Abstract

Observing the August 11th Solar Eclipse near the Edges of the Path of Totality

David Dunham

Amateurastronomen, die auf der Zentrallinie von totalen Sonnenfinsternissen beobachten, bekommen nur einen flüchtigen Blick auf die dynamischen und einzigartigen Ereignisse. Bei einer Randbeobachtung dauern solche Ereignisse etwa ein Drittel der Totalitätsdauer auf der Zentrallinie.
Es wird auf Beobachtungsmöglichkeiten, Vorsichtsmaßnahmen und die Verwendung/Auswertung der Beobachtungen eingegangen.

Most amateur astronomers will crowd onto the central line of the August 11th zone of totality, but there they will have only a fleeting glimpse of all of the dynamic phenomena that make total solar eclipses such unique experiences: Shadow bands, Baily's beads, and visibility of the chromosphere (flash spectrum). Many members of the International Occultation Timing Association (IOTA, and its European Section) will instead observe from locations one to two kilometers inside the edges of the path of totality, carefully computed taking into account the predicted lunar profile. At these locations, which take advantage of the circular geometry of the eclipse shadow, they will still have about a third of the duration of totality that central-line observers will see, while prolonging all of the dynamical edge phenomena by a factor of five to ten.

Moreover, since the mountains in profile near the lunar poles are the same at each eclipse, timings of the contacts and of Baily's beads made at near-edge locations can be compared with timings from past eclipses to measure small changes in the solar radius, greatly reducing the lunar profile error that is a dominant contributor to the uncertainty of the derived results.

Observations by different techniques are encouraged near the path edges. Video cameras used with small telescopes with full-aperature filters have given the best results for recent eclipses. But observations like those used at earlier eclipses are also encouraged, including low-resolution telephoto video observation, observation (visual and video) of a solar image projected with a small telescope, and direct visual observation. Then we can compare the results from these other techniques (which are the only ones available for many past eclipses) with the modern video observations. Also encouraged are observations by lines of direct visual (be careful for eye safety) and camcorder observers across each limit, to try to define their locations, as has been done during some past eclipses.

Such visual observers can simply be asked, "did you see or not see any totality, and if you did see it, maybe you could time the duration with a watch or stopwatch (or stopwatch feature available on many watches now)". Visual observers do not need (and should not have, or else their timings will be worthless) eye protection during totality, but when the last bead (before totality) or the first bead (after totality) is more than 10 times brighter than Venus, then a mylar filter or other good eye protection should be employed.

With such observations, we can determine the different heights in the solar photosphere sampled by different techniques to derive corrections and error estimates for the different methods, thereby obtaining a more reliable history of solar radius variations from all past eclipse observations.

Dr. David Dunham, President of the International Occultation Timing Association & Johns Hopkins Univ. Applied Physics Lab.
Address: 7006 Megan Ln., Greenbelt, MD 20770-3012, U.S.A. E-Mail: dunham@erols.com Internet: http://iota.jhuapl.edu
Telephone: +1 301 474 4722 Fax: +1 240 228 1093

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