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George Ellery Hale: Solar Physics and Related Inventions, 1927


The spectroheliograph in use at the Yerkes Observatory brought proof of the distribution of elements such as calcium, at various densities, in layers of the Sun.s atmosphere. Hale named the billowing clouds of calcium and hydrogen vapor "flocculi."

Hale made his greatest discovery while working at the Mt. Wilson Observatory. In laboratory experiments he matched the spectral lines emitted by sunspots, the dark areas of the Sun where the temperature drops hundreds of degrees below that of the surrounding photosphere. By applying the Zeeman effect of magnetic displacement to the spectral data, Hale proved the shifting magnetic field present in sunspots. Further observation and experimentation led Hale and his group to discover the Sun's entire magnetic field and its polarity reversals with the sunspot cycle.

Hale's instrumentation plus the observatories he designed and built are landmarks in the history of astrophysics and cosmology.