The Science of the Stars

by E. Walter Maunder

VENUS is nearly of the same size as the Earth, and the conditions as to the arrangement of its atmosphere, the force of gravity at its surface, must be nearly the same as on our own world. But we know almost nothing of the details of its surface; the planet is very bright, reflecting fully seven-tenths of the sunlight that falls upon it. It would seem that, in general, we see nothing of the actual details of the planet, but only the upper surface of a very cloudy atmosphere. Owing to the fact that Venus shows no fixed definite marking that we can watch, it is still a matter of controversy as to the time in which it rotates upon its axis. Schiaparelli and some other observers consider that it rotates in the same time as it revolves round the Sun. Others believe that it rotates in a little less than twenty-four hours. If this be so, and there is any body in the solar system other than the Earth, which is adapted to be the home of life, then the planet Venus is that one.

THE SUN, like the Moon, presents a visible surface to the naked eye, but one that shows no details. In the telescope the contrast between it and the Moon is very great, and still greater is the contrast which is brought out by the measurements of its size, volume, and weight. But the really significant difference is that the Sun is a body giving out light and heat, not merely reflecting them. Without doubt this last difference is connected most closely with the difference in size. The Moon is cold, dead, unchanging, because it is a small world; the Sun is bright, fervent, and undergoes the most violent change, because it is an exceedingly large world.

The two bodies—the Sun and Moon—appear to the eye as being about the same size, but since the Sun is 400 times as far off as the Moon it must be 400 times the diameter. That means that it has 400 times 400, or 160,000 times the surface and 400 times 400 times 400, or 64,000,000 times the volume. The Sun and Moon, therefore, stand at the very extremes of the scale.

The heat of the Sun is so great that there is some difficulty in observing it in the telescope. It is not sufficient to use a dark glass in order to protect the eye, unless the telescope be quite a small one. Some means have to be employed to get rid of the greater part of the heat and light. The simplest method of observing is to fix a screen behind the eyepiece of a telescope and let the image of the Sun be projected upon the screen, or the sensitive plate may be substituted for the screen, and a photograph obtained, which can be examined at leisure afterwards.

As generally seen, the surface of the Sun appears to be mottled all over by a fine irregular stippling. This stippling, though everywhere present, is not very strongly marked, and a first hasty glance might overlook it. From time to time, however, dark spots are seen, of ever-changing form and size. By watching these, Galileo proved that the Sun rotated on its axis in a little more than twenty-five days, and in the nineteenth century SCHWABE proved that the sunspots were not equally large and numerous at all times, but that there was a kind of cycle of a little more than eleven years in average length. At one time the Sun would be free from spots; then a few small ones would appear; these would gradually become larger and more numerous; then a decline would follow, and another spotless period would succeed about eleven years after the first. As a rule, the increase in the spots takes place more quickly than the decline.

Most of the spot-groups last only a very few days, but about one group in four lasts long enough to be brought round by the rotation of the Sun a second time; in other words, it continues for about a month. In a very few cases spots have endured for half a year.

An ordinary form for a group of spots is a long stream drawn out parallel to the Sun's equator, the leading spot being the largest and best defined. It is followed by a number of very small irregular and ill-developed spots, and the train is brought up by a large spot, sometimes even larger than the leader, but by no means so regular in form or so well defined. The leading spot for a short time moves forward much faster than its followers, at a speed of about 8000 miles per day. The small middle spots then gradually die out, or rather seem to be overflowed by the bright material of the solar surface, the "photosphere," as it is called; the spot in the rear breaks up a little later, and the leader, which is now almost circular, is left alone, and may last in this condition for some weeks. Finally, it slowly contracts or breaks up, and the disturbance comes to an end. This is the course of development of many long-lived spot-groups, but all do not conform to the same type. The very largest spots are indeed usually quite different in their appearance and history.

In size, sunspots vary from the smallest dot that can be discovered in the telescope up to huge rents with areas that are to be counted by thousands of millions of square miles; the great group of February 1905 had an area of 4,000,000,000 square miles, a thousand times the area of Europe.

Closely associated with the maculæ, as the spots were called by the first observers, are the "faculæ"—long, branching lines of bright white light, bright as seen even against the dazzling background of the Sun itself, and looking like the long lines of foam of an incoming tide. These are often associated with the spots; the spots are formed between their ridges, and after a spot-group has disappeared the broken waves of faculæ will sometimes persist in the same region for quite a long time.

The faculæ clearly rise above the ordinary solar surface; the spots as clearly are depressed a little below it; because from time to time we see the bright material of the surface pour over spots, across them, and sometimes into them. But there is no reason to believe that the spots are deep, in proportion either to the Sun itself or even to their own extent.