12. THE SUN.
One of Galileo’s most striking discoveries, when he pointed his telescope to the heavenly bodies, was that of the irregularly shaped spots on the sun, with the dark central umbra and the less dark, but more extensive, penumbra surrounding it, sometimes with several umbrae in one penumbra. He has left us many drawings of these spots, and he fixed their period of rotation as a lunar month.
It is not certain whether Galileo, Fabricius, or Schemer was the first to see the spots. They all did good work. The spots were found to be ever varying in size and shape. Sometimes, when a spot disappears at the western limb of the sun, it is never seen again. In other cases, after a fortnight, it reappears at the eastern limb. 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.
SOLAR SURFACE.
As Photographed at the Royal Observatory, Greenwich, showing sun-spots with umbræ, penumbræ, and faculæ.
Speculations as to the cause of sun-spots have never ceased from Galileo’s time to ours. He supposed them to be clouds. Scheiner[[1]] said they were the indications of tumultuous movements occasionally agitating the ocean of liquid fire of which he supposed the sun to be composed.
A. Wilson, of Glasgow, in 1769,[[2]] noticed a movement of the umbra relative to the penumbra in the transit of the spot over the sun’s surface; exactly as if the spot were a hollow, with a black base and grey shelving sides. This was generally accepted, but later investigations have contradicted its universality. Regarding the cause of these hollows, Wilson said:—
Whether their first production and subsequent numberless changes depend upon the eructation of elastic vapours from below, or upon eddies or whirlpools commencing at the surface, or upon the dissolving of the luminous matter in the solar atmosphere, as clouds are melted and again given out by our air; or, if the reader pleases, upon the annihilation and reproduction of parts of this resplendent covering, is left for theory to guess at.[[3]]
Ever since that date theory has been guessing at it. The solar astronomer is still applying all the instruments of modern research to find out which of these suppositions, or what modification of any of them, is nearest the truth. The obstacle—one that is perhaps fatal to a real theory—lies in the impossibility of reproducing comparative experiments in our laboratories or in our atmosphere.