Various attempts have been made to determine the amount of light reflected by the Moon, and also the question whether it yields any measurable amount of heat. As regards the light of the full Moon compared with that of the Sun, the estimates range from 1/300000 to 1/800000, a discrepancy not perhaps greater than might be expected under the circumstances of the case.
With respect to the heat possessed by, or radiated from the Moon’s surface, the conclusions of those who have attempted to deal with the matter are less consistent. As regards the surface of the Moon itself Sir John Herschel was of opinion that it is heated at least to the temperature of boiling water, but that owing to the radiant heat having to pass through our atmosphere, which acts as an obstacle, it is no wonder that it should be difficult for us to become conscious of its existence. In 1846 Melloni, by concentrating the rays of the Moon with a lens 3 feet in diameter, thought he detected a sensible elevation of temperature; and in 1856 C. P. Smyth at Teneriffe, but with inferior instrumental appliances, arrived at the same conclusion. Though Professor Tyndall in 1861 obtained a contrary result, yet the most recent experiments by the younger Earl of Rosse, Professor Langley, and others, all tend to show that the Moon does really radiate a certain infinitesimally small amount of heat. Perhaps, however, it will be best to give Langley’s ideas as to this in his own words:—“While we have found abundant evidence of heat from the Moon, every method we have tried, or that has been tried by others, for determining the character of this heat appears to us inconclusive; and without questioning that the Moon radiates heat earthward from its soil, we have not yet found any experimental means of discriminating with such certainty between this and reflected heat that it is not open to misinterpretation.” It is obvious from the foregoing that we on the Earth need not concern ourselves very much about lunar heat; and I will only add that F. W. Very, by an ingenious endeavour to localise the Moon’s radiant heat, has been able, he thinks, to establish the fact that on the part of the Moon to which the Sun is setting, what he calls the heat-gradient (using a phrase suggested by terrestrial meteorology) appears to be steeper than on that part to which the Sun is rising. Generally, Very’s observations accord fairly with Lord Rosse’s.
The Moon revolves round the Earth in 27 d. 7 h. 43 m. 11 s. at a mean distance of 237,300 miles, in an orbit which is somewhat, but not very, eccentric. Its angular diameter at mean distance is 31′ 5″, or, say, just over ½°. The real diameter may be called 2160 miles.
A few words will probably be expected by the reader on the subject of lunar influences on the weather, and generally; this being a matter highly attractive to the popular mind. The truth appears to lie, as usual, between two extremes of thought. The Moon, of course, is the main cause of the tides of the Ocean, and it is not entirely inconceivable that tidal changes imparted to vast masses of water may be either synchronous with, or may in some way engender, analogous movements in the Earth’s atmosphere; though no distinct proofs of this, as a determinate fact, can be brought forward.
There is no doubt whatever that at or near the time of full Moon, evening clouds tend to disperse as the Moon comes up to the meridian, and that by the time the Moon has reached the meridian a sky previously overcast will have become almost or quite clear. Sir John Herschel has alluded to this by speaking of a “tendency to disappearance of clouds under a full Moon”; and he considers this “fully entitled to rank as a meteorological fact.” He goes on, not unnaturally, to suggest the obvious thought that such dissipation of terrestrial clouds is due to the circumstance that, assuming heat really comes by radiation from the Moon (and we have seen on a previous page the probability of this) such radiant heat will be more potential if it falls on the Earth perpendicularly, as from a Meridian Moon, than if it comes to us at any one locality from a Moon low down in the observer’s horizon, and therefore has to pass through the denser strata of the Earth’s atmosphere and suffer material enfeeblement accordingly. I am aware that Mr. Ellis, late of the Royal Observatory, Greenwich, has sought to show by a seemingly powerful array of statistics that the idea now under consideration is unfounded, but I consider that we have here only one more illustration of the familiar statement that you can prove anything you like by statistics. I am firmly convinced, as the result of more than 30 years’ observation, that terrestrial clouds do disperse under the circumstances stated. Sir J. Herschel added that his statement proceeded from his own observation “made quite independently of any knowledge of such a tendency having been observed by others. Humboldt, however, in his Personal Narrative, speaks of it as well known to the pilots and seamen of Spanish America.” Sir John Herschel further remarked:—“Arago has shown from a comparison of rain, registered as having fallen during a long period, that a slight preponderance in respect of quantity falls near the ‘new’ Moon over that which falls near the ‘full.’ This would be a natural and necessary consequence of a preponderance of a cloudless sky about the ‘full,’ and forms, therefore, part and parcel of the same meteorological fact.”
Bernadin has asserted it to be a fact that many thunderstorms occur about the period of “new” or “full” Moon. But what I want most to warn the reader against is that popular idea (wonderfully wide-spread it must be admitted) that at the epochs of what are called, most illogically, the Moon’s “changes,” changes of weather may certainly be expected. There is absolutely no foundation whatever for this, and still more void of authority (if such a phrase is admissible) is a table of imaginary weather to be expected at changes of the Moon, often met with in books published half a century ago, and still occasionally reprinted in third-rate almanacs, and designated “Dr. Herschel’s Weather Table.” This precious production is not only devoid of authenticity as regards its name, but may easily be seen to be fraudulent in its reputed facts any month in the year.
It would be beyond both my present available space and the legitimate objects of this work to attempt even an outline of the influences over things terrestrial ascribed to, or associated, rightly or wrongly, with the Moon, and of which the word “lunatic” perhaps affords the most familiar exponent.
CHAPTER VII.
MARS.
Mars, though considerably smaller than the Earth, is commonly regarded as the planet which, taken all in all, bears most resemblance to the Earth, though only one-fourth its size. Under circumstances which have already been briefly alluded to in Chapter I., Mars exhibits from time to time a slight phase, but nothing approaching in amount the phases presented by the two inferior planets, Mercury and Venus. When in opposition to the Sun, that is to say when on the meridian at midnight, it has a truly circular disc; but between opposition and its two positions of quadrature it is gibbous. At the minimum phase, which is at each quadrature, E. or W. as the case may be, the planet resembles the Moon 3 days from its “full.” These phases are an indication that Mars shines by the reflected light of the Sun. It is a remarkable tribute to Galileo’s powers of observation that with his trumpery telescope, only a few inches long, he should have been able to suspect the existence of a Martial phase. Writing to a friend in 1610 he says:—“I dare not affirm that I can observe the phases of Mars; however, if I mistake not, I think I already perceive that he is not perfectly round.”
