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Re: How good were chronometers?
From: Frank Reed CT
Date: 2006 Mar 14, 21:33 EST
From: Frank Reed CT
Date: 2006 Mar 14, 21:33 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