The Observatory at Chichén Itzá is called el Caracol (or snail in Spanish) because it has an interior staircase that spirals upward like a snail's shell. The round, concentrically-vaulted Caracol was built and rebuilt several times over its use, in part, scholars believe, to calibrate the astronomical observations. The first structure was probably built here during the transition period of the late 9th century and consisted of a large rectangular platform with a stairway on its west side. A round tower of about 48 feet high was built atop the platform, with a solid lower body, a central portion with two circular galleries and a spiral staircase and an observation chamber on the top. Later, a circular and then a rectangular platform was added. The windows in the Caracol point in the cardinal and subcardinal directions and are believed to enable the tracking of the movement of Venus, the Pleides, the sun and the moon and other celestial events.