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Solar Diameter by Solar Eclipses

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:

More literature about this subject can be found if carefully searching databases:

  • Eddy, J.A. and Boornazian, A.A., 1979. Secular decrease in the solar diameter, 1863–1953. Bulletin of the American Astronomical Society, 11:437
  • Shapiro, I.I., 1980. Is the Sun Shrinking? Science, 208:51–53.
  • Dunham, D.W., Sophia S., Fiala, A.D., Herald, D. and Muller, P.M., 1980. Obvservations of a probable change in the solar radius between 1715 and 1979. Science, 210:1243–1245
  • Parkinson, J.H., Morrision, L.V. and Stephenson, F.R., 1980. The constancy of the solar diameter over the past 250 years. Nature, 288:548–551.
  • Gilliland, R.L., 1981. Solar radius variations over the past 265 years. The Astrophysical Journal, 248:1144–1155.
  • Sofia, S., Dunham. D.W., Dunham, J.B. and Fiala, A.D., 1983. Solar radius change between 1925 and 1979. Nature, 304:522–526






  • 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:

    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.

    For more informations please contact Konrad Guhl.