NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Halley's lunar knowledge.
From: George Huxtable
Date: 2007 Nov 27, 09:46 -0000
From: George Huxtable
Date: 2007 Nov 27, 09:46 -0000
I am still having some difficulty in understanding what Mike Daly is telling us.. I had pointed to predictions of the Moon's position, and particularly to the "Connaissance du Temps", available in Halley's time, and asked- | > Are those what Daly appears now to dismiss as "raw data"? And he has replied- | Of course they are raw data. Halley did not go into his friendly, | neighbourhood navigator's supply shop and buy a printed copy of the | lunar information telling him when a particular star would be occulted | _in_the_future_ by the moon. He had data that specified the position | information on the moon - in the past. That's raw data. He has to use | it along with a model of lunar motion to produce the information he needs. Well, that's exactly what he could do, back in 1698. He could buy a copy of Connaissance du Temps, for 1699, at Stationers' Hall, in London, which would provide him with the position of the Moon in the sky, in ecliptic latitude and longitude. I've little doubt that he did. I don't have a copy available to tell me whether it would supply occultations or not (they are rather local phenomena, which would have applied to Paris, not to mid-Atlantic, in any case). But Halley required close appulses, not occultations, as I have explained before. He was perfectly capable of deducing those from predictions of the Moon coordinates and his own detailed knowledge of star positions around the ecliptic. He had been doing that, routinely, for years. But Mike Daly seems to be insisting that only information on past positions of the Moon was available in Halley's time, and not predictions into the future (I have tried to discover some alternative meaning, without success, in his words quoted above, in case I'm accused once again of "misquoting or convoluting" his meaning). If that is what he is claiming, it's nonsense. These, in "Connaissance" and elsewhere, were predictions, not history. It's perfectly true, as I've said before, that those lunar predictions in 1699 were not up to the precision of Mayer's, in Maskelyne's 1767 almanac. They wouldn't have qualified for the longitude prize. There's little doubt that Halley, from his own observations, could do better. But Daly is telling us (isn't he?) that no such predictions existed. If, as it seems, he is describing those predictions in "Connaissance" as "raw data", that's simply abuse of language. Astronomer's real "raw data" of Moon observations had been collected for many years, sifted and assessed, fitted to the best model they could think of, and then that model was run to produce future predictions for coming years. At least, that's how we would describe their process in modern terms. "Raw data"? Halley's proposal was to bypass much of that process by examining data from one Saros cycle previous, if that detailed data had been available. It had some strengths, and some weaknesses, as I have discussed. It was superseded by better mathematicians than was Halley; Clairaut and then Mayer, who had Newton's understanding of the dynamics to build on. George. contact George Huxtable at george@huxtable.u-net.com or at +44 1865 820222 (from UK, 01865 820222) or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK. --~--~---------~--~----~------------~-------~--~----~ To post to this group, send email to NavList@fer3.com To , send email to NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---