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Re: Additional altitude correction for Venus
From: Stan K
Date: 2015 May 14, 16:27 -0400
From: Stan K
Date: 2015 May 14, 16:27 -0400
According to page 259 of the Nautical Almanac, "...additional corrections for parallax (page A2) are required for Venus and Mars." No mention of a phase correction. Later, in an example on the same page, it says, "The phase correction for Venus has been incorporated in the tabulations for GHA and Dec, and no correction for phase is required. The additional corrections for Venus and Mars allow for parallax." But from the 1962 Bowditch, "Because of phase, the actual centers of the planets and the moon may differ somewhat from the apparent centers. Average corrections for this difference are included in the additional corrections for Venus and Mars given on the inside front cover of the Nautical Almanac. They should be applied only when these bodies are observed during twilight. At other times, the magnitude and even the sign of the correction might differ from those tabulated, because of a different relationship between the body and the horizon." Gary's Nautical Information web site agrees with Bowditch, saying "The phase correction corrects the sextant to an equivalent altitude of the center of the body. This correction is a component of the additional corrections for Venus and Mars found inside the front cover (page A2) of the Nautical Almanac."
So, unless I missed something, the Gospel, the Nautical Almanac, that is, is the only one that does not mention a phase correction, and it certainly should because we teach than parallax decreases with altitude, being zero at the zenith. I can picture a student reading the statement in the Almanac that the additional correction is for parallax, and knowing that parallax decreases with altitude, starts banging his head against the wall when presented with the additional correction for Venus in 1978 (or one of the other years that show this phenomenon). And even considering a phase correction, I still do not see how it is a function of altitude, doing more than just cancelling out the effect of parallax, but that might just be a personal problem.
Aha! It just dawned on me to read page 259 of an Almanac that shows this phenomenon. The 1961 Almanac says "..additional corrections (inside front cover) are required for Venus and Mars." Not even a mention of parallax here. But the example says, "The additional corrections for Venus and Mars allow for parallax and phase..." and go on to give the formula and the parameters for 1961. So problem solved - both parallax and phase used to be included in the additional correction for Venus and Mars, but, as Frank indicated, this caused more confusion than it was worth, and later Almanacs only consider parallax. Maybe it is more correct to say that "some years" included the phase correction - Almanacs earlier than 1961, the earliest I have, might not have included it. (I'm not sure where Gary's Nautical Information web site got its information, perhaps from an older Bowditch. The 2002 Bowditch says even less: Mars or Venus Additional Correction: ...The correction is a function of the planet measured, the time of year, and the apparent altitude.")
So now I wonder for what years the phase correction was included and for what years it was not.
FYI, attached is page A2 of the 1978 Almanac.
Stan
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