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Session 2 - Applied History of Astronomy II.
Oral session, Sunday, January 14
Salon del Rey Central, Hilton
Sir William Herschel observed the Sun from 1779 to 1818. Most of his solar observations were made from 1799 to 1806. He used his own terminology to identify many solar features: sunspots were openings; penumbrae, flats or shallows; faculae were ridges; and dark lanes were shallows. Herschel displayed remarkable courage in making these observations as he nearly went blind when the dark glass he observed the Sun through, cracked. The observations were also unique because other solar observers were not active then and because the Sun behaved very strangely during this time interval. The Sun was nearly dormant and the period marked the lowest solar activity since the Maunder Minimum. It has been called the Dalton or Modern Minimum. Observations of Beryllium and Carbon cosmogenic isotopes, as well as auroral and geomagnetic records confirm these conclusions. Further, Wolf deduced what was the longest interval of a sunspot cycle during this period: a lengthy 17 years! With Herschel's observations, the cycle length appears to be 14 years, somewhat closer to the 11 year average and a sunspot number of 38 is found, rather than 45 as Wolf obtained. Wolf, in reconstructing the long-term history of solar activity, did not have Herschel's notebooks for analyses. Thus he was limited to scattered observations. According to Wolf, the Wolf sunspot numbers for these years are very uncertain. By examining Herschel's observations, one may obtain a better reconstruction of solar activity during this interesting time period. Details on Herschel's solar observations may be found in: Hoyt and Schatten, Ap. J. 384, 361-384, 1992, and Ap. J. Supp. 78, 301 - 340, 1992.
