NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Mid XIX century Nav
From: Frank Reed CT
Date: 2005 Nov 22, 01:29 EST
From: Frank Reed CT
Date: 2005 Nov 22, 01:29 EST
George H, you wrote: "A modern Times Atlas gives the longitude of Nagasaki as 129 deg 52 E, which is exactly the same as Raper (in 1864) gives for "Nangasaki". What is remarkable is the agreement, with a minute, between that figure and the long. quoted by Frank Reed from Kruzenschtern of 230 deg 07W, as early as 1805!" I should emphasize again that the longitude on the chart is "maybe" 230 07W. It's right at the limit of readability on the NYPL web site, and I may be influenced by the known longitude of the place. And you wrote: "Frank made the interesting suggestion that a lunar occultation might have been employed to get the "timing" error of the chronometer, and indeed, so it could. " I agree with you about the relative impracticality of using occultations, but actually, I mentioned "moon culminations" which were very popular for land-based determinations of longitude in the first half of the 19th century. These are apparently more practical than occultations. And you wrote: "The answer lies in the stars. If the navigator goes ashore, and sets up a post with a clear view of the Southern sky behind it, and some sort of peep-hole to position his eye well in front of it, he can time low-altitude stars as they disappear, instantaneously, behind that post. The interval between passages of the same star is exactly one sidereal day, which is just 3 minutes 55.91 seconds (and umpteen decimal places) short of a day by GMT." This is a good method for measuring a sidereal day if you have few instruments (or a hundred years earlier), but I think in the mid-19th century the rating was done much more simply. You do time sights a few days apart with an artificial horizon (if possible). If the chronometer is five seconds slow on Monday at 4pm and seven and a half seconds slow on Friday at 4pm, then the rate is 0.6 seconds per day. Then, let's say, three weeks later, at your next port of call with a well-established longitude, you would expect to find that the chronometer is 20 seconds slow if the rate has been constant. If not, you've got a new rate determination, or trouble. -FER 42.0N 87.7W, or 41.4N 72.1W. www.HistoricalAtlas.com/lunars