Ofi Memoir relative to the Annular Eclipse of the Sun, 



before oliserved be set to mean time on the day of the eclipse *„ 

 1 must again repeat liowever, that the neglect of this precaution 

 should not deter the observer from noting dov,'n the duration of 

 the several phases above mentioned, and particularly the times 

 at which the unnulus is formed and dissolved: which may be 

 afterwards compared with more exact observations, and lead to 

 a correction of the true times t- 



It is presumed that the observer will also, from time to time, 

 during the progress of the eclipse, observe and note down the 

 distance and inclination of the cusps in the usual manner :t^. It 

 may likewise be proper to remark that it will be of considerable 

 importance to ascertain, at the time of the middle of the echpse, 

 the magnitude of the annulus on the north and on the south side 

 of the moon, in order to determine how far distant, at that time, 

 the centre of tlie moon is from the centre of the sun §. If at the 

 same moment the observer can determine the diameter of the sun 

 and moon, it will add considerably to tiie importance of the ob- 

 servation ; and tend to determine a much disputed point in prac- 

 tical astronomy I). These observations however should be made 

 with a good telescope furnished with an accurate micrometer: 

 and, in making a report thereof, the observer should describe the 

 kind of telescope made use of, as well as the method employed iu 

 determining the magnitude of the annulus, &c. For the sake of 



* Iti the evening of the same day on whieh this eclipse takes place there 

 will be an eclipfc nf the first satellite of Jupiter: the immersion will take 

 place at Greenwich at S'\ ."J-l . 'M'' mean time. Those persons therefore who 

 are furnished with sufficiently powerful telescopes, may (if the weather prove 

 favourable) have an opportunity of ascertaining the correctness of their clocks 

 or watches. 



•f- M. l)e L'islc states, that if we observe the situation of the cusps, or only 

 their distance, at the time of the middle of the eclipse (when the eclipse is 

 not anmilar, but nearly so), it will serve to determine the apparent route of 

 the penumbra and its limits, as exactly a.s if we had observed the duration 

 of the annular eclipse. 



X There are two modes of observing the phases of an eclipse of the sui; : 

 the one, by looking directly at tlie sun, with a telescope furnished with a 

 micrometer: the other, by I'eceiving the image of the sun, through a tele- 

 scope, on a screen, in a dark chamber, or camera obscura. Each has its ad- 

 vantages, and may be practised according to circimistances. See Lalande's 

 Astronomie, vol. ii. p. 659. M. De L'islc indeed says that " we may deter- 

 mine, with sufficient exactness, the situation of the cusps, without making 

 use either of the dark chamber or the micrometer, by ob.servin* the moment 

 of the passage of the cusps and of the limbs of the sun, by means of simple 

 wires placed iu the focus of the telescope, in any situation whatever; and 

 leaving the telescope in a fixed position, during the time that the sun em- 

 ploys to traverse the field of it." 



§ M. Do L'isle doubts whether this part of the observation can be made 

 with sufficient accuracy in a dark chamber; on account of the indistinctness 

 of the image of the moon. ]| Sec pages 97 and 08. 



greater 



