234 PHILOSOPHICAI, TRANSACTIONS. [aNNO 1685. 



Abstract of a Letter from Mr. J. Flamsteed, Math. Reg. and F. R. S. giving an 

 Account of the Eclipses of Jupiter's Satellites, &c. Anno 1 68(j. N° 1 77, p. 1 2 1 5. 



You have here the eclipses of Jupiter's satellites for the next year )686, de- 

 duced from the same numbers by which I calculated those of the years l684 

 and l685. I shall not need to repeat here again, how to find within what 

 limits on our earth any of them, not visible with us, will be observable ; but 

 shall only observe that all my observations of them since made, have fully veri- 

 fied my assertions concerning them, and that I have not found my numbers at 

 any time since to err above one minute in the eclipses of the first or innermost 

 satellites: so that if a way were found of managing a tube of 8, 10, or 12 feet 

 long at sea, and observing the time as exactly as we can at land, observations 

 of these eclipses would certainly give us the difference of longitude, as exactly 

 as the latitudes are pretended to be observed by our more knowing seamen, and 

 certainly much better than the observations of lunar eclipses. How the differ- 

 ences of longitude are gained by them, you will see by the following examples, 

 the latter of which I have purposely produced to show the good agreement of 

 my numbers with the heavens. 



Anno l680, Oct. 23, O. S. Signor Joseph Ponthia and Marco Antonio Cellio, 

 with a telescope of 25 palms long, observed the total immersion of the first sa- 

 tellite into Jupiter's shadow at Rome, at lOh. 7m. 53s. p. m. which in our ob- 

 servatory here I noted at gh. 15m. 41s. the difference is the difference of our 

 meridians = 52m. 12s. or 13° S'. — Again, Jan. 28, l685, Signor Francis Blan- 

 chini, having received my catalogue of the eclipses of the satellites for that 

 year, observed the total immersion of the first at Rome at lib. IQ-l-m. which 

 I saw not here, but my numbers give it lOh. 27-i-m. therefore the difference of 

 meridians is 52-i-m. agreeing with the former observation, and showing the error 

 of the tables to be insensible. 



Tycho therefore judged not amiss, when he placed Uraniburg and Rome 

 under the same meridian ; for by several observations of satellite eclipses, it is 

 evident, that the difference of meridians between Uraniburg and our observatory 

 is 51m. 10s, of time, so that Rome lies only Im. of time, or ^ of a degree to 

 the east of Uraniburg. 



Some ingenious persons having often wished that such tables of the satellites' 

 motions were published, as might serve to find their appearances at any given 

 time, though I think it not fit, because they are capable of much correction 

 and great improvement, to publish mine at present ; yet for the sake of the dili- 

 gent observer, I have contrived a small instrument, whereby with the sole help 

 of the usual catalogue, and the table of the parallaxes of Jupiter's orbit, their 



