SUN 



805 



the surface of a large mass of molten iron in an 

 open mould gives a fair idea of this process. It 

 must always be remembered, when vapour or gas is 

 spoken of as at the sun's surface, that the enor- 

 mous temperatures and pressures there prevailing, 

 with the scale on which these must vary in short 

 intervals of time, will make vapours behave much 

 more like terrestrial solids than like gases as we 

 know them. The impact of a small jet of solar 

 ' vapour ' would in fact be far more powerful than 

 that of a projectile from a 100-ton gun. The 

 rapidity of these convection currents must there- 

 fore be enormous, and a little careful watching 

 soon shows that the whole solar surface is in a 

 state of constant change. 



In certain regions of the photosphere, between 

 6 and 35 solar latitude, both north and south of 

 the solar equator, large black spots are frequently 

 observed. In size these vary from 150,000 miles in 

 largest diameter to small black dots approaching 

 in appearance the 'pores.' The largest are easily 

 seen by the naked eye when fog or dark glass pro- 

 tects it from the excessive solar glare. The activity 

 of their producing cause is subject to a considerable 

 variation. Schwabe of Dessau in 1843 announced 



Fig. 2. Sun-spots. From a photograph taken February 

 13, 1892, 9 hours 47 minutes. By permission of the 

 Astronomer-royal. The centre of the sun's disc is 

 at a. 



the discovery of this important fact, giving about 

 ten years as its period. Wolf in 1852 corrected 

 this to 11 Ml years. This is generally accepted as 

 the mean period, but individual periods may vary 

 from it considerably. The shortest periods are 

 the most intense. There is an undoubted con- 

 nection between this period and that of terres- 

 trial magnetic phenomena. Aurorse and sun-spots 

 wax and wane together, even in their smaller 

 fluctuations. But the theory that sun-spots depend 



for their frequency on the influence and position of 

 the planets has had to be abandoned. These spots 

 usually have three well-marked areas, distinguished 

 by their different degrees of blackness. The pen- 

 umbra forms the outer border of the spot, and is 

 only grayish compared with the general white of 

 the solar surface. Within this is a much darker 

 area called the umbra, and within this a still blacker 

 spot, the nucleus. While sometimes persisting 

 for months, spots frequently vanish or form in a 

 few days, sometimes even in a few hours. They 

 are the theatre of constant changes. Long fila- 

 ments are often extended from the penumbra across 

 the umbra, forming 'bridges.' In fact the whole 

 penumbra appears filamentary in structure, being 

 composed of the 'granules' drawn inwards from 

 the edge by the force in the spot. Cyclonic move- 

 ments have been observed in spots, but are not 

 usual. The spot is most probably a cavity formed 

 in the photosphere by the pressure of a vast 

 descending mass of vapour. In spot latitudes, for 

 pome unknown reason, these masses collect in 

 unusual size, not descending by the minute ' pores,' 

 but requiring larger openings. Both spots and 

 pores appear dark, not because the uncovered 

 lower solar layers are cooler than those above, 

 but because tlie cool masses of vapour pressing 

 on them from above absorb their light, and pre- 

 vent it reaching us. In reality their blackness 

 or grayness is only such as compared with the 

 intensely white photosphere. It is almost certain 

 that these absorbing vapours are considerably 

 cooler than the neighbouring surface. Great differ- 

 ences of pressure, as well as of temperature, exist 

 in spots. Hence they are accompanied by (or 

 accompany) great disturbances and fierce vapour 

 currents. These affect even the earth, and cause 

 simultaneous disturbances in our magnetic needles. 

 White ridges (called facitlce) are raised in the 

 neighbourhood of spots, indicating enormous pres- 

 sures, and spreading often over a wide area of the 

 solar surface. The spectra of sun-spots are most 

 complex. Their meaning cannot yet be said to be 

 fully understood. But they give certain evidence 

 of vapour movements of enormous rapidity, and of 

 pressures on a like scale. One example of this 

 occurred in the great spot of June 1889, when a 

 dark spectral line of iron was widened to five times 

 its usual thickness, indicating an immense pressure. 

 Displacements of lines from their normal position 

 have also been observed, indicating vertical vapour 

 movements at a velocity as high as 320 miles per 

 second. 



During total solar eclipses certain solar pheno- 

 mena become visible, which bear closely on the 

 problem of the sun's physical condition. Chief 

 among these are the corona, prominences, and 

 chromosphere. The last (sometimes called the 

 sierra) surrounds the sun completely. It consists 

 of a layer of vapours covering the entire photo- 

 sphere. Its depth varies at different times and in 

 different parts, ranging from about 6000 to 2000 

 miles. As seen in eclipses it is of a beautiful rosy 

 hue, and its surface, seen in profile at the edge of 

 the solar disc, appears sharply jagged and broken 

 into waves or spear-like jets of varied altitude. It 

 consists chiefly of hydrogen and an element, till 1895 

 unknown in our laboratories, called ' helium.' ( See 

 the article ARGON ). Sometimes heavier vapours, as 

 of iron, calcium, titanium, magnesium, and others, 

 are projected into it from the true solar atmo- 

 sphere below. There is indeed no marked border 

 between these groups of gases other than a fluc- 

 tuating one due to their varied weight. The 

 chromosphere rises often in local jets of rosy gas 

 to an enormous altitude. These form the pro- 

 minences, first recorded as seen at an eclipse by 

 Captain Stannyan, who observed at Bern, Switzer- 



