VENUS AND MARS. 



523 



vapour exists around, and water in, Venus. No satellite can be found, though 



the ancients reported such an attendant upon this planet. 



The apparent diameter of Venus varies considerably in consequence of 



her varying distances at the inferior and superior 



conjunction. When nearest the earth, if she pre- 

 sented her fully illuminated disc to our gaze, we 



should see a miniature moon, and even under the 



circumstances Venus throws a shadow, so brilliant 



is her light. 



The transits of Venus have been referred to, 



and, like those of Mercury, are simply a passing, 



or a transit," of the planet across the illuminated 



disc of the sun. The transits afford means to Fig. S79 .-Venus, at quadrature. 



ascertain the volume and distance, etc., of the sun, and this year (1882) the 



next transit is expected. There will not be another for more than one 



hundred years. 



Whether Venus has a constitution similar to our globe is of course 

 doubtful. The matter is less dense than the earth, 

 and there is an atmosphere half as dense again as 

 ours. Spots have been noticed crossing the 

 planet, which may have been vapours or clouds, 

 and the rotation of Venus on its axis was calcu- 

 lated from these spots as being 23 h 2i m 22 s . 

 The seasons in Venus must be very different from 

 ours, as her inclination is greater than our earth, 

 and as the sun is so much nearer to her than to 

 us her tropical and polar regions are close, and 

 a vertical sun is scarcely enjoyed by two places 

 for three successive days, and she may have two. 



winters and summers, two springs and autumns ! 



Fig. 580. Venus, near inferior 

 conjunction. 



MARS. 



Having already considered the earth, we pass on from Venus to Mars. 

 The orbit of the latter planet is exterior to the earth's, as is proved by his 

 never appearing " horned," nor ever passing across the sun's disc. Therefore 

 no " transits " of Mars can take place as transits of Venus and Mercury. 



Yet Mars is most favourably situated for astronomical observation by 

 us, because it turns its full disc to us. Venus is nearer to us than Mars 

 but, as we have explained, when she comes nearest to us she is quite invisible. 

 Astronomers have been enabled to ascertain a good deal concerning the 

 planet of war " the red planet Mars." 



Mars has been considered very like the earth. We perceive seas and 

 continents, and the shape of Mars is like the earth. But our globe is larger 

 than Mars, which is much less dense, so the force of gravitation is less also. 



