------------------+------------------------------------------------+ PLANETS | STARS | -------+----------+-------------------+-------------------+--------+ | | | |Distance| Name | Distance | Name | Distance |(light- | | (miles) | | (miles) | years)| -------+----------+-------------------+-------------------+--------+ Venus |26,000,000| { Proxima Centauri| 25,000,000 million| 4·27 | | | { α Centauri | | 4·31 | | | | | | Mars |35,000,000| Munich 15040 | 36,000,000 ” | 6·06 | | | | | | | | { Wolf 359 | 47,000,000 ” | 8·07 | Mercury|47,000,000| { Lalande 21185 | 49,000,000 ” | 8·33 | | | { Sirius | 51,000,000 ” | 8·65 | -------+----------+-------------------+-------------------+--------+ As it is almost impossible to visualise a million, the mere statement that the stars are a million times as remote as the planets gives only a feeble indication of the immensity of the gap that divides the solar system from its nearest neighbours in space. Perhaps the apparent fixity of the stars gives a more vivid impression. The earth performs its yearly journey round the sun at a speed of about 18½ miles a second, which is about 1200 times the speed of an express train. The sun moves through the stars at nearly the same rate—to be precise, at about 800 times the speed of an express train. And, broadly speaking, the nearer planets and the majority of the stars move with similar speeds. We shall not obtain a bad approximation to the truth if we imagine that all astronomical bodies move with exactly equal speeds, let us say, to fix our thoughts, a speed equal to 1000 times the speed of an express train. The distances of astronomical objects are now betrayed by the speed with which they appear to move across the sky—the slower their apparent motion the greater their distances, and _vice versa_. Now the planets move across the sky so rapidly that it is quite easy to detect their motion from night to night and even from hour to hour; the stars move so slowly that, except with telescopic aid, no motion can be detected from generation to generation, or even from age to age. Even the conspicuous constellations in the sky, which on the whole are formed of the nearer stars, have retained their present appearance throughout the whole of historic times. The contrast between the planets which change their positions every hour, and the stars which fail to shew any appreciable change in a century, gives a vivid impression of the extent to which the stars are more distant than the planets.