The Science of the Stars

by E. Walter Maunder

Sunspots are not seen in all regions of the Sun. It is very seldom that they are noted in a higher solar latitude than 40°, the great majority of spots lying in the two zones between 5° and 25° latitude on either side of the equator. Faculæ, on the other hand, though most frequent in the spot zones, are observed much nearer the two poles.

It is very hard to find analogies on our Earth for sunspots and for their peculiarities of behaviour. Some of the earlier astronomers thought they were like terrestrial volcanoes, or rather like the eruptions from them. But if there were a solid nucleus to the Sun, and the spots were eruptions from definite areas of the nucleus, they would all give the same period of rotation. But sunspots move about freely on the solar surface, and the different zones of that surface rotate in different times, the region of the equator rotating the most quickly. This alone is enough to show that the Sun is essentially not a solid body. Yet far down below the photosphere something approaching to a definite structure must already be forming. For there is a well-marked progression in the zones of sunspots during the eleven-year cycle. At a time when spots are few and small, known as the sunspot minimum, they begin to be seen in fairly high latitudes. As they get more numerous, and many of them larger, they frequent the medium zones. When the Sun is at its greatest activity, known as the sunspot maximum, they are found from the highest zone right down to the equator. Then the decline sets in, but it sets in first in the highest zones, and when the time of minimum has come again the spots are close to the equator. Before these have all died away, a few small spots, the heralds of a new cycle of activity, begin to appear in high latitudes.

This law, called after SPÖRER, its discoverer, indicates that the origin and source of sunspot activity lie within the Sun. At one time it was thought that sunspots were due to some action of Jupiter—for Jupiter moves round the Sun in 11.8 years, a period not very different from the sunspot cycle—or to some meteoric stream. But Spörer's Law could not be imposed by some influence from without. Still sunspots, once formed, may be influenced by the Earth, and perhaps by other planets also, for MRS. WALTER MAUNDER has shown that the numbers and areas of spots tend to be smaller on the western half of the disc, as seen from the Earth, than on the eastern, while considerably more groups come into view at the east edge of the Sun than pass out of view at the west edge, so that it would appear as if the Earth had a damping effect upon the spots exposed to it.

But the Sun is far greater than it ordinarily appears to us. Twice every year, and sometimes oftener, the Moon, when new, comes between the Earth and the Sun, and we have an Eclipse of the Sun, the dark body of the Moon hiding part, or all, of the greater light. The Sun and Moon are so nearly of the same apparent size that an eclipse of the Sun is total only for a very narrow belt of the Earth's surface, and, as the Moon moves more quickly than the Sun, the eclipse only remains total for a very short time—seven minutes at the outside, more usually only two or three. North or south of that belt the Moon is projected, so as to leave uncovered a part of the Sun north or south of the Moon. A total eclipse, therefore, is rare at any particular place, and if a man were able to put himself in the best possible position on each occasion, it would cost him thirty years to secure an hour's accumulated duration.

Eclipses of the Moon are visible over half the world at one time, for there is a real loss to the Moon of her light. Her eclipses are brought about when, in her orbit, she passes behind the Earth, and the Earth, being between the Sun and the Moon, cuts off from the latter most of the light falling upon her; not quite all; a small portion reaches her after passing through the thickest part of the Earth's atmosphere, so that the Moon in an eclipse looks a deep copper colour, much as she does when rising on a foggy evening.

Total eclipses of the Sun have well repaid all the efforts made to observe them. It is a wonderful sight to watch the blackness of darkness slowly creeping over the very fountain of light until it is wholly and entirely hidden; to watch the colours fade away from the landscape and a deathlike, leaden hue pervade all nature, and then to see a silvery, star-like halo, flecked with bright little rose-coloured flames, flash out round the black disc that has taken the place of the Sun.

These rose-coloured flames are the solar "prominences," and the halo is the "corona," and it is to watch these that astronomers have made so many expeditions hither and thither during the last seventy years. The "prominences," or red flames, can be observed, without an eclipse, by means of the spectroscope, but, as the work of the spectroscope is to form the subject of another volume of this series, it is sufficient to add here that the prominences are composed of various glowing gases, chiefly of hydrogen, calcium, and helium.

These and other gases form a shell round the Sun, about 3000 miles in depth, to which the name "chromosphere" has been given. It is out of the chromosphere that the prominences arise as vast irregular jets and clouds. Ordinarily they do not exceed 40 or 50 thousand miles in height, but occasionally they extend for 200 or even 300 thousand miles from the Sun. Their changes are as remarkable as their dimensions; huge jets of 50 or 100 thousand miles have been seen to form, rise, and disappear within an hour or less, and movements have been chronicled of 200 or 300 miles in a single second of time.

Prominences are largest and most frequent when sunspots and faculæ are most frequent, and fewest when those are fewest. The corona, too, varies with the sunspots. At the time of maximum the corona sends forth rays and streamers in all directions, and looks like the conventional figure of a star on a gigantic scale. At minimum the corona is simpler in form, and shows two great wings, east and west, in the direction of the Sun's equator, and round both of his poles a number of small, beautiful jets like a crest of feathers.

Some of the streamers or wings of the corona have been traced to an enormous distance from the Sun. Mrs. Walter Maunder photographed one ray of the corona of 1898 to a distance of 6 millions of miles. LANGLEY, in the clear air of Pike's Peak, traced the wings of the corona of 1878 with the naked eye to nearly double this distance.