Hi Brian,

Observation, observation, observation. The people who built ancient sites such as Stonehenge in England or Chichen Itza in Mexico were careful observers of the skies. They would observe the motions of the Sun, moon, stars and planets over long periods of time. We know that some cultures, such as the Maya kept records of their observations in writing. In building monuments that were aligned with celestial objects, there were sometimes different phases of building in which alignments were added or made more accurate.

-Nancy Ali