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Re: Lunar eclipse report
From: Frank Reed CT
Date: 2004 Oct 31, 21:17 EST
From: Frank Reed CT
Date: 2004 Oct 31, 21:17 EST
Herbert Prinz wrote:
"In actual practice, however, the REAL problem was to measure
elapsed time between the celestial signals (eclipse phases, occultation,
whatever) and the events that established local time (meridian
transits). In antiquity, local time would more likely have been
established by sun observation than from the stars."
I understand that the Sun was prefered historically, but the RA of the zenith is easy to observe and *could* have been used without any form of clock in that "what if they had needed it" scenario I mentioned earlier. There doesn't have to be a star right at the zenith for this to work. An accurate sketch of the five degree patch of sky around the zenith will work just fine. A report would consist of the event, e.g., "begin totality" or "end totality" and the sketch of the zenith area with the actual zenith at the time of the event marked as carefully as possible based on visual extension of plumb lines. If I can estimate the location of my zenith among the stars within half a degree, then that is also the error of the longitude (from the "time sight" side of this --this is in addition to the error that arises from estimating the event itself). Note that the end of totality would be better suited to this system since your eyes have had time to dark-adapt during totality. This means you would be able to see fifth magnitude stars near the zenith and you would stand a much better chance of getting the sidereal time accurate to a fraction of a degree.
Here in Chicago, many rather tall buildings have narrow "gangways" between them, and many are oriented north-south. Each is like an old transit observatory. By standing in the middle and looking at the narrow gap overhead, it's possible to observe the local time quite accurately.
Frank R
[ ] Mystic, Connecticut
[X] Chicago, Illinois
"In actual practice, however, the REAL problem was to measure
elapsed time between the celestial signals (eclipse phases, occultation,
whatever) and the events that established local time (meridian
transits). In antiquity, local time would more likely have been
established by sun observation than from the stars."
I understand that the Sun was prefered historically, but the RA of the zenith is easy to observe and *could* have been used without any form of clock in that "what if they had needed it" scenario I mentioned earlier. There doesn't have to be a star right at the zenith for this to work. An accurate sketch of the five degree patch of sky around the zenith will work just fine. A report would consist of the event, e.g., "begin totality" or "end totality" and the sketch of the zenith area with the actual zenith at the time of the event marked as carefully as possible based on visual extension of plumb lines. If I can estimate the location of my zenith among the stars within half a degree, then that is also the error of the longitude (from the "time sight" side of this --this is in addition to the error that arises from estimating the event itself). Note that the end of totality would be better suited to this system since your eyes have had time to dark-adapt during totality. This means you would be able to see fifth magnitude stars near the zenith and you would stand a much better chance of getting the sidereal time accurate to a fraction of a degree.
Here in Chicago, many rather tall buildings have narrow "gangways" between them, and many are oriented north-south. Each is like an old transit observatory. By standing in the middle and looking at the narrow gap overhead, it's possible to observe the local time quite accurately.
Frank R
[ ] Mystic, Connecticut
[X] Chicago, Illinois
