Popular Science Monthly/Volume 23/September 1883/Faculae and Sun-Spots
THE sun when examined with a suitable telescope, properly adjusted as to the power used, exhibits, scattered over its disk, great facular waves, which are elevated portions of its surface, and are composed of luminous matter which has extended through its denser atmosphere. In order that these waves may be seen, it is requisite that they attain a height of at least forty-five times that of the Himalayas. Their appearance is very rare in polar regions, but very abundant in the close neighborhood of spots; in fact, they generally precede the formation of a spot. The faculæ, at a distance from the spots, change somewhat slowly, remaining for several days without much variation in their appearance. But it is quite otherwise near a spot, for here these waves change with a rapidity which renders it exceedingly difficult to make a draft of them. Movements not less than one thousand miles in an hour are not uncommon. The faculæ are generally round, though sometimes they appear in long strips of light. When they take the shape of a wreath, a group of spots quite soon appears, as a rule. With the discovery of solar spots may be said to have commenced our knowledge of the physical condition of the sun. Kepler was of the opinion that, in lines 441 and 454 of Virgil's first "Georgic," the solar spots were referred to. We also find in the annals of the Chinese, made many centuries ago, that spots were observed by the unaided eye, and in the year 807 an exceedingly large spot was seen for eight days. A solar spot consists, in the main, of two parts—the central part, called the umbra, surrounded by a less dark portion called the penumbra; and, as Professor Young has said, "The appearance is as if the umbra were a hole, and the penumbra filaments overhung and partly shaded it from our view, like bushes at the mouth of a cavern"; the umbra being a depression below the photosphere, filled with less luminous matter, while the penumbra may be seen around the edges.
It is observed that the spot, when half through its existence, is circular in shape, but, as it approaches disruption, it is subject to great change, sudden and violent. Respecting the average life of a spot, we may say it is from two to three months. The spot, however, observed in 1840 and 1841 lasted eighteen months, the longest time on record. Again, some may last but a few hours, being suddenly formed and rapidly shattered in pieces. They appear generally in groups, though single spots are seen at times. When a large one is divided into parts, they seem to repel one another, and move away in various directions with great velocity; it is not uncommon that this separation is equal to one thousand miles in an hour. Respecting what is sometimes seen about or crossing spots, we call attention to the simultaneous observations of Mr. Carrington and Mr. Hodgson upon two luminous objects, resembling in shape two new moons, each 8,000 miles in length and 2,000 miles wide: these, separated by at least 12,000 miles, came instantly into view at the edge of a large sun-spot, with a brightness several times greater than the surrounding photosphere, then passed eastward over the spot, and disappeared in about five minutes, having gone not less than 36,000 miles. In no way did the disturbance apparently affect the shape of the spot (passing probably above it). On the following evening a great magnetic storm and aurora were decidedly manifest, and there can be little doubt that they were connected with the event seen in the sun the day preceding. Likewise we know that a great solar storm is in progress when our northern heavens are so beautifully illuminated with the northern light. As to the formation of some spots they have been very slow, requiring no small amount of time after the disappearance of the facular waves; again, as stated, some come and go in a few hours, and assume in some cases huge dimensions as compared with any one thing on the earth. A single spot has measured from 40,000 to 50,000 miles in diameter, in which, as will be readily seen, we could put our earth for a standing point of observation, and note how the vast facular waves roll and leap about the edge of the spot, and also how the metallic rain is formed from the warmer portions of the sun. In June, 1843, a solar spot remained a week visible to the naked eye, having a diameter of about 77,000 miles; and in 1837 a cluster of spots covered an area of nearly 4,000,000,000 square miles. When we call to mind that the smallest spot which can be seen with the most powerful telescope must have an area of about 50,000 miles, we can readily see how large a spot must be in order to be visible to the unaided eye. Pasteroff, in 1828, measured a spot whose umbra had an extent four times greater than the earth's surface. In August, 1858, a spot was measured by Newall, and it had a diameter of 58,000 miles—more, as you will see, than seven times the diameter of the earth. The largest spot that has ever been known to astronomy was no less in diameter than 153,500 miles, so that across this you could have placed side by side eighteen globes as large as the earth; and, when the depth of this cavity is considered mathematically, the result shows that probably not less than one hundred globes the size of our earth would have been requisite to bring it up to the photosphere of the sun. So numerous have been the observations and measurements, that any attempt to present them all would require many pages.