SOLAR ACTIVITY AND THE EARTH Solar radiation is characterized by high stability. A drop of its intensity to one half its present value would reduce the temperature on the Earth much below the freezing point. All water bodies would turn to ice, and no life would be possible. A fourfold increase in the flux of solar heat and light would bring the oceans to the boiling point, and similarly destroy all life. However, geological findings show that life has existed on our planet for at least a billion years. Hence we come to the conclusion that during this entire period the flux of solar energy reaching the Earth did not change, or varied but insignificantly. Therefore the radiation of the Sun as a whole also remained constant or fairly constant.
Theoretical calculations show, in fact, that the solar radiation increased during this time by some 10%. This variation, of course, is not highly significant. It is moreover possible that the Earth's atmosphere, which greatly changed during the evolution of our planet, offset to a certain extent the increase in the radiation of the day luminary and that the flux of solar radiation reaching the surface of the Earth increased by less than one tenth.
Not all stars are as calm as the Sun. The radiation flux of variable stars changes considerably. It sharply increases when novae flare up; when supernovae erupt, their brightness increases in a very short time many millions of times.
Although the luminosity of the Sun remains fairly constant, its surface is by no means immutable. We have known of the appearance of dark spots on the Sun for a long time. In far antiquity our forefathers saw them with the naked eye, and the first written record of a sunspot observation dates back to 188 A. D. Observations of sunspots are recorded in ancient Chinese chronicles. One of the Russian annals mentions that in 1371 "spots dark like nails" were visible on the Sun. An analysis of perennial observations of the Sun carried out at the end of the first half of the last century showed that the number of sunspots on the solar surface varies from year to year, reaching a high approximately every 11 years. Distinct cyclic repetition was observed on the Sun. Most of these changes are quite rapid and some are violent.
A solar activity maximum was last observed in 1958, when a multitude of variable phenomena were recorded on the Sun. At about that time, in
1957-1958, the solar research in many countries followed a single program set for the International Geophysical Year (IGY). The day luminary was almost always in the field of vision of observers in one or several obser vatories, i. e. , almost continuous observations were made. The solar activity will reach a low, as the astronomers expect, in 1963-1964. Solar research in almost all the countries of the world will then again follow the program of the International Year of the Quiet Sun. According to some researchers, the minimum of solar activity will occur somewhat later, in the first quarter of 1965.
Having reached a maximum, the solar activity mostly decreases rapidly and no outstanding solar phenomena are observed. However, the 1958 peak was exceptionally high, the most powerful of all recorded until now. There fore, apparently, even by the middle of 1961 the Sun has not quieted down by any means.
Sunspots, which are relatively cold regions of the solar surface, have been known for so long because they are so easy to detect. While the temperature of the visible surface, the photosphere, reaches almost 6 thousand degrees, the temperature in a sunspot is a mere 4.5 thousand degrees. Although the sunspots in themselves are fairly bright, they seem dark in comparison with the intensely luminous surrounding surface. A sunspot can be regarded as some sort of a recess in the photosphere. The depth of this recess may reach several thousands of kilometers, but apparently no more than 10,000 km.
A large group of sunspots was discovered on 9 July 1961 at the Mountain Astronomical Station near Kislovodsk. Many of these spots measured up to 40,000 km in diameter, i. e. , more than thrice the diameter of our planet. Some spots may be up to 200,000 km long, i.e., their diameters are equal to half the distance to the Moon. According to observations of Soviet astrono mers, the largest number of sunspots during July 1961 was recorded on the 19th. The smallest number was observed on 30 July. The Wolf number, which characterizes the number of individual sunspots and spot groups*, was 144 for 19 July and 32 for 30 July.