On the true Measure of a Lunar Cycle. 85 



1 00 solar vears, and from the corresponding quarters ascertain 

 their exact' times, it would be vastly satisfactory; but since that 

 is not to be known but from tables, we must be content to deter- 

 mine the nearest possible by the aids we are possessed of. 



The quantity of a mean lunation obtained from 1237 com- 

 plete lunations from the memorable solar eclipse April 22d, 1715 

 O. S. to its corresponding New moon in May 1815, N. S., and 

 computed by me at 29 days, 12 hoius, 44 min. 38 sec. 51 thirds, 

 in page 25 of this volume, allows 6939 days, 18 hours, 52 min., 

 10 sec, for the completion of the cycle of 235 lunations, and 

 corresponds exactly and to a minute with the middle time of that 

 eclipse and the instant of the new moon in May 1815, as I shall 

 now show. 



In 100 Julian years are 36525 days, and in 36526 are an even 

 number of weeks : the eclipse happened on a Friday at 51 min. 

 past 9 o'clock in the morning, and the corresponding New moon 

 was on Tuesday, Mav 9th, at 20 minutes past 6 o'clock in the 

 morning. 



36525 100 Julian years H. M. 

 1715 36526 Friday 9 51 morn. 



36527 Saturday 



36528 Sunday 



36529 Monday 20 29 



1815 36530 Tuesday 6 20 morn. 



.\mong the ancient historical eclipses pul)]ishod in Mr. Fer- 

 guson's Astronomy, is a total eclipse of the sun at Wittemburg, 

 on June 6th, in the year 1415, at 43 minutes past 6 o'clock. I 

 find this same eclipse mentioned in Fox's Acts and Monuments, 

 vol. i. page 792, and according to this historian the eclipse hap- 

 pened on Thursday, the seventh of June, when the sun was al- 

 most wholly eclipsed somewhat after seven of the clock : now, 

 that the seventh day of June 1415 was Thursday, is proved from 

 a document in the same history, page 824, signed, Thursday/ 

 the 28' day of June Anno 1415. I presume the date in Fer- 

 guson's Catalogue was computed and inserted from some 

 Ephemeris, but the latter the observed time when this remark- 

 able eclipse did happen. From June 1415 to June 1S15, is 

 400 vears, and from the new moon at the time of this memorable 

 eclipse to its corresponding New moon in 1815, there are 4918 

 complete lunations, or 4 x 1237 = 4948. The day of the week 

 when this eclipse happened being ascertained, 1 shall show the 

 importance ot this date in determining the precise measure of 

 4948 lunations in solar days. 



The historicul date June 7tl>, according to the old calendar, 



iinist 



