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 (~$150 US), their
positional accuracy of ±100 meters is about three times larger than
the minimum accuracy required by grazing eclipse measurements.
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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