NavList:

Dave, you wrote:
"I seem to need delta T to get RA and Dec from any ephemeris program. (including; the new French INPOP08a, VSOP87/ELP2000-82B, DE405/LE405, and DE406/LE406)"

Well sure. Of course you need delta-T at some point --just as you need to the obliquity of the ecliptic at some point. You don't find the obliquity of the ecliptic in those ephemerides either, right?

Delta-T is part of the calculation of any topocentric ephemerides. The standard ephemerides are published in non-topocentric coordinates because there's greater certainty in them (by this I mean for purposes far more accurate than those made even in the most delicate calculations of navigation and nautical astronomy). The time variable that governs the motions of the planets is nearly that perfect time variable of Newtonian physics. It is the time of omniscient physics. The post-Newtonian correction is quite small. But the time in navigational ephemeris calculations is time based on the rotation of the Earth which is wobbly and variable and ultimately based on presently unknown or poorly understood phenomena in geophysics. We know the difference from direct observations, not theoretical calculations. For the late 18th century, those observations are primarily observations of the Moon's position relative to the local meridian made at the small handful of well-equipped astronomical observatories that existed back then, including, at the top of the heap, the Greenwich Observatory. Those observations have been analyzed and re-analyzed in detail and they give the value of delta-T to the nearest second or two of time in the late 18th century and the nearest tenth of a second or a few by the late 19th century. There's nothing more to know about it. It's as much a given and as much an unknown as the obliquity of the ecliptic and for the very same reasons (the uncertainty in the latter, since it is not cumulative in the same sense, is far less important for these calculations but the cause is the same).

Speaking of all these various ephemerides with their acronymic names, just to clarify for anyone contemplating doing their own calculations, it's all the same as far as navigation is concerned. Just make sure you're using data sets published in the past twenty years or so without intentional reduction in accuracy. The positions of the Sun, Moon, stars, and navigational planets are known quantities to the nearest arcsecond or better for the period of modern navigation post-1767. There is no benefit re-calculating these things every time they're needed unless you're programming a device with unusually low memory (like an old programmable calculator). There is no benefit (except amusement! --nothing wrong with that) using the various algorithms published by Jean Meeus and others in several books twenty to thirty years ago every time somebody needs the position of the Moon. Just tabulate it to the nearest arcsecond for every hour and look it up as needed. Note that I am talking strictly about present and historical celestial navigation and traditional terrestrial position-finding here. Other applications, like the view of the Solar System from the surface of Mars, to pick a random example, would have different requirements.

-FER

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