66

early in the seventeenth century. In 1609, Galileo made his first telescope, and late in 1610, or early in 1611, with one of his improved instruments, he discovered some dark objects on the body of the sun. A little later, a Jesuit Father, Christopher Scheiner, professor of mathematics at Ingolstadt, commenced a regular series of observations of these dark spots, which he termed Maculae," and he also noted that some parts of the sun appeared to be brighter than the rest of the surface, and he, therefore, called them "Faculae," a name which is still universally given to them. Though Scheiner was able to show these spots to his pupils, he was at first forbidden to publish his observations, except anonymously, as his superiors were suspicious of his discovery. Thus, when he announced his discovery to the Provincial of his Order, the latter replied: “I have read Aristotle's writings from end to end many times, and I can assure you that nowhere have I found anything similar to what you describe. Go, my son, and tranquillize yourself; be assured that what you take for spots on the sun are the faults of your glasses or of your eyes.

وو

Sunspots, in appearance are, as their name suggests, dark stains on the intensely brilliant surface of the sun. And a sunspot, when fully developed, shows two (or more) grades of darkness; an outer shaded ring known as the penumbra, and an inner, much darker, core which we call the umbra. The penumbra is generally striated; the umbra in very large spots shows points within itself darker still, which are usually called nuclei. It is characteristic of sunspots that they are associated together in groups, and these groups tend to conform to a certain type of development. At the first appearance, a pair of very small spots. are seen near each other. The two members of the pair appear to be repelled, and they move apart; the two spots increasing in size, and other smaller spots forming between them. The group, therefore, tends to become a stream more or less parallel to the sun's equator. The leader spot is generally very well defined in outline, with its umbra dark and distinct; the rear spot is usually not so regular in form, or so dark as the leader, though it often is, for a time, the larger of the two. In the early days of the development of a stream, the leader moves rapidly forward from the rear spot, at the speed of about five miles a minute that is to say, five or six times the speed of an express train. The photosphere, or bright surface of the sun, appears to be heaped up in front of the leader, and to overflow the small spots

in the middle of the group, soon concealing them from sight. Then the rear spot begins to be engulfed, and many streams of bright matter cross it, forming "bridges," or flow down into it. Finally, the whole stream disappears, with the exception of the leader, which has now become large and circular. In many cases the leader now begins to move backwards towards the region from which it originally sprang; sometimes it breaks up into small fragments, but more often it simply contracts on itself, and so gradually disappears.

Groups of spots differ largely as to their size and duration. The largest group in my experience covered an area of 4000 millions of square miles; a thousand times the area of Australia or of Europe. The smallest spots which would be counted worthy of the name, would cover about a million square miles. Fullydeveloped circular spots very seldom exceed 600 millions of square miles in area, and more generally run to a quarter or a fifth of that extent. Whatever theory of the constitution of a sunspot is accepted, it remains marvellous that a formation covering hundreds of millions of square miles should be able to plough its way through the solar photosphere at such tremendous speed, without breaking up or suffering deformation of its outline. The differences in the duration of spots are equally marked; many spots only last a few minutes; groups of a hundred million miles or more in area frequently last a fortnight or more; and occasionally a group has been known to persist for half a year.

The nature of sunspots may be briefly explained in the following manner. The general surface of the sun is intensely hot and bright, its temperature being about 6000° Centigrade. It appears mottled in character, minute granules of intensest brilliance being thickly clustered on a slightly less luminous background. The bright granules are supposed by many to be luminous clouds, produced by the condensation of carbon at the sun's surface. Sunspots, then, are areas where these clouds have not been able to form or have been torn asunder; areas which offer a very distant analogy to the storms of our own atmosphere. Thus, in terrestrial storms, we have regions where great differences of atmospheric pressure are striving to adjust themselves; in sunspots, probably, we have regions where great differences of temperature are similarly in action. Consequently, sunspots are areas both of lowered temperature-say, 3500° Centigrade, instead of 6000°-and of unusual heat. The cooling is evidenced by the formation of compounds, the characteristic spectra of

which have been clearly recognised, such as, for instance, titanium oxide, magnesium hydride and water vapour, and the absorption due to these accounts for much of the apparent darkness of the spot. Round the spot the photosphere is noticeably brighter, and in cases of great activity, very brilliant "bridges" flow into the penumbra, and cross the umbra, and the spectroscope shows many of the dark lines strongly reversed over or near the spot; all symptoms of increased temperature.

