Untitled _one half mile_ away. The hours, days, and years are the measures of time as we use them; that is: Jupiter's "Day" (one rotation of the planet) is made in ten of _our hours_; Mercury's "Year" (one revolution of the planet around the Sun) is eighty-eight of _our days_. Mercury's "Day" and "Year" are the same. This planet turns always the same side to the Sun.] [Illustration: THE COMPARATIVE SIZES OF THE SUN AND THE PLANETS (Drawn approximately to scale) On this scale the Sun would be 17-1/2 inches in diameter; it is far greater than all the planets put together. Jupiter, in turn, is greater than all the other planets put together.] Except when the winds rise to a high speed, we seem to live in a very tranquil world. At night, when the glare of the sun passes out of our atmosphere, the stars and planets seem to move across the heavens with a stately and solemn slowness. It was one of the first discoveries of modern astronomy that this movement is only apparent. The apparent creeping of the stars across the heavens at night is accounted for by
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