NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Lunar longitudes, not by lunar distance.
From: George Huxtable
Date: 2009 Aug 9, 10:07 +0100
From: George Huxtable
Date: 2009 Aug 9, 10:07 +0100
Hanno has come round to accepting the theodolite as a suitable instrument for his project. That seems wise. It's quite good enough for his purposes. The astronomer's transit instrument was designed to achieve the ultimate in precision. All he needs to do, once it's been placed firmly on a well-set tripod, is to set up his instrument so that its vertical axis is truly vertical, for which the built-in levels should be quite adequate. Then to set it so that its telescope swings in a near-meridian plane. One way to do this is to establish North (in the Northern hemisphere), by bisecting the most-Eastern and most-Western azimuth swings of Polaris. However, that's only possible at certain times of year; at others, two different stars may have to be used, but this presents no problem now that stars have been catalogued so precisely. Then swing by 180� to look Southwards. The time of the Moon's centre passing the central hairline is needed, but as it's only the outer-limb that can be observed, corrections for semidiameter are needed. The time of a nearby well-known star crossing the hairline is also needed, so a timepiece with known rate is required. The closer Moon and star are together, the more relaxed can be the requirements for timing and for theodolite alignment. From that time difference, longitude and GMT can be deduced. If star altitude is measured and corrected for refraction, latitude will become accurately known. Chauvenet, in his vol.2, describes such observations, and the use of the transit, in great detail, with engravings of instruments, but as he is writing for astronomers who are after the ultimate in precision, he goes into unnecessary detail (for us). It's easy to get distracted by his accounting for the most minute corrections. The advantage of timing such Moon culminations is that there's no effect of parallax, refraction, or dip to bother about. It's all very simple. Pity it can't be done at sea. Such a technique will benefit from more accurate predictions than those of the Nautical Almanac, as are to be found in an astronomical almanac or on a computer-predicted website. Please note that I've never actually made the type of observation described here, nor even do I possess a theodolite, so others that know better may be able to make corrections to those words. George. contact George Huxtable, at george@hux.me.uk or at +44 1865 820222 (from UK, 01865 820222) or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK. --~--~---------~--~----~------------~-------~--~----~ NavList message boards: www.fer3.com/arc Or post by email to: NavList@fer3.com To , email NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---