NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Nautical astronomy was different
From: Bruce Stark
Date: 2004 Oct 19, 15:16 EDT
From: Bruce Stark
Date: 2004 Oct 19, 15:16 EDT
For the last hundred years or so, navigation authorities have been trying to explain the old nautical astronomy in twentieth-century terms. It hasn't worked very well. The logic of the old system is simple, but doesn't fit the present way of thinking. The world has changed, and the facts a navigator deals with have changed accordingly. Here are some of the facts that, for navigators from James Cook to Joshua Slocum, were too obvious to mention:
(1) A lunar distance didn't give you the time. It couldn't. What it did was give you the longitude, so you could correct the dead reckoning.
(2) A chronometer, set to GMT, didn't give you the time either. Not unless it had been "regulated" with a time sight. Like a lunar, it gave you the longitude.
(3) You found time by time sight. The hour angle between you and the sun was THE time. That's what you based your calculations on. NOT Greenwich time.
(4) You had no need of accurate Greenwich time when taking out data for working observations. A crude estimate did the job.
(5) As a navigator (as opposed to an astronomer) you needed very little from the Nautical Almanac beyond the predicted lunar distances. If you'd been able to rely on a chronometer you probably wouldn't have bothered with an Almanac. Your navigation manual had all the astronomical data you were apt to use.
The "Navigator's Newsletter," in issue 82, published a paper of mine called "Tin Clock and Sextant." The paper demonstrates the self-sufficiency you gain with the old approach, and shows how to use that approach today.
I'm hoping it will stir up a controversy.
Bruce
(1) A lunar distance didn't give you the time. It couldn't. What it did was give you the longitude, so you could correct the dead reckoning.
(2) A chronometer, set to GMT, didn't give you the time either. Not unless it had been "regulated" with a time sight. Like a lunar, it gave you the longitude.
(3) You found time by time sight. The hour angle between you and the sun was THE time. That's what you based your calculations on. NOT Greenwich time.
(4) You had no need of accurate Greenwich time when taking out data for working observations. A crude estimate did the job.
(5) As a navigator (as opposed to an astronomer) you needed very little from the Nautical Almanac beyond the predicted lunar distances. If you'd been able to rely on a chronometer you probably wouldn't have bothered with an Almanac. Your navigation manual had all the astronomical data you were apt to use.
The "Navigator's Newsletter," in issue 82, published a paper of mine called "Tin Clock and Sextant." The paper demonstrates the self-sufficiency you gain with the old approach, and shows how to use that approach today.
I'm hoping it will stir up a controversy.
Bruce
