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Updated: May 14, 2025
Hewlett, however, in 1894, after thirty years of work, showed that the spots are not always depressions, being very subject to disturbance. The Kew photographs contributed a vast amount of information about sun-spots, and they showed that the faculae generally follow the spots in their rotation round the sun.
By using a high dispersion the faculae which give off K light can be correctly photographed, not only at the sun's edge, but all over his surface. The actual mechanical method of carrying out the observation is not quite so simple as what is here described.
The sun himself represents a crowd of pending problems. His peculiar mode of rotation; the level of sunspots; the constitution of the photospheric cloud-shell, its relation to faculae which rise from it, and to the surmounting vaporous strata; the nature of the prominences; the alternations of coronal types; the affinities of the zodiacal light all await investigation.
When the ascending currents are powerful, they create those appearances which astronomers designate the nuclei, the penumbrae, the faculae. Such was Herschel's explanation of the mode of formation of the solar spots; and allowing it to be well-founded, we must expect to find what is, indeed, the case that the Sun does not always and regularly pour forth equal quantities of light and heat.
In 1861 he similarly combined two photographs of a sun-spot, the perspective effect showing the umbra like a floor at the bottom of a hollow penumbra; and in one case the faculae were discovered to be sailing over a spot apparently at some considerable height. These appearances may be partly due to a proper motion; but, so far as it went, this was a beautiful confirmation of Wilson's discovery.
In other years I saw the zodiacal light augment in the southern hemisphere half an hour before its disappearance. Cassini admitted "that the zodiacal light was feebler in certain years, and then returned to its former brilliancy." He thought that these slow changes were connected with "the same emanations which render the appearance of spots and faculae periodical on the solar disk."
The faculae, or bright areas, which are seen all over the sun's surface, but specially in the neighbourhood of spots, and most distinctly near the sun's edge, were discovered by Galileo. A high telescopic power resolves their structure into an appearance like willow-leaves, or rice-grains, fairly uniform in size, and more marked than on other parts of the sun's surface.
When several spots coalesce into one, how do they do it? When a spot breaks up into several pieces, what is the seeming nature of the process? How do the groups of brilliant points called faculae come, change, and grow?
Perhaps one of the most interesting and useful pieces of astronomical work which an amateur can perform will consist of a record of the origin and changes of form of the solar spots and faculae. What does a spot look like when it first comes into sight? Does it immediately burst forth with considerable magnitude, or does it begin as the smallest visible speck, and gradually grow?
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