Accuracy of Solar Radius Determinations from Solar Eclipse Observations, and Comparison with SDS and SOHO Data

David W. Dunham [david.dunham@jhuapl.edu], Johns Hopkins University/Applied Physics Laboratory, Laurel, Maryland; James R. Thompson; Sabatino Sofia, Yale University, New Haven, Connecticut;  David R. Herald;, Reinhold Buechner; Alan D. Fiala; Wayne H. Warren, Jr.; and Harry E. Bates.

    On intermediate to long (yearly to decadal and longer) time scales, the Sun’s total (TSI) irradiance may be correlated with its diameter1. If so, measurements of the solar diameter could be used as a proxy for measuring its irradiance. Observations of total and annular solar eclipses provide the most accurate ground-based determination of the solar radius since the geometry of the fast-moving shadow is set in space. Thus, atmospheric seeing has only a small secondary effect on the observations. Useful observations made near the edges of the central eclipse paths extend back to 1715. Consequently, this technique could provide TSI data for nearly three centuries, and as far back as the end of the Maunder minimum.

    Defining the edge of the Sun, that is, when it “completely” disappears or first reappears during an eclipse, is the most difficult part of these observations, limiting their accuracy. We have analyzed video observations of recent eclipses made with equipment of different sensitivities to assess the accuracy of the eclipse solar radius determinations. The errors are larger than expected and show the need for calibrations to determine the small variations of the solar radius accurately enough to be useful.

    From 1992 to 1996 the solar radius has been measured with a balloon-based experiment called the Solar Disk Sextant (SDS)2,3, and  since 1995, on several occasions, it has been determined with data from the Michelson Doppler Imager on SOHO4. The comparison with SDS is made approximately (the dates are not very close) with eclipses observed on 1994 May 10 and 1995 October 24. The SOHO comparison is made for eclipses observed on 1998 February 26, 1999 August 11, and 2004 December 4. In addition, we have new results from the eclipse of 1991 July 11 that also adds to earlier eclipse radius determinations5.

1. Sofia, S., Okeefe, J., Lesh, J. R., and Endal, A. S. 1979, Science, Vol. 204, p. 1306.

2. Sofia, S., Heaps, W., and Twigg, L. 1994, Ap. J., Vol. 427, p. 1048.

3. Egidi, A., Caccin, B., Sofia, S., Twigg, L., Heaps, W., and Hoegy, W. 2005, submitted to Sol. Phys.

4. Kuhn, J. R., Bush, R. I., Emilio, M., and Scherrer, P. H. 2004, Ap. J., Vol. 613, p. 1241.

5. Fiala, A. D., Dunham, D. W., and Sofia, S. 1994, Sol. Phys., Vol. 152, p. 97.