NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Lunar Distance in Wikipedia
From: Fred Hebard
Date: 2007 Aug 2, 08:42 -0400
From: Fred Hebard
Date: 2007 Aug 2, 08:42 -0400
Two minor edits. On Aug 2, 2007, at 6:53 AM, George Huxtable wrote: substituting "per" for "in an" in the sentence below. > A lunar distance changes with time at a rate of roughly half a > degree, or 30 > arc-minutes, per hour. Move the sentence on calculation precision to the end of this paragraph, to dovetail with the discussion of overall error. This also sets up a discussion of the various methods of clearing, which all aimed to simplify the clearing process, hence Bowditch's claim to fame. > In the early days of lunars, predictions of the Moon's position > were good > only to half an arc-minute, so to obtain the required overall > precision of > one arc-minute, only half a minute could be allowed for errors in > measuring > the lunar distance, and in calculation, combined. The best sextants > could > indicate angle to one-sixth of a minute, but in practice at sea, > actual > errors were somewhat larger, good observers typically achieving > overall > accuracy within half a minute in favourable conditions. From the > unstable > deck of a small vessel, in bad weather, with the sextant elevated > and tilted > at an awkward angle to get both objects simultaneously in view in > its small > mirrors, and with much of the sky obscured by square sails, it > became a > severe test of a navigator's skill. Because of the half-arc-minute > errors in both the moon's position predicted by almanacs and in the > measurement, great precision was required in the calculation of > Greenwich time. The calculations, which were complex, called for > five-figure log tables of trig functions. --~--~---------~--~----~------------~-------~--~----~ To post to this group, send email to NavList@fer3.com To , send email to NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---