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A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Using any star for a lunar
From: George Huxtable
Date: 2005 Mar 12, 17:50 +0000
From: George Huxtable
Date: 2005 Mar 12, 17:50 +0000
Jim Thompson asked- >I am undoubtedly revealing my failure to spend enough time learning lunars >(my limp defense is that it is still bloody cold around here at night), but >can any navigational star or planet be used to work a lunar distance, as >long as the altitudes of the body and moon are within the window for a given >method? I know the old Almanacs had lunar distance data for only a few >selected stars, and I think that Frank's online calculator has solutions for >some selected stars, but can I just go outside, shoot a set of sights >between the moon and a convenient body, and then come back indoors to reduce >the sight? Or do I have to select from a short list of nagivational bodies? > >I have a run of sights on a star from last fall, taken with chilled but not >frozen fingers, but I recall that when I tried to do a preliminary reduction >using Frank's online calculator, that star was not available. > >What am I missing? ===================== If it's at a useful separation from the Moon, any identifiable planet can be used to work a lunar distance, because all planets are close to the plane of the ecliptic, as is the Moon, so that the planet always lies close to the path of the Moon. Two effects need to be considered in obtaining the apparent position of the planet in the sky, however. First, parallax can be appreciable, especially for Venus and Mars when they get close to the Earth. The planet correction table in the almanac allows for this. Second, there's a small "phase correction", because the surface of a planet is only partly-lit by the Sun. It appears to the eye to be at its "centre of brightness", which is slightly displaced from the geometrical centre. The almanac allows for this; a computer prediction of the planet may or may not have done so. As for stars, any identifiable star at a useful separation that's somewhere near the path of the Moon (i.e. near to the ecliptic) will do fine. You can find the ecliptic marked on Norton's Star Atlas, for example. Otherwise, if it's well out of that plane, then the Moon won't be travelling toward it (or away from it) but at an oblique angle, in which case the effective speed of the Moon will be significantly reduced. Historically, to minimise the labour of computation and the space in the almanac, only a minimum number of stars were selected, chosen for their brightness and identifiability and for roughly equal spacing around the ecliptic. And really, there's no great call for a wider selection of stars than those chosen in Maskelyne's day, though it's true that patchy cloud may hide some for a time, some nights. Nowadays, computation being so easy, any star near the ecliptic can be chosen, but make sure that it has been correctly identified and named! Having found its coordinates at certain moments near the time of a lunar distance observation (and the Moon's) then calculating the predicted lunar distances between them is no big deal. George. ================================================================ contact George Huxtable by email at george@huxtable.u-net.com, by phone at 01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK. ================================================================