IOTA's Solar Eclipse Research
The International Occultation Timing Association (IOTA), leads a long term study in to measure possible solar radius variations. It is extremely difficult to measure the diameter of the Sun using conventional photographic techniques, even with the most sophisticated Earth based equipment. This is due to the Earth's atmosphere and its continuous turbulent motions, both near to the ground and in its upper layers. Constant motion of the air gives blurry images of the Sun even under the best atmospheric seeing.
So IOTA astronomer Dr. David Dunham devised a sensitive technique to measure accurately the diameter of the Sun during a solar eclipse. Along with colleagues Drs. Joan Dunham, Wayne Warren, Jr., Alan Fiala (deceased), David Herald of Australia, Paul Maley and Richard Nugent, and other IOTA members, observers are stationed near the umbral limits (shadow extremes) during a solar eclipse. By stationing the observers at the shadow limits, we only need to determine whether or not the observer actually has totality at that point. Being at the shadow limits and that the Moon's surface is full of craters, mountains and valleys, these observers will experience a long period of Baily's Beads, sunlight shining through the lunar mountains and valleys. If no complete totality is seen, (That is, Baily's Bead's are continuously visible) then the observer is outside the Moon's shadow. If totality is seen, then the observer is within the Moon's shadow. A line of observers can easily establish the geographical location of the edge of the Moon's shadow to within 100 meters. This uncertainty corresponds to an error in the Solar diameter of 0.05 arcseconds.
Early results using this technique showed a precision of better than 0.1 arcsecond, and although this is quite an impressive figure, it was not enough to detect any changes in the Solar diameter. But comparison with a historical eclipse observed and timed by Sir Edmund Halley (of Halley's Comet fame), the resultant change in the Sun's size was computed to be -0.34 ± 0.2 arc second or approximately 492 ± 290 km. This figure is subject to updating as measurement and reduction techniques have been refined.
IOTA's method for determining the Solar Diameter.
As can be seen in the above diagram, prior to the eclipse, all quantities are known except for the Unknown solar diameter and the "Measure Umbra" distance on the Earth's surface. Following the eclipse, the size of the Moon's umbral shadow is known on Earth using the techniques described above, thus the Unknown solar diameter can be computed. Results indicate that the solar diameter may be shrinking and calibration with Picard satellite data is now being done.
from the total eclipse on August 1, 2008 from
Results from the January 15, 2010
annular eclipse from
Results from the July 11, 2010 Total eclipse from the French Polynesia appears here: