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 USA
Phone: (301) 474-4722
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 USA
Next section
Table of Contents