The Longitude of Lisbon, and the Fort of New York, from Wansted and London, Determin'd by Eclipses of the First Satellite of Jupiter. By the Reverend Mr. James Bradley, M. A. Astron. Prof. Savil. R. S. S.

I. The Longitude of Lisbon, and the Fort of New York, from Wansted and London, determin'd by Eclipses of the First Satellite of Jupiter. By the Reverend Mr. James Bradley, M. A. Astron. Prof. Savil. R. S. S. Some curious Astronomical Observations having lately been communicated to this Society from Lisbon, among which were several Eclipses of the first Satellite of Jupiter*; I was willing to examine whether I had made any at Wansted which tallied with them, that by comparing such together, the true Difference of Longitude between those Places might be found. But looking over my Observations of the first Satellite, made last Year and the beginning of this, I meet only with Two Emerisions that were observed the same Night both at Lisbon and Wansted. There are others, indeed, made within a few Days of each other, which may likewise be made use of to determine the Difference of Longitude; but not with the same degree of Certainty, by reason of the irregular Motion of the Satellite; which I presume, chiefly arises from the Gravity of the other Satellites towards it. For altho' the Effect of the Influence that the Satellites have on each other, is most remarkable in the Second, whose Motion will sometimes be accelerated or retarded thereby, as much as amounts to 30 or 40 Minutes in time, in the space of about seven Months, or in half the Period in which the three innermost Satellites return, to have nearly the same Position with respect to themselves, and the Shadow of Jupiter; yet the first * Vide Page 90. seems also liable to Inequalities that cannot well be accounted for, but from some such Cause as is before-mentioned, the effect of which will not easily be reduced to any Rule, but from a long and exact Series of Observations. And till some better and more certain Rule can be found out, we may suppose, that the Effect produced by this Cause, is, during small Intervals, proportionable to the time. On this Supposition I have compared some Observations with others not made the same Nights; and the result is nearly the same as in those which were observed at the same time in both Places, as will appear by the following Particulars. The Immersion of the First Satellite was observed at Wansted with Mr. Hadley's Reflecting Telescope on August 4, N.S. 1725, about $45''$ after the time of the Immersion, as calculated from my Tables. By another Observation made August 29, N.S. the true Immersion preceded the Calculation from the same Tables $1' 10''$. So that in 25 Days the Satellite's Motion was accelerated as much as answer'd to $1' 55''$ in time. Supposing therefore the Acceleration to have been in the same proportion between July 28, and August 4, N.S. then the true Immersion July 28, N.S. would have happen'd at Wansted about $1' 15''$ after the time by the Tables, which make the Immersion at $12h. 48' 45''$ App. Time. The true Immersion therefore was at Wansted July 28, N.S. $12h. 50' 0''$ App. Time; and at Lisbon 'twas observed at $12h. 12' 26''$ App. Time, the Difference being $37' 34''$. September 28, N.S. the First Satellite was seen emerging in the Reflector at Wansted $3' 50''$ sooner than the Tables make the Emersion; and by the Mean of two more Observations made at the same Place, and with the same Telescope, on the 14th and 16th of October, N.S. the true Emersion preceded the Calculation lation $4'30''$. We may therefore from hence conclude, that on Sept. 21, N.S. the true Emersion at Wansted preceded the Calculation by the Tables about $3'35''$, and that the true Emersion there was at 12h. $1'15''$ Apr. 1; but this Emersion was observed at Lisbon at 11h. $24'55''$, the Difference being $36'20''$. The Observations at Wansted being made with Mr. Hadley's Reflecting Telescope (by which one may see the First Satellite near $\frac{1}{4}$ of a Minute sooner when 'tis Emerging, than in a Refracting Telescope of 15 Feet, and the contrary when 'tis Immerging) there ought to be some Allowance made on account of different Telescopes made use of at Lisbon and Wansted, by deducting $10$ or $15''$ from the Difference of Time collected from the Immersions, and adding as much to the Difference deduced from the Emersions. Such Correction being made, the Difference of Meridians by the Immersion observed July 28, will be $37'20''$, and by the Emersion Sept. 21, $36'35''$. The Emersion observed at Lisbon, Decemb. 