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    Longitude by lunar altitudes (1854)
    From: Frank Reed CT
    Date: 2005 Jun 3, 00:27 EDT
    From the Monthly Notices of the Royal Astronomical Society, 1854:
     
    Lt. E. D. Ashe, Royal Navy writes:
    "notwithstanding my experience of more than twenty years' sea time, I can recollect only one instance of the chronometers having been checked by lunar distances"
     
    This is interesting because it suggests that lunars were out of favor earlier in the Royal Navy than elsewhere. This would make good sense since the R.N. would have had very early access to good chronometers.
     
    The article goes on to describe Ashe's discovery that lunar altitudes can be used to determine longitude (as a check on the chronometers) instead of lunar distances, and he suggest that navigators might be more comfortable with such a method and therefore more likely to try it at sea. Of course, this is actually a re-discovery and one that has been made many times before and since. John Letcher, in his great little book "Self-Contained Celestial Navigation with H.O. 208", published in the 1970s, made almost the same comments that Lt. Ashe wrote 125 years earlier noting that longitude by lunar altitudes involves tools that are much more familiar to today's navigators. So why not use lunar altitudes? The limitations have been discussed on the list before, but I think the editor of the MNRAS summed up the situation very nicely. Here are his comments, from 1854, on Ashe's ideas:
     
    "The practical objection to using altitudes of the moon at sea, for getting the longitude, is, that the horizon is seldom so well defined as to allow of great accuracy, and that, unless the moon's orbit makes a considerable angle with the horizon, her motion in her orbit may not be shown satisfactorily by motion in altitude. The lunar distance observation is capable of much greater accuracy; and by using stars on both side of the moon, a large portion of the necessary errors of observation are diminished; the motion in her orbit is more favourably shown. The calculations are by no means laborious or complicated; but it must be admitted that great nicety is required to make the observations well, and that the instrument must be of the best kind. On land, perhaps, lunar altitudes would be available in lowish latitudes, as the angle is doubled by the mercurial horizon, and the observation is easy and of great exactness. If a seaman wished to use altitudes of the moon as a coarse check on his chronometer, the easiest method would be to calculate the sidereal time, using the moon as a star, upon two approximate suppositions of the longitude. If that were pretty nearly known, a simple interpolation would show the true longitude; i.e. that which gives the true sidereal time, supposed to be already ascertained. But we should guess, in the absence of actual trial, that a very bad lunar distance would give more trustworthy results than a very good lunar altitude."
     
    Well-said.
     
    -FER
    42.0N 87.7W, or 41.4N 72.1W.
    www.HistoricalAtlas.com/lunars
       
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