Hildebrandson makes the objection that Tycho Brahe’s observations were confined to an abnormally cold period judging by the tables Speerschneider has prepared showing the ice formation in Danish navigable waters. Nine out of the sixteen years in which Tycho Brahe gathered his data were notable for abnormally cold winters, while only nineteen out of the hundred years composing the sixteenth century were characterized by equally severe winters.
Thus, the conclusion is not warranted that the winters of the sixteenth century as a whole averaged colder than those of the nineteenth. Later investigations (in 1917) in regard to the dates when the ice would break up in Lake Mälar at Vesterås, in Neva River at Petrograd, and in Dwina River at Riga, have led Ekholm to believe that he has found a periodicity in winter temperatures of not less than 212 years, a conclusion which would agree with Speerschneider’s statistics. If so, we are at present living in a period remarkable for its mild winters while a series of extremely severe winter seasons occurred at the time of Tycho Brahe. This law would also have a bearing on the preceding table of temperatures in Stockholm, Lund, London, and Paris as a succession of severe winters came around in the beginning of the nineteenth century while the reverse is true towards its end. On the whole climatic variations during historic time have been insignificant, if present at all, provided we extend our comparisons over two or more centuries. Such is also the opinion of Hildebrandson.
The idea of a slow deterioration of the climate due to increasing desiccation is of old lineage and is most likely related to the venerable conception of a bygone golden age. Aristotle even at that early date believed that a gradual arefaction of the Earth took place. In recent times this faith has been particularly fostered by Huntington in a number of treatises where he endeavours to prove that Asia, represented for instance by Palestine, Syria, and Persia, and further Africa and North America are subject to a rapid exsiccation clearly traceable through historic time. The contrary, however, is true about Western Europe. It was often said that Southern Russia in recent times suffered a slow arefaction manifesting itself in the formation of steppes. This led to careful investigations showing the assertion to be erroneous and culminating in the work of Leo Berg. Rather, a slight shifting in the opposite direction is detectable as the forest region has expanded at the expense of the steppes in conformity with the development toward the end of prehistoric time. The renowned American astronomer, the late Dr. Lowell, embraced the idea of increasing aridity which he observed himself in Arizona where his observatory is located. The drying out of Arizona undoubtedly took place in long bygone prehistoric time. The disappearance of a high culture in Syria and Mesopotamia was the result of hostile devastation of their waterworks; a compensation is now offered in the reclamation of the deserts along the Nile, in California and Arizona, and in numerous other places.
CHAPTER IV
ATMOSPHERE AND PHYSICS OF THE STELLAR BODIES
In a certain sense, we are justified in speaking of an atmosphere of suns and stars. These bodies consist mainly of a comparatively dense mass surrounded by a layer of very attenuated gas. The density of our Sun is about 1.4 times that of water. In other stars it is considerably lower, in some cases only a few hundredths of the density of water. This applies particularly to those stars of variable magnitude, the Cepheid type, named after their longest and best known representative, the mysterious star Delta in the constellation Cepheus, and in general to young stars. In any case the stars are gaseous throughout on account of their high temperature. An exception must be made for the clouds of matter precipitated by easily condensed vapours, such as gaseous carbon, which clouds float in the outer strata, and are responsible for the bright astral light.
The stars just mentioned belong to the comparatively young stellar bodies, while the Sun in common with other yellow stars is considerably older. Correlated with their age is undoubtedly the greater mean density of the yellow stars. Around many of the young stars, for instance around the brilliant Altair, the principal member of the constellation Aquila, a gas shell of great expansion has been observed, usually consisting of hydrogen but frequently also of helium. These extensive gas appendages may be considered as a kind of atmosphere surrounding the stars in question. Their density is no doubt exceedingly small. Our own central orb, the Sun, is also endowed with rarefied gases outside of the luminous clouds. By their absorption of light they cause the dark lines in the Sun’s spectrum, after their discoverer named Fraunhofer lines. The greatest height from the surface of the Sun is reached by hydrogen, mixed with a small quantity of helium and with a gas, unknown on Earth, which we call coronium because it has been observed in the Sun’s corona. These gases may be looked upon as the atmosphere of the Sun.
Similar conditions obtain no doubt on the major planets, which possess a density not essentially different from that of the Sun. They have, moreover, practically the same period of revolution around their own axis, Jupiter 9.9 hours, Saturn 10.3, and Uranus (probably) 10.8 hours. Judging by their density, they are, in all probability, like the Sun, gaseous throughout except for the heavy cloud formations which appear to constitute the outer limit of these stellar bodies. Their interior, like that of the Sun, may contain comparatively sluggish gas-masses, as certain peculiar patches appear on their exterior, similar to the sunspots and persisting for long intervals, sometimes over a year. The best known example of the kind is the so-called red spot on Jupiter, which has remained since 1878, although it is not so pronounced now as during its early days (see [Fig. 11]). Characteristic for these planets are certain bands of a marked delineation and running parallel with the equator (see [Figs. 11] and [12]). They are caused by the rapid peripheral motion of these planets, with Jupiter 28 and with Saturn 24 times that of the Earth.
Fig. 11. The planet Jupiter in 1909 in Mercator’s projection by F. le Coultre of Geneva. The “red spot” over which the clouds are dispersed, is found at 355° Long. and 20° S. Lat. in the outcurving of the dark band. South direction is upward, as on all pictures obtained with astronomical tubes.