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Measuring and Monitoring the Solar Diameter
 
Due to the astrophysical relevance of the constance or variability of the solar diameter, many efforts have been made to gain precise results. While conventional techniques of measuring the sun's angular diameter failed to yield the required accuracy, IOTA developed a sensitive method to determine small variations of the solar diameter. Some of the literature available online can be find in the following:
 
Richard Nugent explains the principle in his article at 
http://weblore.com/richard/Solareclipse.htm 
     and reports about an expedition at
http://sunearth.gsfc.nasa.gov/eclipse/TSE98reports/TSE98Nugent.html 
For more detailed descriptions of the observational efforts, please read Paul Maley's article at
http://eclipsetours.com/science.html
David Dunham held the following presentation on a science meeting in September 2005: 
http://lasp.colorado.edu/sorce/2005ScienceMeeting/posters/Dunhams.pdf
A bibliography about historical and current measurements of the solar diameter is given at
http://www.icra.it/Research_area/research_area6.htm#4.%20Solar%20Diameter
Another Article about the determination of the solar diameter during solar eclipses, published at
 PASJ, 45,6, 819-829 (1993)

More literature about this subject can be found if carefully searching databases
Some other technologies to determine  the solar diameter can be found here:
Using drift-time measurements for the years 1990 to 2000 Wittmann and Bianda published the following paper:
www.irsol.ch/Papers/Solspa2000.pdf
In The Astrophysical Journal, Volume 543, Issue 2, pp. 1007-1010 an article "On the Constancy of the Solar Diameter" by Emilio et al has been published.

And finally please read the author's article about a solar eclipse expedition to Tunisia in October 2005, and see Torsten Schaefer's video of Baily's beads on this website: 
     http://www.iota-es.de/eclring.html
 
The use of the moon as a standard gauge by recording Baily's beads caused by the topography near the moon's rim requires the precise knowledge of the lunar limb. The observation of stellar occultations can refine the lunar profile better than any astronautic exploration, yet. Especially grazing occultations steadily improve the precision of the lunar profile in its polar regions. The MOONLIMB project of IOTA-ES (by D. Büttner) incorporates the data of lunar occultations into a new database of the lunar limb profile.

Due to librations the lunar profile periodically changes. But during a solar eclipse the libration in latitude is always close to zero, while the longitude libration may have any value. Therefore the projected outline of the polar regions of the moon only slightly varies, while other regions may completely alter their appearance. This strongly preferes the polar regions for the analysis of the solar diameter.

During a total or anular sun eclipse, the moon's polar regions are projected on the edges of the eclipse path. Another advantage is the slow proceeding of bead phenomena along the edge of this track. Besides the less accurate knowledge of the lunar profile far from the poles, Baily's beads observed from deep inside the eclipse path develop too rapidly for a sufficient time resolution. Any observer who records thoroughly timed Baily's beads from the edges of an eclipse path with video equipment can gain valuable data and contribute to the knowledge about the variability of the sun's dimensions.
 
IOTA/ES has reinstalled a working group on the reduction of solar eclipse data. 
For more informations please contact Hans-Joachim Bode (H.-J.B@ONLINE.de),  and Dr. Wolfgang Beisker (wbeisker@iota-es.de),                                    
as well as Konrad Guhl (kguhl@ASTW.de)
IOTA/ES
 
November 2005