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, 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.
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. There exists now a significant amount of solar eclipse data that has yet to be reduced, but the preliminary results indicate that much more work needs to be done in this field.
Results from the total eclipse on August 1, 2008 from Hami, China appear here:
http://weblore.com/richard/Aug_1_2008_Eclipse.htm
Recent Eclipse Expeditions
A list of eclipse expeditions I have been to which include researching solar eclipse radius variations:
1. Total Solar Eclipse, March 7, 1970, Long Island, New York. First eclipse at age 14. Clear skies.
2. Total Solar Eclipse, July 11, 1991- Puerta Valerta, Mexico. With Paul Maley, NASA/JSC
3. Annular Solar Eclipse, January 5, 1992-Truk Lagoon, Micronesia. IOTA trip to record Bailey's Beads phenomena along northern shadow limit on a small 400 meter size island. Led by Paul Maley, NASA/JSC.
4. Total Solar Eclipse, June 30,1992-Punte De Este,Uruguay. Houston Museum of Natural Science trip led by Dr. Carolyn Sumners.
5. Volcano Expedition, September 1, 1992-Kamchatka, Russia. Assisted Russian volcanologists and geologists to take magnetic field readings and Global Positioning Satellite (GPS) readings around the active Karymskii volcano. Brought over American equipment not available to Russian scientists for data acquisition. Volunteer trip with Paul Maley NASA/JSC.
6. Total Lunar Eclipse, November 29, 1993, La Paz, Baja Mexico. IOTA graze expedition to record the star ZC 646 in joint effort with Canadian team to measure lunar polar diameter. With Paul Maley, NASA/JSC.
7. Annular Solar Eclipse, May 10, 1994, Demming, New Mexico. IOTA expedition to record the Baily's Beads in the continuing study to measure the solar radius variations.
8. Total Solar Eclipse, November 3, 1994, Arequipa, Peru. IOTA expedition to record the Baily's Beads in the continuing study to measure solar radius variations. Science Team leader.
9. Annular Solar Eclipse, April 29, 1995, Rio Napo, Peru.IOTA expedition to record the Baily's Beads in the continuing study to measure solar radius variations. With Paul Maley.
10. Total Solar Eclipse, October 24, 1995, Agra, India. IOTA expedition to record Baily's Beads in the continuing study to measure solar radius variations. Science Team leader, south limit.
11. Total Solar Eclipse, March 7, 1997, Darhan, Mongolia. Observed this eclipse under extremely cold temperatures (11°F). Early morning blizzard forced us to relocate. Saw end of eclipse through clouds/haze.
12. Total Solar Eclipse, February 26, 1998, Curacau, IOTA expedition to record the Baily's Beads in the continuing study to measure solar radius variations. Science Team Leader for southern umbral limit. Video frames of Baily’s Beads published in several publications including the book, “Eclips” by Sky and Telescope Editor Govert Schilling.
13. Annular Solar Eclipse, August 22, 1998, Kota Tinggi, Malaysia. IOTA expedition to record the Baily's Beads in the continuing study to measure solar radius variations.
14. Total Solar Eclipse, August 11, 1999, Diyarbikir, Turkey. IOTA expedition to record the Baily's Beads in the continuing study to measure solar radius variations. Science Team leader.
15. Annular Solar Eclipse, December 25, 2000, Conroe, Texas. Obtained the only known photo of this partial eclipse through rain clouds over Houston. Photo published on internet at www.spaceweather.com.
16. Total Solar Eclipse, June 21, 2001, Chinyingi, Zambia. Expedition to the northern limit canceled upon arrival due to unknown, difficult terrain to reach site in jungle. Observed from center line off Zambezi river near center line. Video/slides obtained presented to international audiences. Science Team leader.
17. Annular Solar Eclipse, December 14, 2001, Costa Rica. IOTA expedition to record the Baily's Beads in the continuing study to measure solar radius variations. Poor weather forced expedition to go 10k further inside southern limit then desired, but we did view the eclipse!
18. Annular Solar Eclipse, June 10, 2002, Puerta Vallerta, Mexico. Poor weather forced expedition to go to center line where we were clouded out, except for 45 seconds just before 2nd contact when the partially eclipsed Sun’s horns were viewed.
19. Total Solar Eclipse, December 4, 2002, Pafuri Gate ,South Africa. IOTA expedition. Set up at North limit. Clear all morning but clouds rolled in 1 hour before start of eclipse. No data acquired.
20. Annular Solar Eclipse, May 31, 2003. North of Dalvik, Iceland overlooking the Arctic Ocean. Another mad dash to find holes in the clouds. We observed the eclipse successfully through thin clouds with the Sun just 3.7 degrees above the horizon. Read about it at: http://weblore.com/richard/Annular_Eclipse.htm
21. Annular Solar Eclipse, April 8, 2005. Panama Canal zone. This was a hybrid eclipse. It was annular for most of the ground path except for the point of greatest eclipse where the Moon's shadow touched the Earth. Our expedition was successful and we saw the eclipse through thin cirrus clouds.
22. Hami, China. Total eclipse, Science team of Richard Nugent and Chuck Herald obtained GPS time inserted video of Baily's Beads at 1.4 and 3.4 km inside the south eclipse limit under clear skies.