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Re: Eclipse timings and theodolite accuracy
From: Roger W. Sinnott
Date: 2023 Feb 9, 02:15 +0000
From: Roger W. Sinnott
Date: 2023 Feb 9, 02:15 +0000
Robin,
You raise interesting questions, and I don't know any details of this proposal beyond what is said in the full S&T News Note from 1948 -- namely, that the National Geographic Society, the Army Map Service, Army Corps of Engineers, Signal Corps, Navy, Air Force, and National Bureau of Standards were all involved.in the project, as were such well-known astronomers as Rev. Francis J. Heyden, Charles H. Smiley, and George Van Biesbroeck. So it doesn't sound like a crazy idea of crackpots. (The more recent efforts at using eclipses to study possible long-term changes in the Sun's diameter weren't crazy either -- the International Occultation Timing Association was involved with that over a number of years.)
I'm just guessing, but my hunch is that solar eclipse timings are somewhat better than stellar occultation timings for refining the Earth's shape, simply because eclipse contacts automatically select low points among irregularities along the lunar limb, while stellar occultations take place at random heights on the lunar profile. (And back in 1948 the lunar limb profile was poorly known.)
A theodolite on a stable tripod or pier would certainly be much better than any marine sextant for measuring altitudes, but I don't have one of those either. Regardless, wouldn't altitudes of celestial bodies measured that way be subject to the Earth's local (and poorly known) gravitational anomalies?
Roger
-----Original Message-----
From: Robin Stuart <NoReply_Stuart@navlist.net>
To: rsinnott@post.harvard.edu
Sent: Wed, Feb 8, 2023 12:45 pm
Subject: [NavList] Eclipse timings and theodolite accuracy
From: Robin Stuart <NoReply_Stuart@navlist.net>
To: rsinnott@post.harvard.edu
Sent: Wed, Feb 8, 2023 12:45 pm
Subject: [NavList] Eclipse timings and theodolite accuracy
My online copy of the April 2023 edition of Sky & Telescope arrived today and as usual the first thing I read was Roger Sinnott's column 75, 50 & 25 YEARS AGO. The April 1948 edition describes expeditions to "Burma, Siam, China, Japan, Korea, and the Aleutians" to observe an annular eclipse. It was claimed that this would allow the separation of the observers' positions to be determined with errors not exceeding 150 feet and hence measure the figure of the Earth.
(I admit to a rather cynical view of some proposed eclipse projects. In the early 80's, as a student, I attended a talk by someone who was measuring the size of the Sun using total eclipse timings to refute suggestions that it was shrinking at an alarming rate. I came away with the impression that he had come up with a great way to get free trips to exotic locations and observe a rare and magnificent celestial spectacle. He did apparently get results and found that the Sun isn't shrinking. Whew!)
This raises the question as to what can be learned by observing an annular eclipse that cannot be done at least as well with stellar occultations. Both will presumably be subject to uncertainties arising from limb features.
In the column Roger notes "In those days, star sights with a sextant yielded positions accurate to about a mile. LORAN radio navigation was not even that good." But I wonder how well can you do in practice armed with a theodolite? Unfortunately I don't have one to try with.
Robin Stuart
(I admit to a rather cynical view of some proposed eclipse projects. In the early 80's, as a student, I attended a talk by someone who was measuring the size of the Sun using total eclipse timings to refute suggestions that it was shrinking at an alarming rate. I came away with the impression that he had come up with a great way to get free trips to exotic locations and observe a rare and magnificent celestial spectacle. He did apparently get results and found that the Sun isn't shrinking. Whew!)
This raises the question as to what can be learned by observing an annular eclipse that cannot be done at least as well with stellar occultations. Both will presumably be subject to uncertainties arising from limb features.
In the column Roger notes "In those days, star sights with a sextant yielded positions accurate to about a mile. LORAN radio navigation was not even that good." But I wonder how well can you do in practice armed with a theodolite? Unfortunately I don't have one to try with.
Robin Stuart
