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23.1: Venus and Mercury

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    The five planets known to the ancients were named after principal Greek gods, later replaced by their Roman equivalents: Mercury, Venus, Mars, Jupiter and Saturn. They were relatively bright -- Venus and Jupiter can be brighter than any fixed star -- though their brightness seemed to vary. Venus and Mercury never appear far from the Sun and (outside the polar regions, at least) are only visible just after sunset or before sunrise, suggesting that those planets were confined near the Sun. The Greeks called Venus “Hesperus" when it appeared as the evening star and “Phosphorus" when it rose before sunrise as the morning star, though they realized both were the same object. Mercury, which is fainter and closer to the Sun, is particularly hard to detect by eye and is visible only when its position is far from the Sun's.

    All planets seemed to move among the stars in the same direction as the Moon (and of the Sun) -- with one strange variation: sometimes their apparent motion is temporarily reversed (“retrograde motion"). That is most evident with Mercury and Venus, which shuttle back and forth across the position of the Sun. As the Sun moved among the stars -- along the constellations of the zodiac -- these planets sometimes move the same way and add their motion to that of the Sun, but sometimes their apparent motion opposes the one of the Sun, causing them to seem to move backwards or in “retrograde."

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