254 ELEMENTS OF GENERAL SCIENCE 



creatures analogous to those on the earth is greater than in the 

 case of any other planet. Its year is only a little more than 

 half that of the earth, while it receives per unit area about 

 twice as much light and heat fBom the sun as does the earth. 

 The length of its day is not known, but if it is approximately 

 the same as that of the earth the conditions on Venus are in 

 a general way similar to those on the earth, the chief dif- 

 ference being that the seasons are shorter, while the climate 

 is considerably warmer. In fact, if the earth were at the 

 distance of Venus its average temperature would be 150 F. 

 instead of 60, as it is at present. 



Venus is the most conspicuous object in the heavens besides 

 the sun and moon. When it is at its greatest brilliancy, as 

 evening or morning star, it gives enough light to make build- 

 ings cast perceptible shadows. It is not always visible ; for 

 when it passes between the earth and the sun or around to 

 the part of its orbit opposite the sun, it is obscured, or lost 

 in the sun's more brilliant rays. Venus was an evening star 

 in the late autumn of 1917, and will be thereafter at intervals 

 of one year and seven months. Its appearance as morning 

 star is intermediate between its appearances as evening star. 



258. The red planet Mars. The best known of all the 

 planets is Mars. The reason for this is that it is relatively 

 near the earth and well situated for observation, and also that 

 it has a thin atmosphere, which permits us to obtain good 

 views of its surface. The diameter of the planet is about 

 4000 miles, or one half that of the earth. Conspicuous mark- 

 ings on its surface have been recorded for about two hundred 

 years, and it has been found by watching them that the day 

 of the planet is only a little longer than that of the earth. 

 Its axis of rotation, too, is inclined to the plane of its orbit 

 about the same as the axis of the earth is inclined to the 

 plane of its orbit. Consequently the day and night and sea- 

 sons of Mars are much like those of the earth except that the 

 seasons are twice as long. The principal difference is due 



