Next message: Frank Reed: "Re: Venus"
Bill you asked:
"Does your online lunars calculator take phase into account for Venus or
other bodies?"
No. There is no "obvious" way to do this that is unambiguously correct.
Also, if phase is large enough to worry about, you can *see* it (once you know
what you're looking for at least). As Venus gets closer to us, take a look at
it with your sextant's telescope. If it's approaching one arc-minute in
diameter, then through a 7x telescope, it will have an apparent diameter of more
han five arc-minutes. That's plenty big enough to resolve with the human eye.
You'll actually see a tiny crescent through the sextant. So you should still
be able to estimate the location of the planet's center and park that spot
right on the lunar limb when you're shooting lunars. And by the way, when in
doubt about the size and phase, pull out a little telescope and take a look!
Historically, no correction was made for phase. I think that's still
appropriate, but John Letcher in his book "Self-Contained Celestial Navigation with
H.O. 208", which includes some good info on shooting modern lunars,
recommends avoiding Venus and also Mars completely. This is partly because of the
phase issue, but more importantly, his simplified method for clearing lunar
distance sights could not handle planetary parallax. This is a unique feature of
his method, so I don't think it's really necessary to follow that advice.
-FER
42.0N 87.7W, or 41.4N 72.1W.
www.HistoricalAtlas.com/lunars