8, N.S. at 8h. $32'40''$ Apparent Time, was likewise seen at Wansted in a 15 Foot Tube at 9h. $10'5''$ Apparent Time, the Air being a little hazy, which may probably make the Difference $37'25''$ a little too great. The Emersion seen at Lisbon Jan. 16, 1726, N. S. at 6h. $51'10''$, which seems accompanied with Circumstances that argue its Exactness, was likewise very well observed at Wansted in a 15 Foot Tube at 7h. $28'22''$ Apparent Time, the Difference being $37'12''$. These are the only Observations among those which were last communicated, that I could compare with any degree of Certainty with my own: But I find others printed in the Philosoph. Transact. No. 385, which were likewise made by the same curious Persons, who observed an Emersion of the First Satellite at Lisbon September 2, 1724, N.S. at 9h. 36' 57''. This was seen also at Wansted in the Reflector at 10h. 13' 28'' Apparent Time. Hence, allowing for the different Telescopes, the Difference of Meridians is 36' 45''. This Emersion at Wansted preceded the Calculation by the Tables 4' 40''. And another Emersion observed with the same Telescope on Sept. 18, N.S. preceded the Calculation 5' 10''. We may therefore suppose, that on Sept. 9, N.S. the true Emersion at Wansted preceded the computed about 4' 52''. The Emersion that Day by the Tables was at 12h. 15' 34'' App. Time; therefore the true Emersion at Wansted was at 12h. 10' 42''. At Lisbon twas observed at 11h. 34' 26''. So that allowing for the Difference of Telescopes, the Difference of Meridians by this Observation is 36' 30''. The Mean of all these Differences is about 36' 58'', from which subtracting 28'' for the Difference of Meridians between London and Wansted, the remainder will be the Difference of Meridians between London and Lisbon, viz. $36\frac{1}{2} = 9^\circ 7'\frac{1}{2}$, Lisbon being so much to the Westward of London. This Difference of Longitude is about 5' $\frac{1}{2}$ greater than what is determined in the forementioned Transaction: But as the Gentlemen to whom we are indebted for these Observations, have given us hopes that they will continue to make and communicate more, we need not doubt but their exact Care and Diligence will soon enable us to judge yet more nicely of the true Situation of those Cities with respect to each other. The same Transaction containing some Observations of Eclipses of the same Satellite made in the Fort of New York, communicated by his Excellency William Burnet, Esq; Governor of New York, I shall take this Oppor- Opportunity of determining the Longitude of that Fort more exactly than it can be supposed to be there done, by the bare Comparison of the Observations with the Tables; having two Observations made at Wansted, which tally with two made at New York, on Aug. 25, and Sept. 10. By the Observation made Aug. 25, 1723, O.S. which is esteemed the most distinct and best, the Satellite Emerged at 9h. 35' 14'' by the Clock, which went about 1' 4'' too fast for the Apparent Time at the Emersion, as appears by the Altitudes of the Sun's Limb taken the Morning before and after the Observation; so that the Emersion at New York was at 9h. 34' Apparent Time; that is, 9h. 32' 20'' Mean Time. August 27, 8h. 57' 40'' Mean Time, the Satellite was seen emerging at Wansted in the Reflector; and Sept 12, 7h. 17' 15'' M.T. 'twas seen emerging again in the same Telescope: So that in 15d. 22h. 19' 35'' there were 9 Emersions; and the Interval between each was about 1 d. 18h. 28' 50''. This subtracted from the Time of the Emersion observ'd at Wansted August 27, will give the true Emersion at Wansted on August 25, 14h. 28' 50'' M.T. that is, 4h. 56' 30'' later than it was observed at New York. September 10, 8h. 0' 10'' by the Clock, another Emersion was observed at New York. From the Altitudes of the Sun's Limb taken the Morning before, I compute the Error of the Clock at the time of the Emersion to be 1' 10'', and that the Emersion happen'd at 7h. 59' App. T. that is, 7h. 51' 52'' Mean Time at New York. But subtracting the aforementioned Interval of 1 d. 18 h. 28' 50'' from the Time of the Emersion observed at Wansted September 12, 7h. 17' 15'' M.T. we shall have the time of the true Emersion at Wansted on Sept. 10, at 12h. 48' 25'' M.T. which which is $4\text{h}.56'\ 33''$ later than 'twas observed at New York. The Difference therefore of Meridians between Wansted and New York, allowing about $15''$ for the Difference of Telescopes, is about $4\text{h}.56'\ 45''$, and between London and New York, $4\text{h}.56'\ \frac{1}{4}$. So that the true Longitude of New York from London is $74^\circ\ 4'$ West. II: Observationes Astronomicae habita Ulyssipone, Anno 1725, & sub init. 1726, à Rev. P. Johanne Baptista Carbone, Soc. Jes. Communicante Isaaco Sequeyra Samuda, M.D., R.S., S. Coll. Med., Lond. Lic. R Arò cælum hoc anno nubibus expers contemplari licuit. Tunc verò vel maximè turbatum sensimus, cum aliquid spectatu dignum propius immineret; ut meritò crederem, omnes nobis hoc anno observationes Astronomicas suisse interdictas. Perpaucas tandem habere datum est circa consuetas intimi Jovis Satellitis Eclipses, quas hic subneceto, Lunari Eclipse, die 21 Octobris, Martisque transitu per Lunam, die 18 Septembris, omnino inobservatis. Mens. Dies. Jul. 28. Immergi visus est in umbram Jovis veram, 12 12 26 telescopio consueto Josephi Campani palmonum Rom. 30. Ceperat verò debilitari lumen, 12 11 35 Emerit