8 OUR PHYSICAL WORLD 



Since Mercury and Venus are nearer the sun than is the earth, 

 their orbits are included within that of the earth, and they can 

 never appear on the opposite side of the earth from the sun, but 

 are always seen near the sun, either rising just ahead of it, 

 when they are called " morning stars," or setting shortly after it, 

 when they are known as " evening stars." Mercury is as bril- 

 liant as its namesake, the liquid metal familiar in the thermometer 

 bulb. Its orbit is so small that it is usually obscured by the sun's 

 intense light, since it can never get far from it. Venus, however, 

 with its larger orbit, may precede the sun or follow it at greater 

 distance, and therefore is not commonly obliterated by the glare 

 of the sun when it is a morning or evening star. Shining as it does 

 with a silvery sheen, it has ever been a noted object in the sky, and 

 may even be seen by day when one knows just where to look for it. 



Mars glows with a ruddy light. Its blood-red appearance has 

 always associated it with war. Mars was the war-god. While 

 the surfaces of Mercury and Venus can never be studied with our 

 telescopes very satisfactorily because they are so near the sun, 

 Mars may be seen at times with a round disk, like a full moon; 

 and since it is our next-door neighbor, distant when nearest to us 

 only about half as far as the sun, its surface is plainly visible. 

 What appear to be polar snow caps may be seen, which increase 

 and decrease in size as the seasons change. Numerous straight 

 markings radiate from the Pole in various directions, often 

 intersecting. These have been thought by some astronomers to 

 indicate a complicated system of canals built by the inhabitants 

 of the planet to conduct water from the melting polar snows to 

 irrigate their lands, or possibly since they change color seasonally 

 they are lines of vegetation along such canals or along areas of 

 maximum rainfall. Since Mars is much smaller than the earth, 

 the force of gravity on its surface is only 38 per cent of that on 

 the earth, so that an object weighing 100 pounds here would 

 weigh only 38 pounds there. The inhabitants may grow, there- 

 fore, proportionately larger, and these giants might really dig 

 such great canals, since the material excavated would be so 



