On the Figure of the Earth. 407 



certained) if the clock or watch can be depended upon for the 

 few minutes that the star is hid behind the moon: since the 

 duration will be the same whether the clock is set to mean time, 

 or not*. 



In observing occultations, similar to those which have been 

 the subject of the preceding Memoir, the observer should be care- 

 ful not to withdraw his eye from the telescope before he is fully 

 satisfied that the star is completely hid by the body of the moon. 

 For the star passes behind so small a segment of the moon's disc, 

 that it may undergo a partial occultation by the projecting moun- 

 tains of the moon, before it is wholly hid by that body ; as ap- 

 pears from the following singular phaenomenon, noticed by 

 M. Koch at Dantzic, when observing the occultation of Alde- 

 laran on March 7, 1794. " He was looking out for the immer- 

 sion of the star, near the upper crescent of the moon. It disap- 

 peared at first: but 10" afterwards it re-appeared suddenly in 

 all its brilliancy. It was soon afterwards hid a second time. It 

 re-appeared however again : but, presently after, its immersion 

 took place for the third time at 8^ 28' 21" apparent time. The 

 observer then counted 30"; and, finding that the star did not 

 make its appearance again, quitted the telescope in order to 

 write down the observation. He came back immediately ; but 

 the star had already emerged from behind the moon. It is to be 

 regretted that he was not present at the. instant of emersion, in 

 order to complete an observation which had never been made be- 

 fore. However, it was sufficient to show that the star did not 

 pass a single second of a degree within the moon s disc. For, 

 the semidiameter of the moon being 15'. 52", and its apparent 

 motion 29" in a minute, it is found that, if the duration of the 

 occultation had been one minute, the line joining the centre of 

 the moon and the middle of the chord (traversed by the star) 

 ought to be 15'. 51"." Connaissance desTems,Anneev\, page 253. 

 The writer of that article justly adds that " this is one of the 

 very interesting but very rare cases that M. Cagnoli has proposed 

 to select for the purpose of determining the true figure of the 

 earth." 



It is not necessary to inform the practical astronomer that in 

 many occultations the star, immediately before its immersion, and 

 immediately after its emersion, is observed to change its colour f, 



and 



• In calculating the exact time of the immersion or emersion of a star, in or- 

 der to deduce any practical and very accurate results therefrom, I do not find 

 that any allowance is ever directed to be made for the time that light takes to 

 travel from the moon to the eartli ; which is about one second and a quarter. 



\MeiiKter, who observed the occultation of /lldebnrau on Sept. l!5, 1755, 

 states that just before the immersion, on the illuminated side of the moon's 

 disc, he saw the star sensibly diminish in light, change its colour and bc- 



C c4 come 



