NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Lunars for dummies like me
From: Frank Reed CT
Date: 2004 Sep 24, 23:20 EDT
From: Frank Reed CT
Date: 2004 Sep 24, 23:20 EDT
Jim T asked:
"Did navigators of the day set a ship's clock to GAT obtained from the lunar distance, and then therefore know the difference in time between when they took their lunar observation of GAT, and when they took their morning or afternoon time sight of the sun? Or did they do this without any ship's timepiece at all?"
Let's consider two cases:
1) You have no timepiece, not even an ordinary watch. In a case like this, you would shoot your lunar and your time sight simultaneously. That is, the altitude of the Sun is used to clear the lunar *and* to generate Local Apparent Time. This requires lunars where the Sun is well away from the meridian. And you're basically limited to using the Sun, but that was by far the prefered "other body" for lunars in any case.
2) You have a watch, but it's not a chronometer. This was the common situation. The lunar and the time sights can be separated in time by as much interval as you can trust your watch. So for example, you take a time sight at 08:00:00 in the morning and set the watch (to 8:00). At 2pm (make it 14:00:00 exactly) on the watch, you shoot a set of lunars and reduce them to find that the apparent time in London is now 8pm (20:00:00). So you're 6 hours apart or 90 degrees difference in longitude. Well, not quite... You've been sailing in between so you have to adjust the local time. Imagine you've been sailing west at 10 knots near the equator (you're whaling near the Galapagos in 1840 perhaps). You're chasing the Sun so the local time when you did the lunar wasn't quite 2 o'clock. You've moved west 60 miles or one full degree of longitude since the morning time sight so the local time was actually four minutes earlier, 13:56:00. The difference in time between Greenwhich and your present location is therefore 6 hours and 4 minutes. Equivalently, you can view your 6 hour longitude difference as applying to your longitude at 8am local time. Either way you look at it, your present longitude is 91d 00' W. I have a feeling that this business was somewhat confusing to navigators back then. But they didn't go very fast, so maybe they didn't worry!
Incidentally, although sights due east/west are certainly prefered for time sights, there's a lot of leeway.
Frank R
[ ] Mystic, Connecticut
[X] Chicago, Illinois
"Did navigators of the day set a ship's clock to GAT obtained from the lunar distance, and then therefore know the difference in time between when they took their lunar observation of GAT, and when they took their morning or afternoon time sight of the sun? Or did they do this without any ship's timepiece at all?"
Let's consider two cases:
1) You have no timepiece, not even an ordinary watch. In a case like this, you would shoot your lunar and your time sight simultaneously. That is, the altitude of the Sun is used to clear the lunar *and* to generate Local Apparent Time. This requires lunars where the Sun is well away from the meridian. And you're basically limited to using the Sun, but that was by far the prefered "other body" for lunars in any case.
2) You have a watch, but it's not a chronometer. This was the common situation. The lunar and the time sights can be separated in time by as much interval as you can trust your watch. So for example, you take a time sight at 08:00:00 in the morning and set the watch (to 8:00). At 2pm (make it 14:00:00 exactly) on the watch, you shoot a set of lunars and reduce them to find that the apparent time in London is now 8pm (20:00:00). So you're 6 hours apart or 90 degrees difference in longitude. Well, not quite... You've been sailing in between so you have to adjust the local time. Imagine you've been sailing west at 10 knots near the equator (you're whaling near the Galapagos in 1840 perhaps). You're chasing the Sun so the local time when you did the lunar wasn't quite 2 o'clock. You've moved west 60 miles or one full degree of longitude since the morning time sight so the local time was actually four minutes earlier, 13:56:00. The difference in time between Greenwhich and your present location is therefore 6 hours and 4 minutes. Equivalently, you can view your 6 hour longitude difference as applying to your longitude at 8am local time. Either way you look at it, your present longitude is 91d 00' W. I have a feeling that this business was somewhat confusing to navigators back then. But they didn't go very fast, so maybe they didn't worry!
Incidentally, although sights due east/west are certainly prefered for time sights, there's a lot of leeway.
Frank R
[ ] Mystic, Connecticut
[X] Chicago, Illinois
