NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Historical Lunars : take in account 'delta-T' or ignore it ?
From: George Huxtable
Date: 2009 Dec 12, 00:04 -0000
From: George Huxtable
Date: 2009 Dec 12, 00:04 -0000
Antoine asks a lot of interesting questions about delta-T and lunar observations. I'm not sure I can answer them, but let me have a try at some aspects, to make sure that I am following his reasoning. He wrote (section 2)- "how should we best proceed if we are to reprocess / re-compute historical Lunars with to-day's computation tools? I would like to offer the following comments and will certainly be very glad to hear feed-back from the NavList community, especially if view points are different from the one I am detailing here-after. -*-*- First, we can state that, when they cleared their Lunars, the 18th, 19th and (if any) the early 20th centuries Navigators were not aware of the "necessity" of using 2 time scales. In other words - and unless they used data amended with 'observed corrective terms'(see �6 here-above) - they (unknowingly) cleared all their Lunars with the Value delta-T=0.0 seconds of time." ========================== As I see it, that was exactly the right thing to do. Mayer's predictions were based on the contemporary observations of his epoch, the length of the day (then) and the length of the month (then). They were related to the observed instantaneous position of the Sun and Moon at some moment, and predicted what their values would be subsequently, for some coming years. Until many years had passed, and the slowing Earth rotation increased the day-length, and so give rise to a significant change in the time-base, there was no delta-T to take into account. So the navigators of the 1760s didn't need to worry about any delta-T correction. Just like the way we use an almanac for 2009 today, indeed. There is no absolute value for delta-T. We only use the value we do, of 65 seconds or whatever, because Simon Newcomb worked it all out in around 1893, and chose to set delta-T to have a zero value in (I think) 1895. There's nothing magic about the actual value of delta-T; all that matters is the CHANGE in delta-T, between the epoch for which predictions are made, and the date of the observations to which those predictions are being applied, or compared. If you like, you can think of it as delta-delta-T, between those dates. If we wanted to check predictions made by Mayer, say, or Maskelyne, to determine their accuracy, by comparing with the modern JPL or Chapront-Touzet ephemeris, then we should work out what the modern ephemeris predicts for that date and time in the 18th century, and then apply the change in delta-T, between this date and that one. If we were foolish enough to check Mayer's predictions by extrapolating them to the present day, that would be doomed to fail, because those predictions were nowhere near accurate enough to take such a strain. But, in theory at least, if we did so, we would then have to allow for the change in delta-T, in the same way. Because the Moon is the object that moves fastest in the sky, with respect to the stars, its position is by far the most affected by changes in delta-T. For that reason, study of the history of such changes involves looking at the dating and timing of ancient eclipses and conjunctions, and I suppose lunar distances too, made by observers whose longitude can be known. I wonder if this is addressing the questions Antoine is asking? Perhaps he'll say. George. contact George Huxtable, at george@hux.me.uk or at +44 1865 820222 (from UK, 01865 820222) or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK. -- NavList message boards: www.fer3.com/arc Or post by email to: NavList@fer3.com To , email NavList+@fer3.com