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Re: Jack Aubrey's fixing of longitude
From: Geoffrey Kolbe
Date: 2011 Jun 25, 11:48 +0100
From: Geoffrey Kolbe
Date: 2011 Jun 25, 11:48 +0100
>At the moment I tend to another interpretationn >of this literary fantasy. As no lunar data for >planets was available, I think that the >occultation of Venus can also be ruled out, as >an occultation (please correct me) is a special >case of lunar (distance = 0). Nor was it a >normal lunar, as the team was waiting for >something to happen and a lunar can be taken any time. > >To sum up: they waited for the occultation of a >star. And again, here comes the "azimuth man"� > >By measuringg the exact azimuth you can reduce >the local hour (angle), as now you have the >precise declination and the local latitude. The >Nautical Almanac gives you the Greenwich time for this event. Albert, a couple of points. The Nautical Almanac of the period gave the positions of the moon and also the positions of Venus. The declination of the moon and Venus at any given time would have been known. It would have been possible to calculate the time of an occultation of Venus using the data in the Nautical Almanac. It follows that it would have been possible for a mathematically inclined master of a naval ship of the day to use a convenient Venus occultation to fix the position of an island. The second point is that if he had his local latitude, which our mythical master would doubtless have had written down to the nearest second from a noon altitude of the sun, then a measurement of azimuth of the moon at the moment of occultation is not necessary. In any case, when the observer is near the equator (as our mythical master was) then the azimuth would change very slowly with time - especially if it was around "two seven four" degrees - and so would not be of much use in fixing a position. Geoffrey Kolbe