IS TEE MOON DEAD? 



2S3 



ders than take place in the case of our own 

 earth. Thus the great length of the lunar day, 

 and the moon's waterless condition and rare 

 atmosphere, must help to cause a comparatively 

 rapid crumbling of the moon's surface. During 

 the long and intensely hot lunar day the rock- 

 substance of the moon's surface must expand 

 considerably, for it is raised to a degree of heat 

 exceeding that of boiling water. During the long 

 lunar night the surface is exposed to a degree of 

 refrigeration far exceeding that of the bitterest 

 winter in the arctic regions, and must contract 

 correspondingly. This alternate expansion and 

 contraction must gradually crumble away all the 

 loftiest and steepest portions of the moon's sur- 

 face, and will doubtless, in the long-run — that is, 

 some few hundreds of millions of years hence — 

 destroy all the most marked irregularities of the 

 moon's surface. 



The cases of change which have been recog- 

 nized by telescopists who have carefully studied 

 the moon's surface may all, without exception, 

 be referred to this process of gradual but steady 

 disintegration. The most remarkable case hith- 

 erto known, for example, the disappearance of 

 the lunar crater Linne, is far better explained in 

 this way than as the result of volcanic outburst. 

 This case has recently been described as fol- 

 lows, by the present writer : In the lunar Sea of 

 Serenity there was once a deep crater, nearly 

 seven miles across, a very distinct and obvious 

 feature, even with the small telescope (less than 

 four inches in aperture) used by Beer and Miid- 

 ler in forming their celebrated chart. But, ten 

 years ago, the astronomer Schmidt, a selenogra- 

 pher of selenographers (who has in fact given the 

 best energies of his life to moon-gazing), found 

 this crater missing. When he announced the 

 fact to the scientific world, other astronomers, 

 armed with very powerful instruments, looked 

 for the crater which had been so clearly seen 

 with Madler's small telescope ; but though they 

 found a crater, it was nothing like the crater 

 described by Madler. The present crater is 

 scarcely two miles in diameter, and only just 

 visible with powerful telescopes ; all around it 

 there is a shallow depression, occupying a region 

 about as large as the whole crater had been be- 

 fore. It seems impossible to doubt that a great 

 change has taken place here, and the question 

 arises whether the change has been produced by 

 volcanic activity or otherwise. Sir John Her- 

 schel pronounced somewhat confidently in favor 

 of the former hypothesis. " The most plausible 

 conjecture," said he, "as to the cause of this dis- 

 appearance, seems to be the filling up of the 



crater from beneath, by an effusion of viscous 

 lava, which, overflowing the rim on all sides, 

 may have so flowed down the outer slope as to 

 efface its ruggeuness, and convert it into a grad- 

 ual declivity casting no stray shadows." "But 

 how tremendous the volcanic energy," we note in 

 the passage referred to, "required to till with 

 lava a crater nearly seven miles in diameter, 

 and more than half a mile deep ! The volcanic 

 hypothesis seems on this account utterly incredi- 

 ble, for if such energy resided in the moon's 

 interior we should find her whole surface con- 

 tinually changing. Far more probable seems the 

 idea that the wall of this crater has simply fallen 

 in, scattering its fragments over what had once 

 been the floor of the crater. The forces at work 

 on the moon are quite competent to throw down 

 steep crater-walls like those which seem formerly 

 to have girt about this deep cavity." 1 



That the kind of vitality evidenced by such 

 changes as these still exists in the moon's frame, 

 is not merely probable but certain. Other 

 changes, however, which were once supposed to 

 have been observed, must be dismissed as having 

 had no real existence. The effects of various 

 kinds of illusion have to be taken into account 

 in considering such phenomena. Thus the theory 

 that a process of monthly change, due perhaps 

 to vegetation, affects the floor of the large lunar 

 crater Plato (called by ITevelius the greater 

 Black Lake), is now rejected, because the sup- 

 posed change has been shown to be a mere effect 

 of contrast. The apparent change is of this 

 nature: As the sun first begins to rise above 

 the floor of the crater — or, in other words, as the 

 light of the filling moon gradually flows over the 

 crater — the floor appears bright, getting brighter 

 and brighter as the sun rises higher and higher, 

 up to a certain point. But afterward the floor 

 darkens, becoming darkest toward lunar mid- 

 day. Lastly, as the lunar afternoon progresses, 

 the floor of Plato gets gradually lighter again. 

 The mid-day darkening was attributed to some 

 process of vegetation or else to chemical changes. 

 It has no real existence, however, but is due 

 simply to the effect of contrast with the great 

 brightness of the crater-wall all around, which is 

 formed of some very white substance, and looks 

 peculiarly bright and lustrous at the time of lunar 

 mid-day, so that contrasted with it the floor looks 

 peculiarly dark. On the other hand, during the 

 morning and evening hours, the black shadow of 

 the crater-wall is thrown across the floor, which 

 by contrast looks brighter tl an it really is This 

 explanation has indeed oeen denied very con 

 1 The present writer, in the Spectator for J une 24, 1ST6. 



