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), may be useful 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 detect 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. Commercially available GPS's (Global Positioning Satellite receivers) have comparable
positional accuracy as long as the U. S. Department of Defense keeps SA (Selective Availability) turned
off, Otherwise, SA degrades the positional accuracy to about ±100. GPS receivers are also a useful source
for accurate UT. The International Occultation Timing Association (IOTA) coordinates observers world-wide
during each eclipse. For more information, contact:
Dr. David Dunham,
IOTA
7006 Megan Lane
Greenbelt MD 20770-3012, USA
+1 (301) 474-4722
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, USA
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