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    Re: Early Land Navigation.
    From: Frank Reed
    Date: 2015 Feb 9, 17:43 -0800

    Frank Atkins:

    Who wants to be staring into a bowl of Mercury at high noon in the outback? That's nap time. :) The simplest solution for latitude when the Sun is too high and the weather is too hot is to use stars at night. Unlike the twilight round of sights at sea, a land-based observer using an artificial horizon can comfortably wait for a first magnitude star at convenient altitude to cross the meridian. 

    There are other tricks that work in daylight. There were various specialty sights for latitude by "double altitudes" and others that are of historical interest, but we can sweep all those aside, at least in principle, and go right for the gold. Use what you know: you can get a latitude by crossing any pair of Sun lines. You could generate two lines of position from any pair of Sun altitudes taken from the same location (or nearly so) so long as they are separated by enough time to get a reasonable "cut" on the chart. These Sun lines can be calculated mathematically using any method you like, from 1875-style Sumner lines to 1975-style Pub.229 intercepts. It's all the same fundamentally. Cross the lines as usual to get your fix. If the longitude is suspect you just ignore it and record the latitude of the fix. Note that a method using two Sun altitudes like this requires a fairly accurate observation of the time interval between the sights but the actual GMT can be off by an hour or more with only a modest impact on the latitude (due to the changing declination of the Sun which is about 1 knot at maximum).

    You should probably locate a copy (or several) of "Hints to Travellers" which was published from 1878 by the Royal Geographical Society as a guidebook for gentleman explorers in an era when trekking across Africa, Australia, and other distant bits of empire was almost a sport in some social circles. There are various copies online. Here's one at archive.org from 1906. And one at Google Books from 1883. The ease of finding such things has certainly improved in a decade... Geoffrey Kolbe first set me the task of finding this book online eight or nine years ago. He later applied the methods in it to some of his celestial navigation experiments and trials in the western desert of Egypt which he described in fascinating detail in a lecture at one of the "Navigation Weekends" I organized at Mystic Seaport Museum in 2008.

    Frank Reed
    Conanicut Island USA


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