An ancient Maya astronomical observatory at the Chichen Itza archaeological site on Mexico's Yucatan Peninsula, built around AD 906 during the Post Classic period. Its spiral interior staircase gives the structure its Spanish name, meaning snail; Maya astronomers used its elevated, unobstructed sightlines to track the planet Venus, recording that it completed a full cycle of appearances in about 584 days and that five such cycles equaled eight solar years, knowledge encoded in the building's own alignment.
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