NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Refraction
From: George Huxtable
Date: 2007 Dec 11, 22:54 -0000
From: George Huxtable
Date: 2007 Dec 11, 22:54 -0000
Nicolas de Hilster wrote- "In the Dutch magazine Cornelis Douwes no. 165, Siebren van der Werf has written an article on sun's observations on Nova Zembla. There is a very nice series of pictures in it showing the sun setting on 30 March 2004 behind Sheringham Point (as seen from Resolute Bay). The sun only disappeared when its altitude was already -51.6 arc minutes! This remarkable effect was caused by what they called the Toboggan effect (a Toboggan is a small sledge) as the light follows a cold layer along the slopes of Sheringham Point. Why was this researched? Simply because in 1597 Willem Barentsz and his crew established their longitude by observing the Jupiter/Moon conjunction at the same spot on January 24th and 25th of that year. Due to the Toboggan effect they saw this conjunction no less than two hours after the real conjunction, resulting in a 30 degree (!) error in longitude." ==================== I'm a bit puzzled as to why a Sun altitude of -51.6 arc minutes, at its moment of disappearance, should be thought so unusual. When the last glimpse of the Sun's upper limb can be seen at the horizon, the apparent altitude of that limb is zero. If we look at the table for standard refraction in a modern Nautical Almanac, it tells us that in that case the refraction is -33.8 arc minutes. And if the semidiameter of the Sun was, say, 16 arc minutes, a rather typical value, that would put the Sun's expected position (of its centre) at -49.8. Not much of a discrepancy with the value Nicolas quotes of 51.6. George. contact George Huxtable at george@huxtable.u-net.com or at +44 1865 820222 (from UK, 01865 820222) or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK. --~--~---------~--~----~------------~-------~--~----~ To post to this group, send email to NavList@fer3.com To , send email to NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---