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From: Frank Reed (no email)
Date: Tue Mar 14 2006 - 21:33:48 EST
Greg, you asked:
"wasn’t the standard practice of the day to do either:
1. Do a simple noon sun shoot for Lat & Lon?
2. If you couldn’t do a noon sun shoot for some reason
say a high cloud deck, then you would try a equal alt.
( 2-4 hr before and after) shoot. And interpolate local noon?
3. If you were really good at the math, and had favorable conditions; a
twilight (morning or evening) sight of Polaris and another star, then work
a full spherical triangle?"
A common "standard practice" was to do Noon Sun for latitude and shoot a
separate "time sight" altitude at 9am or 3pm, roughly, for local time and thus
longitude. At noon, you could set watches (average pocket watches, not
chronometers, of course) to 12:00 but this was not an accurate enough measure of
local time for longitude. Equal altitude sights can get you the time of local
noon rather accurately, but you have to hope that you get clear weather
symmetrically on both sides of noon. You also have to correct for the motion of the
vessel, as in any running fix. Stars, including Polaris, were rarely used in
celestial navigation before the 20th century except occasionally for lunar
distance sights.
So what's a "time sight"? You can look at some examples worked up in the
1896 "Navigation Workbook" of the Charles W. Morgan on Mystic Seaport's library
web site. You can also find them in Bowditch. The name "longitude by
chronometer" in older navigation works usually refers to plain time sights. In short,
a time sight turns a sextant into a very accurate sundial. By measuring the
Sun's altitude and working a spherical triangle calculation, with the Sun's
declination and your dead reckoning latitude as additional inputs, you can
calculate the Sun's Local Hour Angle. Converted to time units, the Sun's LHA
*is* the Local Apparent Time --sundial time. After applying the equation of
time, comparing local time with chronometer time yields the longitude directly.
The sight reduction process takes about five minutes.
To get a Sumner line, you simply work the time sight again with a different
assumption for the dead reckoning latitude.
[Note: I mentioned shooting time sights at 9am, 3pm. I'm not saying that
this was recommended technique, only that it was a common technique (I don't
know why). Recommended technique is to take the time sights when the Sun is as
close to true East or true West as possible, so long as it's not less than 5
degrees in altitude]
-FER
42.0N 87.7W, or 41.4N 72.1W.
www.HistoricalAtlas.com/lunars
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