hemisphere of the planet is presented to the earth so that it presents the form of a full moon. As it moves towards inferior conjunction, the lines from the planet to the sun and to the earth, or the angle sun-earth as seen from the planet, on which the phase depends, continually make a greater angle. At the time of greatest elongation this angle is 90°, and the planet appears one half illuminated, like the moon at first or last quarter. Then, as it approaches inferior conjunction, the visible portion of the disk assumes the crescent form, and while the circle bounding the disk continually increases owing to the approach of the planet to the earth, the crescent becomes thinner and thinner until, near inferior conjunction, the planet is no longer visible. After conjunction the phases occur in the reverse order. The brilliancy of the planet, as measured by the total amount of light we receive from it, goes through a similar cycle of change. The point of greatest brilliancy is between inferior conjunction and greatest elongation. In the case of Venus this phase occurs about three or four weeks before and after inferior conjunction.
* Fig. 3
* Fig. 4
* Fig. 5
In the figures given above are shown the relative orbits of the planets, the orbits of Mars, the Earth, Venus and Mercury (fig. 1) being drawn to a scale twenty times that of the outer ones—Neptune, Uranus, Saturn, Jupiter (fig. 2). The positions of the planets at ten-day intervals; their actual position on the 1st of January 1910 at noon, of their nodes and nearer apses, and the points when they are farthest distant north and south of the ecliptic, are also given. The relative sizes of the planets are also given, orientated in their true axial position with regard to the ecliptic. The nearer planets (and also the Moon) are separately compared (fig. 3); and then shown (on a smaller scale) in comparison w1th the more distant ones (fig. 4). Finally scale diagrams of the distances of the orbits of the satellite systems of Saturn, Uranus, Jupiter and Neptune are given (fig. 5).
​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 disk 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 is 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 Spectra and Atmospheres of the Planets. 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 said, 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 Stability of Planetary Atmospheres. 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 by the mass 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.
* Plate I.
* Plate II.
​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 Temperature of the Planets.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 ​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. 202a, 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 Orbit. 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. (S. N.)