Page:Popular Science Monthly Volume 1.djvu/156

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146
THE POPULAR SCIENCE MONTHLY.

days when the sun was observed, there were only 372 days when spots were not seen.

The spots vary in size and number: sometimes but a single one appears, and as many as 80 have been visible at one time. Nor are they equally distributed over the solar surface. The region of the spots is generally confined to the part of the sun within about 30 of its equator, beyond which they are rarely seen.

The photosphere is the name given to the silver sea of light by which the spots are surrounded, and in which they seem to swim; it is the immediate source of illumination, and, viewed through a glass of low power, it appears flat and smooth, and of a uniform whiteness.

But, when a telescope of high magnifying power is directed to the sun, its aspect is greatly changed: the spots lose their simplicity, and the photosphere its uniformity, and in both there are a revelation of structure, a diversity of parts, and a variety of changes, which at once provoke questions in the mind of the observer, as to the causes of this diversified appearance, and the constitution of the body which presents

Fig. 2.
PSM V01 D156 Solar spots.jpg
Solar Spots, showing Furrows in the Penumbra.

them. The hypotheses put forth are ingenious; but, while the facts of observation are rapidly increasing, and there is a growing agreement on many points, there is still profound uncertainty as to the interpretation to be given to the leading phenomena.

Spots upon the sun's surface have been frequently seen with the naked eye. Galileo saw one at sunrise. It was the observation of a solar spot with unassisted vision that so impressed Sir William Herschel as to determine his attention to the physical nature of the sun. The eminent observer Schwabe has also seen many without the aid of the telescope. Solar spots, therefore, are discernible without glasses at a distance of 91,000,000 of miles; but Lockyer says that the finest telescope enables us to see the sun as we should do with the naked eye at a distance of 186,000 miles, that is, 52,000 miles nearer than the moon. The sun is about 62,000,000 times larger than the moon, and Brande states that the smallest space that can be discovered in the sun's disk must subtend an angle of one second, which is equal to