The Science of the Stars

by E. Walter Maunder

But the rapid changes of sunspots and the violence of some of the prominence eruptions are but feeble indications of the most wonderful fact concerning the Sun, i.e. the enormous amount of light and heat which it is continually giving off. Here we can only put together figures which by their vastness escape our understanding. Sunlight is to moonlight as 600,000 is to 1, so that if the entire sky were filled up with full moons, they would not give us a quarter as much light as we derive from the Sun. The intensity of sunlight exceeds by far any artificial light; it is 150 times as bright as the calcium light, and three or four times as bright as the brightest part of the electric arc light. The amount of heat radiated by the Sun has been expressed in a variety of different ways; C. A. YOUNG very graphically by saying that if the Sun were encased in a shell of ice 64 feet deep, its heat would melt the shell in one minute, and that if a bridge of ice could be formed from the Earth to the Sun, 2-½ miles square in section and 93 millions of miles long, and the entire solar radiation concentrated upon it, in one second the ice would be melted, in seven more dissipated into vapour.

The Earth derives from the Sun not merely light and heat, but, by transformation of these, almost every form of energy manifest upon it; the energy of the growth of plants, the vital energy of animals, are only the energy received from the Sun, changed in its expression.

The question naturally arises, "If the Sun, to which the Earth is indebted for nearly everything, passes through a change in its activity every eleven years or so, how is the Earth affected by it?" It would seem at first sight that the effect should be great and manifest. A sunspot, like that of February 1905, one thousand times as large as Europe, into which worlds as large as our Earth might be poured, like peas into a saucer, must mean, one might think, an immense falling off of the solar heat.

Yet it is not so. For even this great sunspot was but small as compared with the Sun as a whole. Had it been dead black, it would have stopped out much less than 1 per cent. of the Sun's heat; and even the darkest sunspot is really very bright. And the more spots there are, the more numerous and brighter are the faculæ; so that we do not know certainly which of the two phases, maximum or minimum, means the greater radiation. If the weather on the Earth answers to the sunspot cycle, the connection is not a simple one; as yet no connection has been proved. Thus two of the worst and coldest summers experienced in England fell the one in 1860, the other in 1879, i.e. at maximum and minimum respectively. So, too, the hot summers of 1893 and 1911 were also, the one at maximum and the other at minimum; and ordinary average years have fallen at both the phases just the same.

Yet there is an answer on the part of the Earth to these solar changes. The Earth itself is a kind of magnet, possessing a magnetism of which the intensity and direction is always changing. To watch these changes, very sensitive magnets are set up, and a slight daily to-and-fro swing is noticed in them; this swing is more marked in summer than in winter, but it is also more marked at times of the sunspot maximum than at minimum, showing a dependence upon the solar activity.

Yet more, from time to time the magnetic needle undergoes more or less violent disturbance; in extreme cases the electric telegraph communication has been disturbed all over the world, as on September 25, 1909, when the submarine cables ceased to carry messages for several hours. In most cases when such a "magnetic storm" occurs, there is an unusually large or active spot on the Sun. The writer was able in 1904 to further prove that such "storms" have a marked tendency to recur when the same longitude of the Sun is presented again towards the Earth. Thus in February 1892, when a very large spot was on the Sun, a violent magnetic storm broke out. The spot passed out of sight and the storm ceased, but in the following month, when the spot reached exactly the same apparent place on the Sun's disc, the storm broke out again. Such magnetic disturbances are therefore due to streams of particles driven off from limited areas of the Sun, probably in the same way that the long, straight rays of the corona are driven off. Such streams of particles, shot out into space, do not spread out equally in all directions, like the rays of light and heat, but are limited in direction, and from time to time they overtake the Earth in its orbit, and, striking it, cause a magnetic storm, which is felt all over the Earth at practically the same moment.

JUPITER is, after the Sun, much the largest member of the solar system, and it is a peculiarly beautiful object in the telescope. Even a small instrument shows the little disc striped with many delicately coloured bands or belts, broken by white clouds and dark streaks, like a "windy sky" at sunset. And it changes while being watched, for, though 400,000,000 miles away from us, it rotates so fast upon its axis that its central markings can actually be seen to move.

This rapid rotation, in less than ten hours, is the most significant fact about Jupiter. For different spots have different rotation periods, even in the same latitude, proving that we are looking down not upon any solid surface of Jupiter, but upon its cloud envelope—an envelope swept by its rapid rotation and by its winds into a vast system of parallel currents.

One object on Jupiter, the great "Red Spot," has been under observation since 1878, and possibly for 200 years before that. It is a large, oval object fitted in a frame of the same shape. The spot itself has often faded and been lost since 1878, but the frame has remained. The spot is in size and position relative to Jupiter much as Australia is to the Earth, but while Australia moves solidly with the rest of the Earth in the daily rotation, neither gaining on South America nor losing on Africa, the Red Spot on Jupiter sees many other spots and clouds pass it by, and does not even retain the same rate of motion itself from one year to another.

No other marking on Jupiter is so permanent as this. From time to time great round white clouds form in a long series as if shot up from some eruption below, and then drawn into the equatorial current. From time to time the belts themselves change in breadth, in colour, and complexity. Jupiter is emphatically the planet of change.