NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Lunar eclipse report
From: Frank Reed CT
Date: 2004 Oct 31, 21:47 EST
From: Frank Reed CT
Date: 2004 Oct 31, 21:47 EST
George H wrote:
"I think this question has come up previously on Nav-l. How good an assumption is it that an observer can time some moment in an eclipse, and agree with another observer who is timing the same eclipse, to within two minutes?"
If they have some idea what they are looking for, it seems to be about in that range: two to four minutes. But I don't know of any statistics for controlled groups of observers.
And:
"The trouble is that none of the shadow-edges is really sharp, but somewhat fuzzy, due to the fact that the Sun isn't a point-source, but subtends half a degree. Assume that each observer has a timepiece, so that they can estimate the mid-point between the last flash of light at the edge of the Moon, and its reappearance on the other side."
That's why it's two minutes and not two seconds. I should say though that my "two minutes" was based on Alex E's observational experiment. I think two to four minutes is more typical. But changes really are visible quite rapidly. I was amazed myself this time around to see how quickly things changed when I wasn't watching continuously. About halfway through the partial phase of the eclipse, with the shadow band running more or less down the middle of the Moon, I got in my car to drive to dinner and watch the game. Naturally I was watching the road and not the sky. But every two or three blocks, I had to wait at a red light. Each time I checked the Moon, very favorably placed for viewing as I was driving south, it looked distinctly different --the elapsed time between stops was four to five minutes.
And George suggested:
"The recent eclipse would have provided a good opportunity for Nav-l members
to indulge in such an exercise: at least,those enjoying clear sky, which
excluded me. Perhaps, it would be an interesting try-out at the next lunar
eclipse."
As luck would have it, we have no such luck. There is an unusual period which has just started with fewer than average total lunar eclipses.
And concluded:
"So timing an eclipse, though useful for longitude on land, wasn't very helpful out at sea."
Certainly. But you can't have one without the other. I believe rather strongly that one of the reasons that many ocean navigators took little interest in lunars when they became practical in the late 18th century is because they understood all too well that they didn't really require the longitude of their vessels. What they needed was the difference in longitude between vessel and destination. What good was longitude "at sea" unless we knew longitude on land? This essential prerequisite for ocean navigation --mapping the globe's land in longitude-- could have been achieved by any of a variety of methods decades or centuries earlier. I supose this was a classic "chicken and egg" situation. Who needs a map unless you can navigate? Who can navigate without a map?
Frank R
[ ] Mystic, Connecticut
[X] Chicago, Illinois
"I think this question has come up previously on Nav-l. How good an assumption is it that an observer can time some moment in an eclipse, and agree with another observer who is timing the same eclipse, to within two minutes?"
If they have some idea what they are looking for, it seems to be about in that range: two to four minutes. But I don't know of any statistics for controlled groups of observers.
And:
"The trouble is that none of the shadow-edges is really sharp, but somewhat fuzzy, due to the fact that the Sun isn't a point-source, but subtends half a degree. Assume that each observer has a timepiece, so that they can estimate the mid-point between the last flash of light at the edge of the Moon, and its reappearance on the other side."
That's why it's two minutes and not two seconds. I should say though that my "two minutes" was based on Alex E's observational experiment. I think two to four minutes is more typical. But changes really are visible quite rapidly. I was amazed myself this time around to see how quickly things changed when I wasn't watching continuously. About halfway through the partial phase of the eclipse, with the shadow band running more or less down the middle of the Moon, I got in my car to drive to dinner and watch the game. Naturally I was watching the road and not the sky. But every two or three blocks, I had to wait at a red light. Each time I checked the Moon, very favorably placed for viewing as I was driving south, it looked distinctly different --the elapsed time between stops was four to five minutes.
And George suggested:
"The recent eclipse would have provided a good opportunity for Nav-l members
to indulge in such an exercise: at least,those enjoying clear sky, which
excluded me. Perhaps, it would be an interesting try-out at the next lunar
eclipse."
As luck would have it, we have no such luck. There is an unusual period which has just started with fewer than average total lunar eclipses.
And concluded:
"So timing an eclipse, though useful for longitude on land, wasn't very helpful out at sea."
Certainly. But you can't have one without the other. I believe rather strongly that one of the reasons that many ocean navigators took little interest in lunars when they became practical in the late 18th century is because they understood all too well that they didn't really require the longitude of their vessels. What they needed was the difference in longitude between vessel and destination. What good was longitude "at sea" unless we knew longitude on land? This essential prerequisite for ocean navigation --mapping the globe's land in longitude-- could have been achieved by any of a variety of methods decades or centuries earlier. I supose this was a classic "chicken and egg" situation. Who needs a map unless you can navigate? Who can navigate without a map?
Frank R
[ ] Mystic, Connecticut
[X] Chicago, Illinois
