168 



THE NATIONAL GEOGRAPHIC MAGAZINE 



Venus was an unusually interesting ob- 

 ject in the sky during July of this year. 

 Not again until February, 192 1, will it 

 appear as bright and fair in the evening 

 sky. It has phases like the moon, and 

 these can be seen even through a good 

 field-glass. Its day is believed to be the 

 same length as its year, which is 224 of 

 our days. 



WIIvL A STAR FORETELL OUR WEATHER? 



Mars always challenges interest. Its 

 day is about the same length as ours, but 

 its year is nearly twice as long. Al- 

 though astronomers generally take less 

 interest than laymen in the surmise as to 

 whether other planets^ and stars are in- 

 habited, since they, more than laymen, 

 realize that this is a problem that must 

 in all human probability remain un- 

 solved, the question is more often asked 

 about Mars than any other planet. 



It is quite generally believed that Mars 

 has ice-capped poles. The telescope re- 

 veals white spots at the poles that have 

 every appearance of being like our ocean 

 Polar region. They advance toward the 

 Equator in winter and retreat in sum- 

 mer. In the summer of 1916, Pickering, 

 who, with Lowell, has led the school of 

 astronomers who believe they can see 

 canals on Mars, said that he found the 

 white caps stretching farther down to- 

 ward the Equator than he had ever seen 

 them before. 



He said that if there was any connec- 

 tion between the weather of Mars and 

 that of the earth, the winter of 1916-17 

 would be the coldest in many years. 

 And it was. May it yet be possible to 

 do long-range weather forecasting on 

 the earth by studying the waxing and 

 the waning of the ice-cap on the South 

 Pole of Mars ? 



Swinging around the sun at a distance 

 five times as remote as that which sepa- 

 rates the earth from the source of its 

 light, having a year nearly twelve times 

 as long as ours and a day less than half 

 as long, Jupiter is as much bigger than the 

 earth as a tangerine is larger than a pea. 

 1 le lias nine satellites, seven of them re- 

 volving around him in one direction, the 

 other two pursuing an opposite course. 

 Saturn, with its wonderful rings, is one 

 of the finest objects in all the skies 



through a telescope of even moderate 

 size. Uranus is barely visible to the 

 naked eye, while Neptune (see page 157) 

 can be seen only with a telescope. 



Whether studied as the head of the 

 planetary family to which the earth be- 

 longs, or whether as an average member 

 of the great household of suns that dwell 

 in the distant skies, Old Sol has many 

 thrills for the student. 



To the inhabitants of the earth the fact 

 that he shines is the most important phys- 

 ical consideration in life. From him we 

 derive warmth, light, and power ; without 

 him the oceans and even the air itself 

 would freeze ; and, of course, under such 

 conditions, life would be impossible. 



TIES THAT BIND 



With what firm ties he holds his family 

 together well-nigh defies the imagination. 



Prof. Charles G. Abbot estimates that 

 a steel column five hundred miles thick 

 would be required to keep Neptune in its 

 path around the sun if the force of 

 gravity were removed. Sir Oliver Lodge 

 has estimated that the pull between the 

 components of the double star Beta Au-- 

 rigae is twenty million times as great as 

 the force that keeps the earth in its path. 



Prof. F. R. Moulton says that the heat 

 that reaches us from the sun amounts to 

 more than two trillion horsepower, in 

 spite of the fact that two billion horse- 

 power goes off into space for every single 

 horsepower that comes to the earth itself. 



While the stars appear to us about as 

 much like the sun as the fireflies of a 

 summer night, yet the patient investiga- 

 tions of astronomers show not only that 

 the sun is a star, but that it is by no 

 means either the largest or the brightest 

 of the celestial family. 



Assured that it is a star and know- 

 ing that the next nearest one is three 

 hundred thousand times as far away, 

 astronomers addressed themselves to the 

 task of learning about the other stars 

 by studying our own. They found that 

 there are some like it, giving out the 

 same kind of light, though most of them 

 send us, through the spectroscope, mes- 

 sages that tell quite different stories. 



With the fundamental facts about the 

 sun in hand, most astronomers are now 

 engaged on star studies. A photographic 