The peculiarity of sunspots which has inevitably attracted the greatest attention, is the way in which the sun's activity waxes and wanes. Thus, in the year 1913, the sun's disc was free from spots on no fewer than 312 days, and the average daily spotted area for the entire sun was only 8 millions of square miles. In 1917, on the other hand, there were no days upon which no spots were seen, and in August of that year the average daily area under sunspots exceeded 2500 millions of square miles.

This immense change in the extent to which the solar surface is disturbed does not take place capriciously, but appears to follow a progression with many features of regularity. The interval of time from one quiet period to the next, or from one very active period to the next, generally exceeds ten years, and is less than twelve; on the average, the Sunspot Cycle, as it is called, is 111 years in length.

When Galileo first noticed sunspots, he was not content with observing their appearance and form, and the changes which they passed through, but he measured their apparent positions on the sun's disc from day to day, and in this way proved that the spots were not dark bodies floating between the earth and the sun, but were actually upon the sun's surface. From this he learned that the sun rotated on an axis, the position of which he soon determined. Then he learned that spots were not equally distributed over the sun's surface, but were found in zones, north and south of the equator; very few, and those but small, being detected at a distance from the equator of more than 40°. One of the most striking peculiarities of sunspots is that their distribution in latitude varies with the progress of the sunspot cycle. If we were to commence observations of the sun at a time when the solar activity was far advanced in decline, we should find that the spots were practically confined to the zone lying between north latitude 10° and south latitude 10°. Very soon, however, spots would begin to appear at about 30° from the equator, both north and south, and for a time three sunspot

zones would be in evidence-the zone on both sides of the equator which had been first seen, and a zone, the centre of which was about latitude 23° or 24° in the northern hemisphere, and a similar one in the southern. As time went on, spots would increase in number and size in the two zones last named, and would diminish in the equatorial zone, until the latter was left quite free from them, and only the two higher latitude zones would show any spots. In these, however, spots would multiply, but with an increasing tendency to form in lower latitudes. The greatest display of sunspots would take place when the mean latitudes of the spotted areas north and south would lie about 13° or 14° from the equator. After this, the spots would diminish both in size and numbers; their distance also from the equator would decrease, until in eleven years or thereabouts from the first observations practically all spots would be found within a single zone extending about 10° from the equator in both directions. This peculiarity of sunspots is of great interest and importance, as it seems to suggest that the spots must be due to causes lying within the body of the sun itself, and not to any outward influence of planets or combination of planets.

TABLE I.-Change of the Mean Spotted Area and of its Mean Distance from the Equator during the Progress of the Solar Cycle.

A third peculiarity of sunspots is that a different rotation period for the sun is found according to the latitude of spots under observation. Here on the earth, the same rotation period is obtained whatever be the latitude of the observing station; it is 23h. 56m. of time from the transit of a star across the meridian one night to its transit on the next night, whether the observation be made at the equator or at the polar circle or anywhere in between. In other words, the earth rotates on its axis like a solid body. The sun, on the other hand, so far as we can observe its rotation by means of its spots, does not rotate as a solid body, the time of rotation being shortest for the equator and lengthening out as we pass from the equator towards the poles. Thus, the mean siderial rotation period of the sun's equator is 24 65 days, but for latitude 35° it is two days longer.

But these values for the rotation period are averages only, obtained by taking the means from an immense number of independent groups. If we take groups in any one particular latitude, and treat them separately, we find that they differ very widely amongst themselves, so that there is nothing unusual in a high latitude group giving a shorter period than an equatorial group at the same time, or in the same group, whatever its latitude, giving quite different values for the rotation period at different times of its life history.

It has been already pointed out that some groups of spots attain amazing dimensions. Thus, the great sunspot group of 1892 February, had a total length of 166,000 miles, the principal spot being 93,000 miles in length. The breadth of the group was 65,000 miles, and its surface was 18 times as great as the superficial area of the entire earth. The sunspot group of 1917 August was even larger. These dimensions are enormous, but when we compare them with those of the sun itself, they appear quite small. The area of the group of 1892 February was only three parts in one thousand of the entire surface of the visible hemisphere of the sun. If the group had been absolutely black, radiating to us no light or heat at all, the sun's light and heat would have been diminished by only three parts in one thousand, but as the penumbra may be taken as being about two-thirds as bright as the disc, and the umbra as one-fourth as bright, the loss of light would be little more than one-part in a thousand. It must further be remembered that not only was the loss of light due to the spot small, but the spot itself only attained its greatest dimensions on one particular day; it was only visible.