EARTHSHINE

Taranaki Daily News, 22 November 1930, Page 1 (Supplement)

EARTHSHINE

THE MOON’S MOONLIGHT

(By Rev. B. Dudley, F.R.A.-S.)

Few people can have failed to notice that when the moon is a thin crescent, during the first days of her monthly career, the rest of the lunar globe is visible with- more or lees clearness, being lit up with a pale light. This is what the French call lumiere cendree. We speak of it as “the old moon in the

young moon’s arm.” It is, as a matter of fact, caused by the earth itself. The light of the sun which falls upon the earth is shed forth into space; and

the moon, catching some of this, is faintly -illuminated thereby. An. observer located in the moon would at certain times witness “full earth,” should, he Ipok towards us, just as we periodically see “full moon.” The ancients found themselves unable to explain this secondary light, or earthshine on the moon, as we know it to be. They made many guesses, some thinking it might perhaps be inherent in the moon itself. They even suggested that the satellite was transparent, shedding forth a kind of phosphorescence. Kepler wrote that Moestlin, of whom he professed himself a disciple, was the first to attribute this ashy light on our satellite to its correct cause. This was in 1596. There is sufficient evidence, however, that it was 'rightly explained as far back as the early part of the sixteenth century by Leonardo da Vinci, ti e celebrated artist, who died in 1513.

Earthshine on the moon is almost entirely non-existent ...when she is in. quadrature; that is to say, whe she is 90 degrees from the sun, at which time she is in her first or third quarter or “half-moon” phase. There are two reasons for this. The earth then sends four times less light to the moon, and thus her ability to reflect back to us the light of the earth is considerably reduced. And, furthermore, the phase of the moon, becoming four or five times larger (or approaching more and more the state known as “full”), our chance of seeing it is practically nil. The dull light under consideration appears a little brighter after the last quarter, which is of course in the morning hours, because, for one thing, the eastern part of - the earth reflects more solar light than the western, where the waters of the sea absorb the rays; and because the eastern-region of the moon is itself a little darker, a fact-which is due to the dark patches which exist there. It has been-suggested that a further contributory cause of the greater ■ brightness of earthshine on the moon during her last quarter as compared with her earlier phases, lies in the fact-that in! the morning our vision is slightly more sensitive, the pupil of * the eye being rather more extended after the darkness of night than after the bright daylight. This light may therefore be spoken of as the reflection of a reflection; and it is. determined to some extent- by the varying state of the earth’s huminosity. For instance, in winter, at which season the earth’s surface is more abundantly covered with snow, earthlight on the moon is perceptibly brighter than in summer. Prior to the actual discovery of Australia, astronomers suspected the existence of “a vast-continent” in that part of the terrestrial globe because this secondary light was considerably brighter than it ought to be if only ocean water occupied those regions of ‘ the earth where that continent w r as subsequently found. For it was well known the reflective power of water surfaces Is much less than that of land areas; Theoretically, too, • the light of the earth on tlie moon ought- to be much ’stronger than the light of the moon on the earth, judging from the comparative size of ’ the two bodies. The earth- being so very much larger than the moon, the light reflected from the former ought to be about fourteentiines as powerful as that reflected from the latter. But because-of , the terrestrial seas and snowfields earthshine is estimated to be nearly twenty times as strong as moonshine.

It is hot difficult , to detect -in the ashy light ’ which faintly illuminates the dark (or night) part of the moon a slightly ruddy tinge. This is due to th© fact that the sunlight which in th© final issue comes to us has passed no less than three times through the earth’s atmosphere. It has entered the air on fir;st proceeding direct from -the great luminary. It has been sent out through the atmospheric envelope, to the moon, and i again from her surface {the parts mt dhutly lit up by the sun’s rays) it passes through the air to our eyes. Thus the atmosphere absorbs much of,the light. It absorbs, in fact, light of other colours in greater proportions than it absorbs the ruddy hues.

Although our. satellite appears so brilliant in the otherwise dark sky it is not in reality a good reflector. Its seeming luminosity is the result of contrast with the dark background whereon it shines. As seen in full daylight its gleam is but faint in comparison with that of white clouds. Careful tests have shown, as a matter of fact, that the moon’s light is only about onesixth of that which would be reflected, in sunlight, by a perfectly white disc of the same size. The brightness of the lunar surface is approximately that of sandstone rocks under the illumination of the sun’s rays. “I have frequently,” wrote Sir John Herschel, “compared the moon setting behind the grey perpendicular facade of the Table Mountain, illuminated by the sun just risen in the opposite quarter of the horizon, when it has been scarcely distinguishable in brightness from the rock in contact with it.” The light of the full moon bears the proportion to that of the sun of roughly one to six hundred thousand. Proctor used to say that if the moon were covered with black velvet it would still appear white at night. We cannot see the earth as it' looks from a distance in outside space, but estimates as to its likely appearance have been made by measuring the intensity of earthshine on the lunar surface, and by studying the brightness of solar radiation at different altitudes above the surface of the earth, small balloons being released to varying heights up to 15 miles for this purpose. When the moon is a slender crescent the illuminated portion forming the crescent appears to be of a larger diameter than the ash-coloured earth-lit bulk of the moon. It is as if the crescent were not a good fit on the lunar globe. This effect is due to irridiation, a white object always appearing larger to the eye than a dark one. If our satellite is a moon to the earth, the earth is just as truly a moon to our satellite, but enormously larger and brighter, and because of its teeming million forms of life far more wonderful.