An Observation of the End of the Total Lunar Eclipse on the 5th of March 1718. Observed Near the Cape of Good Hope, Serving to Determine the Longitude Thereof. With Remarks Thereon. By E. Halley, R S. Secr.

I. An Observation of the end of the Total Lunar Eclipse on the 5th of March 1718, observed near the Cape of Good Hope, serving to determine the Longitude thereof. With Remarks thereon. By E. Halley, R.S. Secr. This now better than thirty Years since I had a Dispute with some of the French Geographers about the Longitude of the Cape of Good Hope, said to have been observ'd by the Religious Missionaries sent to China in the Year 1685. By an Emersion of the first Satellite of Jupiter, they determined that Cape to be 1h. 11' or 17\(\frac{3}{4}\) grad. more Easterly than Paris, that is 20 grad. from London; which for the reasons I then gave, I concluded could not be more than 17 grad. See Phil. Transact. No 185. Very lately I have fallen upon an Observation which I believe will determine the Controversy in my favour: for I had accidentally a Journal of an Officer of the Ship Emperor put into my Hands, who in his return from India, on the fifth of March 1718, observ'd the End of a Lunar Eclipse, when the visible altitude of the Moons Centre was 13° 25'. he being then in the Latitude of 34° 23' South, and as they found afterwards, just 180 Leagues to the Eastwards of Cape Bonne Esperance. By Calculation I find that in that Latitude the Moon had that height at 7h. 17\(\frac{1}{2}\) P.M. and by comparing this Eclipse with that we observ'd with great exactness on Febr. 11°. 1682. (which agrees perfectly well with our Numbers) I conclude the middle of this to have been at London at 3h. 48' P.M. To which adding 1h. 46' for the the Semiduration (this being very certain from the observ'd Continuance of the Eclipse of 1682.) the End will be found to have been at London at $5^h\ 34'$. The Ship was therefore in a Meridian $26^\circ$ to the Eastwards of London: But she was at that time 180 Leagues to the Eastwards of the Cape, which distance in that Latitude gives eleven Degrees of Longitude; this therefore being deducted from the Longitude of the Ship, leaves just $15$ grad. or one Hour, for the difference of Meridians between London and the Cape. So that by this account the Cape is yet nearer our Meridian than I had formerly made it, and near six Degrees nearer than M. De la Hire places it in his Tables. This Eclipse was attended with all the Circumstances requisite to make the Conclusion as certain as the nature of the thing will admit of: For the Moon was nearly in Perigee, and the Eclipse almost central; for which reasons she emerged out of the Shadow as swiftly as possible: The Sea was very smooth, there having been little Wind for above 30 Hours before; and the Moon was not too high to be well observed with a Forestaff. Nor were they long at Sea before they made the Land, for in less than five Days, on the tenth of March at Noon, they had past Cape d'Agulhas the most Southerly Promontory of Africa, which then bore from them North East, about seven Leagues distant. The End of this Eclipse, though not visible here, might have been seen in Germany, both at Nurenburg, Leipzig and Berlin, but we cannot learn that it was any where observed there; however our Numbers in this Case may be securely relied on. On this occasion it may not be amiss to insert an Observation or two I procured to be made at the Cape, by Mr Alexander Brown a Scotch Gentleman, who went to reside in India on our Companies account. He carried with him a very good Brass Quadrant of above two Foot Radius, and at the Dutch Settlement at Table Bay, having rectify'd his Pendulum-Clock by correspondent Altitudes, on the 4th of August 1694, at 5h. 59' Manè, the distance of the bright Limb of the Moon from the right Shoulder of Orion was observ'd to be $25^\circ 3'$. And the next Morning Aug. 5. at 5h. 21'. 12", the same Limb was distant from Procyon $25^\circ 57'$, and at 5h. 36'. 48" from the Lucida Arctis $58^\circ 29'$. It were much to be wish't that the Moon had, either of these Mornings, been accurately observ'd at Greenwich or Paris, or at some Place in Europe, whose Longitude from them is well known. But that failing us, I had recourse to the Period of the Lunar Motions, which is perform'd in 18 Years and ten or eleven Days, after which the Errors of our Lunar Computations return very nearly the same; and I found among my own old Observations, one that tally'd well with that of the 4th of August. Viz. Anno 1676. July $23^\circ$. 13h. 11'. 35". at Oxford, I observ'd the Moon to apply to the Star in medio Collo Tauri, by Bayer markt A. The Star at that time was distant from the Southern and nearest Cusp of the Moon by the Micrometer $20'$. 32". and at $13^\circ$. 17'. 15". when it seem'd to immerge upon the bright Limb of the Moon, it was distant from the Northern Cusp $23'$. 20"; but this less certain by reason of the hazey Air. The Star at that time was in $\varphi$ $28^\circ$. 56'. with $1^\circ$. 13'. 20". North Lat. whereby I found that our Lunar Tables, founded on Sir Isaac Newton's correct Theory of her Motion, gave her place at that time only two Minutes too slow; which Error being allowed on the 4th of August 1694. the result was, that 5h. 59'. at Cape Bonne Esperance was at London $4^\circ$. 53'. whence the difference of Longitude $16\frac{1}{2}$ degrees, sufficiently near what we had before determin'd.