AuoDST 1, 1893.] 



KNOWLEDGE 



149 



THE GREAT LUNAR CRATER TYCHO. 



By A. C. Eanyard. 



THE late Prebendary Webb used to speak of Tycho 

 as the metropolitan crater of the moon. Though 

 by no means the largest of the luuar craters, it is 

 one of the most striking features of the lunar 

 landscape, especially when the moon is near to 

 the full, and the shadows of the mountains have all dis- 

 appeared. The crater of Tycho is then seen as a con- 

 spicuously white spot, from which radiate in all directions 





many of the moon's craters are evidently intimately con- 

 nected. Many able men have doubted whether there is 

 any true analogy between terrestrial volcanoes and the 

 gigantic lunar ring mountains and circular depressions 

 which we ordinarily speak of as craters. The ring of 

 Tycho is 54 miles in diameter, and the great crater Clavius, 

 which lies to the south of it, is more than 140 miles in 

 diameter ; but Clavius is by no means the largest of the 

 lunar craters. If the lunar Apennines and the other 

 mountains forming a broken ring round the Mare Imbrium 

 are the remnants of a crater, it must have had a diameter 

 of over 600 miles, while the largest terrestrial craters are 



— , . ^ not more than 15 or 10 miles 



1 in diameter." Vesuvius and 

 ( the Monte Sumna would 

 appear as insignificant little 

 hills if they were dropped 

 into the centre of the crater of 



Clavius, from a drawing by Mr. Cr. K. Grilbert. 



a great number of whitish rays that extend over more than 

 a third of the visible hemisphere of the moon, indicating 

 that the crater has been the centre of a colossal disturbance 

 which seems to have shattered the lunar crust in all 

 directions. We have, as far as I am aware, no evidence 

 in the terrestrial geologic record that a corresponding 

 cataclasm has ever similarly shattered the earth's crust ; 



Arzficlu'I 

 Alpoti-agius 



M|lh0U3U3 



Cla^•ius, from a ]ilioto- 

 graph by MM. Henry. 



Tycho, whose ring wall towers 

 to a height of 17,000 feet 

 above the plain it encloses. 



Eobert Hooke compared, 

 the lunar craters to the 

 cup-shaped pits formed on the surface of boiling mud by 

 escaping vapour, and the idea has been a fascinating one 

 to many minds since his day, though it needs but little 

 consideration to recognize that bubbles or blisters formed. 

 in a plastic material on a scale corresponding with that 

 of the lunar craters would rapidly sink down and be 

 obliterated. 



Ptolemaeua 



Thebit 



3iilliiil(liH 



but our terrestrial volcanoes are puny things compared 

 with the giant craters of our smaller companion planet. 



As might be expected, the strange phenomena presented 

 in so unparalleled a degree by Tycho have been a fruitful 

 stimulus to speculation as to the origin of the lunar 

 craters and the radiating systems of rays with which 



Tbe Mare iSubium, 



Mr. S. E. Peal has ingeniously advocated a theory which 

 seems to me almost equally untenable. He assumes that 

 the lunar surface consists entirely of ice, and that the 



* According to Mr. G-. K. Gilbert, in a paper published in tlie 

 Snlletin of the Philosophical Sociefi/ of Washitigtov^ vol. xii., p. 247, 

 (1) the old crater containing Lake Bombon, Isle of Luzon, i.^ 



