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    Re: Scrap paper with time sights c.1870
    From: Lars Bergman
    Date: 2013 Feb 23, 02:13 -0800

    Henry, the declination and equation of time values given in the excel sheet are those used in the actual calculations, thus they are (or should have been) interpolated to the GMT of observation. So I don't think that 1870 makes a good fit at all, 1871 is better but not spot on. The first observation for Dec 27th is made very close to Greenwich noon so the values for this obs are probably taken directly, without interpolation, from the NA, or from some simplified almanac in e.g. Bowditch.

    After some pondering I believe more and more in Brad's suggestion to search for such a "short almanac". Maybe a "real" NA was not often carried onboard in those days, whenever they were? I don't know, but maybe Frank has some insight into this after his studies of many an old log book.

    In the calculations discussed I found a few errors made, sometimes arithmetical, sometimes a logarithm from a neighbouring row in the nautical tables, and some other errors, but not large enough to get noticed.

    In some places you can see that the latitude is deliberately changed one minute, in order to get an even value of the sum of altitude, latitude and polar distance. Thus half-minutes are avoided in the half sum. The tables used must have been very similar in layout to my Bowditch of 1851. Thus from the halved log of the sum the navigator directly gets the local apparent time, either am or pm. In some cases interpolation to the nearest second is made, in other cases no interpolation is made.

    Another observation of interest is the chronometer error. The last observations, January 9th, seems to be based on the same chronometer rate as during the previous part of the voyage. This indicates that the used rate must have been more or less perfect, as otherwise a jump in the error (or correction) was to be expected after passing Gibraltar. My impression is that the landfall at Gibraltar was very close to the reckoning and the navigator saw no need to adjust the chronometer correction and rate.

    Lars


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