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A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Longitude by Sunset
From: Gary LaPook
Date: 2012 May 8, 00:52 0700
From: Gary LaPook
Date: 2012 May 8, 00:52 0700
The answer to your question is "nothing." Mr. van Asten really does not understand celnav. The "Parallax in altitude" correction for the moon merely is a computation to convert the altitude actually measured by an observer on the earth's surface to what would have been measured by an observer located at the center of the earth since this is the angle that is needed for celnav and, for the Moon, this can amount to 61'. For bodies other than the Moon we can (for practical navigation) ignore this adjustment because it is very small due to the much greater distances to the other celestial bodies. For precise navigation you can make the same adjustment for Venus, which can reach 0.5', and for Mars, which can reach 0.3'. The P in A correction for the sun is incorporated into the sun correction tables and this factor can reach 0.15'. But none of this affects the actual observation of the body. Mr. van Asten does bring up a theoretically interesting point in that the observed altitude (Ho) of the Moon can never be zero even though the sextant altitude (Hs) can be (assuming that it is visible and not dimmed out by the atmosphere as we are discussing.) But the Ho is the altitude measured at the center of the Earth and there are not very many observations actually taken at that spot. For example, the semidiameter of the Moon now is 17' and the refraction at the horizon is 34.5'. So without allowing for P in A (or in this case the horizontal parallax (H.P.)) an upper limb observation of the moon at Moonset, Hs = 0, would place the center of the Moon at 51.5' below the horizon and that is where it really is. But as observed from the center of the Earth we have to add the H.P. of 61' so the Ho will be 9.5' above the theoretical horizontal. So Mr. van Asten must be taking his observations from a very deep hole. gl  On Mon, 5/7/12, Alexandre E Eremenko <eremenko.edu> wrote:
