feature of the surface comprises the craters, which are scattered
everywhere, and generally surrounded by an approximately
circular elevated ring. Yet another remarkable feature comprises
bright streaks, branching out in various directions and
through long distances from a few central points, especially
that known as Tycho.
The height of the lunar mountains is a subject of interest. It cannot be stated with the same definiteness that we can assign heights to our terrestrial mountains, because there is no fixed sea-level on the moon to which elevations can be referred. The only determination that can be made on the moon is that of the height above some neighbouring hollow, crater or plain. The most detailed measures of this sort were made by Beer and Mädler, who give a great number of such heights. These generally range between 500 and 3000 toises, or 3000 and 20,000 English feet. The highest which they measured was Newton, 3727 toises, or 24,000 ft.
The general trend of lunar investigation has been against the view that there is any resemblance between the surfaces of the moon and of the earth, except in the general features already mentioned. No evidence has yet been found that the moon has either water or air. The former, if it existed at all, could be found only in the more depressed portions; and even here it would evaporate under the influence of the sun’s rays, forming a vapour which, if it existed in considerable quantity, would in some way make itself known to our scrutiny. The most delicate indication of an atmosphere would be through the refraction of the light of a star when seen coincident with the limb of the moon. Not the slightest change in the direction of such a star when in this position has ever been detected, and it is certain that if any occurs it can be but a minute fraction of a second of arc. As an, atmosphere equal to ours in density would produce a deviation of an important fraction of a degree, it may be said that the moon can have no atmosphere exceeding in density the 15000 that of the earth.
Devoid of air and atmosphere, the causes of meteorological phenomena on the earth are non-existent on the moon. The only active cause of such changes is the varying temperature produced by the presence or absence of the sun’s rays. The range of temperature must be vastly wider than on the earth, owing to the absence of an atmosphere to make it equable. Elaborate observations of the heat coming from the moon at its various phases were made and discussed in 1871–1872 by Lord Rosse. Among his results was that during the progressive phases from before the first quarter till the full moon the heat received increases in a much greater proportion than the light, from which it followed that the former was composed mainly of heat radiated from the moon itself in consequence of the temperature which it assumed under the sun’s rays. So far as could be determined, 86% of the heat radiated was by the moon itself, and 14% reflected solar heat. But it seems probable that this disproportion may be somewhat too great. Rosse’s determinations, like those of his predecessors, were made with the thermopile. After S. P. Langley devised his bolometer, which was a much more sensitive instrument than the thermopile, he, in conjunction with F. W. Very, applied it to determine the moon’s radiation at the Allegheny observatory. His results for the ratio of the total radiation of the full moon to that of the sun ranged from 1 : 70,000 to 1 : 110,000, which were in substantial agreement with those of Rosse, who found 1 : 82,000. When Langley published his work the law of radiation as a function of the temperature was not yet established. He therefore wrongly concluded that the highest temperature reached by the moon approximated to the freezing-point of water. Stefan’s law of radiation, on the other hand, shows that the temperature must have been about the boiling-point in order that the observed amount of heat might be radiated. This is in fair agreement with the computed temperature due to the sun’s radiation upon a perpendicular absorbing surface when no temperature is lost through conduction to the interior. The agreement thus brought about between the results deduced from the law of radiation and the most delicate observations of the quantity of heat radiated is of great interest, as showing that the theory of cosmical temperature now rests upon a sound basis. There is, however, still room for improved determinations of the moon’s heat by the use of the bolometer in its latest form.
Possibility of Changes on the Moon.—No evidence of life on the moon has ever been brought out by the minutest telescopic scrutiny, nor does life seem possible in the absence of air and water. Some bright spots are visible by the earth-light when the moon is a thin crescent, which were supposed by Herschel to be volcanoes in eruption. But these are now known to be nothing more than spots of unusual whiteness, and if any active volcano exists it is yet to be discovered. Still, the question whether everything on the moon’s surface is absolutely unchangeable is as yet an open one, with the general trend of opinion toward the affirmative, so far as any actual proof from observation is concerned. The spot which has most frequently exhibited changes in appearance is near the centre of the visible disk, marked on Beer and Mädler’s map as Linne. This has been found to present an aspect quite different from that depicted on the map, and one which varies at different times. But the question still remains open whether these variations may not be due wholly to the different phases of illumination by the sunlight as the latter strikes the region from various directions.
Intensity of Moonlight.—An interesting and important quantity is the ratio of moonlight to sunlight. This has been measured for the full moon by various investigators, but the results are not as accordant as could be desired. The most reliable determinations were made by G. P. Bond at Harvard and F. Zöllner at Leipzig, in 1860 and 1864. The mean result of these two determinations is the ratio 1 : 570,000. We may therefore say that the intensity of sunlight is somewhat more than half a million times that of full moonlight. A remarkable feature of the reflecting power of the moon, which was made known by Zöllner’s observations, is that the proportion of light reflected by a region on the moon is much greater when the light falls perpendicularly, which is the case near the time of full moon, and rapidly becomes less as the light is more oblique. This result was traced by Zöllner to the general irregularity of the lunar surface, and the inference was drawn that the average slope of the lunar elevation amounts to 47°.
Motion of the Moon.—The orbit of the moon around the earth, though not a fixed curve of any class, is elliptical in form, and may be represented by an ellipse which is constantly changing its form and position, and has the earth in one of its foci. The eccentricity of the ellipse is in the general average about 0·055, whence the moon is commonly more than 110 further from the earth at apogee than at perigee. The line of apsides is in continual motion, generally direct, and performs a revolution in about 12 years. The inclination to the ecliptic is a little more than 5°, and the line of nodes performs a revolution in the retrograde direction in 18·6 years. The parallax of the moon is determined by observation from two widely separated points; the most accurate measures are those made at Greenwich and at the Cape of Good Hope. The distance of the moon can also be computed from the law of gravity, the problem being to determine the distance at which a body having the moon’s mass would revolve around the earth in the observed period. The measures of parallax agree perfectly with the computed distance in showing a mean parallax of 57′ 2·8″, and a mean distance of 238,800 miles. The period of revolution, or the lunar month, depends upon the point to which the revolution is referred. Any one of five such directions may be chosen, that of the sun, the fixed stars, the equinox, the perigee, or the node. The terms synodical, sidereal, tropical, anomalistic, nodical, are applied respectively to these months, of which the lengths are as follow:—
Length. |
Deviation from sidereal month. | |
| Synodic month | 29·53059 days. | +2·20893 days. |
| Sidereal month | 27·32166 ,, | 0·00000 ,, |
| Tropical month | 27·32156 ,, | −0·00010 ,, |
| Anomalistic month | 27·55460 ,, | +0·23294 ,, |
| Nodical month | 27·21222 ,, | −0·10944 ,, |
