Untitled_one half mile_ away. The hours, days, and years are the measures oftime as we use them; that is: Jupiter's "Day" (one rotation of theplanet) is made in ten of _our hours_; Mercury's "Year" (one revolutionof 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 tothe Sun.][Illustration: THE COMPARATIVE SIZES OF THE SUN AND THE PLANETS (Drawnapproximately to scale)On this scale the Sun would be 17-1/2 inches in diameter; it is fargreater than all the planets put together. Jupiter, in turn, is greaterthan all the other planets put together.]Except when the winds rise to a high speed, we seem to live in a verytranquil world. At night, when the glare of the sun passes out of ouratmosphere, the stars and planets seem to move across the heavens with astately and solemn slowness. It was one of the first discoveries ofmodern astronomy that this movement is only apparent. The apparent
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