On September 21st and March 21st you can witness the incredible accuracy of Maya astronomy as it was integrated into their architecture, and share a moment in time with the ancient Maya.
The equinox is the moment when the earth is illuminated by the sun in equal form in both the southern and northern hemispheres.
The Maya, stargazers that they were, planned their lives around the sun; their daily lives centered on the sowing and harvesting of their crops.
At the spring equinox they planted the crops, and the autumn equinox was their signal to begin the harvest.
The Maya built complex structures using advanced geometry and astronomy to map the cycles of the sun. During the equinox the sun casts its rays on the Chichén Itzá pyramid during the afternoon, forming seven isosceles triangles that resemble the body of a serpent 37 yards long slithering downwards until it joins the huge serpent’s head carved in stone at the bottom of the stairway.
It is said this snake is trying to make it to the well of sacrifice which is in the same direction. This phenomenon expresses the amazing cultural advancement of the Maya, for it supposes the amalgamation of astronomy, mathematics, chronology, geometry, and religion. You can also see the equinox display at the site of Dzibilchaltún, at dawn on the mornings of September 21 and March 21, in the temple of “Las Siete Muñecas” (seven dolls).
The site opens at 4 am on these two days of the year only. Because the equinox event at Chichén Itzá takes place in the afternoon, it is possible to see both events on the same day.
The phenomenon in Chichén Itzá can actually be seen a day or two before and after the official equinox date as well. http://yucatantoday.com/autumn-equinox/?lang=en