Contact Timings From The Path Limits
Precise timings of beading phenomena made near the northern and southern
limits of the umbral path (i.e. - the graze zones), are of value in determining
the diameter of the Sun relative to the Moon at the time of the eclipse. Such
measurements are essential to an ongoing project to monitor changes in the
solar diameter. Due to the conspicuous nature of the eclipse phenomena and
their strong dependence on geographical location, scientifically useful
observations can be made with relatively modest equipment. A small telescope,
short wave radio and portable camcorder are usually used to make such
measurements. Time signals are broadcast via short wave stations WWV and CHU,
and are recorded simultaneously as the eclipse is videotaped. If a video
camera is not available, a tape recorder can be used to record time signals
with verbal timings of each event. Inexperienced observers are cautioned to
use great care in making such observations. The safest timing technique
consists of observing a projection of the Sun rather than directly imaging the
solar disk itself. The observer's geodetic coordinates are required and can be
measured from USGS or other large scale maps. If a map is unavailable, then a
detailed description of the observing site should be included which provides
information such as distance and directions of the nearest towns/settlements,
nearby landmarks, identifiable buildings and road intersections. The method
of contact timing should be described in detail, along with an estimate of the
error. The precisional requirements of these observations are +/-0.5 seconds
in time, 1" (~30 meters) in latitude and longitude, and +/-20 meters (~60 feet)
in elevation. Although GPS's (Global Positioning Satellite receivers) are
commercially available (~$500 US), their positional accuracy of +/-100 meters
is about three times larger than the minimum accuracy required by grazing
eclipse measurements. The International Occultation Timing Association (IOTA)
coordinates observers world-wide during each eclipse. For more information,
contact:
Dr. David W. Dunham, IOTA
7006 Megan Lane
Greenbelt, MD 20770-3012 Phone: (301) 474-4722
U. S. A. Internet: David_Dunham@jhuapl.edu
Send reports containing graze observations, eclipse contact and Baily's bead
timings, including those made anywhere near or in the path of totality or
annularity to:
Dr. Alan D. Fiala
Orbital Mechanics Dept.
U. S. Naval Observatory
3450 Massachusetts Ave., NW
Washington, DC 20392-5420
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