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A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Longitude by lunar altitudes (1854)
From: Frank Reed CT
Date: 2005 Jun 3, 00:27 EDT
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"
"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
42.0N 87.7W, or 41.4N 72.1W.
www.HistoricalAtlas.com/lunars