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    Re: Latitude by two stars ...on a Party Boat
    From: Bob Crawley
    Date: 2018 Jan 5, 00:15 -0800

    While you've all been enjoying yourselves I've been stuck on this so called party boat trying to get back. I'd decided to turn back but then got to reconsidering the problem. I think we were a lot further East than Bermuda so I've just activated the EPIRB as I'm worried that the water for tea is running out.

    I first assumed that we'd be somewhere in the box bounded by South and East of NY and North and West of Bermuda. This gave me an upper and lower limit on time of twilight etc so bounded UT to between 1744 and 1831. This was a mistake in NavWorld and just confused things.

    The sights conveniently averaged to give me Procyon, Moon and Sirius at the same time (with a smidge of adjustement to the Moon. I didn't bother about temp and pressure and probably needn't have bothered with dip. Did the High pressure tell me I was near the Azores - who knows? Next I realised that I could calculate Latitude regardless of anything above. This calculation proved to be a tough one and unless one was well practised it would be hard. I did it eventually with a spreadsheet. I got 34d40m from Sirius and Procyon. I then double checked with Sirius and Moon and got something different which made me smell a bilge rat although it could well have been a mistake.

    A Sirius Procyon fix without time tells me nothing new. However, the Moon changes its altitdue a different rate to the stars and moves about an hour a day relative to them so I thought let's find a longitude where for the given altitude of the Moon I get the same altitudes for the two stars. I could not come up with a way of doing this other than iteratively so calculated altitudes for my box at the know Latitude. The Moon was too high on 29th and too low on 31st so the 30th seemed to be the date. I found that the Moon was too high at the Eastern end so needed to be earlier in UT and thus further east. By this time I cheated and used the usno site to do the calculation. This got me to a position of 34d30m N 22d W which is about 2400nM from NY and 300nM S of the Azores (maybe).

    There has been thick cloud as well as thick heads since the sights. A further concern is that as I don't know where I am with certainty I don't know magnetic variation so I'm heading into the wind assuming that's West and I'll find North America somewhere that way. Maybe there was a smarter way of doing this and having the confidence to turn North to find the Azores before the tea runs out.

    I'm not too sure about this which is why the EPIRB is bleeping. Some party boat!

    Frank, I probably missed quite a lot in this and may not be where I think but it was a good workout.

    Regards

    BobC

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