314 



Observation of the Lunar Spot Aristarchus. [April 



Kater has supposed that a volcano in the act of eruption was the cause; 

 whilst others, again, presuming on the Moon being destitute of an atmo- 

 sphere, have doubted if, in the absence of oxygen, combustion could 

 take place. It has otherwise been accounted for by supposing this part 

 of the Moon to consist of a somewhat polished surface, and that the 

 varieties of brilliancy were due to changes in the powers of transmission 

 possessed by our atmosphere. However true this latter supposition may 

 be for the variable climate of England, where the observations of thia 

 spot have chiefly been made, it cannot, with any degree of propriety, be 

 applied to the fine months of December, January or February in this 

 country, where an uniformity of temperature and pressure, and conse- 

 quently tranquil sky almost mechanical exists. 



It is somewhat remarkable that, during the last ten years, notwith>- 

 standing that observers and well constructed telescopes have very much 

 increased in number, still there has not a single notice appeared of this 

 phenomenon having been observed by any one. From my own experi- 

 ence at Madras during the last six years I can state, that on one or two 

 occasions only have I ever even fancied that I could distinguish any 

 thing particular in the appearance of this part of the Moon, although 

 my observations for this purpose have been unremitted. 



To those who entertain the idea of a volcano in the Moon, the pre- 

 sent notice would seem to favour their supposition exactly. The 

 volcano has been quiet during the past ten years and has now burst 

 forth again ! A little reflection, however, upon the nature of the motion 

 of the Moon's orbit will show us, that such a supposition may be 

 dispensed with. In the case of a highly polished plateau of land being 

 the cause of the phenomenon in question, its reappearance might be 

 deferred during an interval corresponding to half the period of the 

 regression of the nodes (9| years)— but, conjointly with this cause, 

 taking account of the other various disturbances to which the Moon 

 and her orbit are subjected, it is easy to perceive that the occasional 

 appearance and disappearance of Aristarchus may be fully accounted 

 for. 



