Mayan astronomers knew from naked-eye observations that Venus appeared on the western and disappeared on the eastern horizons at different times in the year, and that it took 584 days to complete one cycle.
They also knew that five of these Venus cycles equaled eight solar years.Venus would therefore make an appearance at the northerly and southerly extremes at eight-year intervals.
Of 29 possible astronomical events (eclipses, equinoxes, solstices, etc.) believed to be of interest to the Mesoamerican residents of Chichén Itzá, sight lines for 20 can be found in the structure. Since a portion of the tower resting on El Caracol has been lost, it is possible that the other measurements will never be ascertained to have been observed.