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    Compare Methods: Lat/Lon Near Noon
    From: Frank Reed
    Date: 2021 Aug 28, 07:27 -0700

    It's May 22, 2022 and you're sailing from southern New England towards Bermuda. Your speed through the water is 4 knots on a heading of 185° true, and you are confident that you are in a current carrying you east at 2 knots. Your estimate of the current's direction is 90°+/- 20° and your estimate of the speed of the current is 2+/- 0.5 knots. What would you estimate for your true course and speed over the ground? It should be something generally towards SSE and the net speed should be somewhat more than 4 knots.

    Starting 20 minutes before local noon, you begin shooting Sun sights. You grab a couple before noon, a pair very close to local noon (showing nearly the same altitude), and two more afternoon sights more or less symmetrical to the pre-noon sights, ending about 20 minutes after local noon. Your dip+SD correction is 12.7'. The Sun throughout the sights is above 72° high. Refraction at these high altitudes adds another 0.3'. IC is zero. So the net altitude correction for all six sights should be about 13.0'.

    Your Sun sights, UT and altitude (raw, Hs):
    16:17:20 -- 72°28.5'
    16:22:40 -- 72°44.5'
    16:36:45 -- 73°04.0'
    16:39:15 -- 73°04.5'
    16:51:25 -- 72°49.0'
    16:56:20 -- 72°35.5'

    What was your latitude at local noon? And for that matter when was local noon (what was UT when the Sun's azimuth was exactly 180° true)? Trickier: what was your longitude at local noon (and don't forget, you're moving!). And what is your best estimate of your lat/lon at 17:00 UT?

    How many methods can you apply to this problem? There are traditional paper tabular approaches. There are graphical approaches... We can throw the whole thing into an app and see what comes out. Does your app handle a case like this? 

    Note: I have done the set up and tested for inconsistencies, but I haven't "solved" this, so I don't have an answer. Give it a go...

    A little preview of the next puzzle... you lose your rigging in a squall overnight. On May 23 you're adrift, but you still have your sextant And that opens another can of worms... I'll have that set up in a couple of days. :-)

    Frank Reed
    Clockwork Mapping / ReedNavigation.com
    Conanicut Island USA

       
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