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    Latitude by Lunar Distance
    From: Frank Reed CT
    Date: 2006 Sep 24, 06:33 -0500
    That's no typo. Historically, lunar distances determine longitude, not latitude. So why latitude?
    Consider this scenario:
    I am becalmed on the Grand Banks. I look up in the sky and see the Moon close to due south about 45 degrees high, very roughly. The horizon is obscured by fog; I cannot measure ordinary altitudes. Above the Moon, I see Vega about 70 or 80 degrees high. I measure the angle between them with my sextant as exactly as possible, recording the exact GMT. I do a little math, and I have my latitude. Latitude by lunars?? Is such a thing possible?!  Sure, it's possible. By inverting the usual problem of lunar distances, the measured distance yields the Moon's altitude, even when the real horizon is quite invisible. Of course, it has fairly severe accuracy limits... So how bad is it?? Could you determine your latitude by measuring a lunar distance? Generally, could a certain class of lunar distance observations be equivalent to ordinary altitude observations for lines of position? Can we dispense with the horizon?
    42.0N 87.7W, or 41.4N 72.1W.

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