Evocative and impressive at any time of year, the massive, well preserved ruins of Chichen Itza make for an unforgettable day trip into the world of the ancient Maya. The site upon which Chichen Itza stands has been occupied for millennia, thanks to the deep “cenotes,” or natural limestone wells, that provide a city’s worth of water during the long, dry, Yucatán summer. By 600 AD, it was one of the most powerful cities, planned along astronomical lines, in the Mayan Empire.

The Mayans are well known for their mathematical and observational prowess—most famously, for their cyclical calendar often (mis)interpreted as predicting the end of the “Fifth Sun” on December 21, 2012. While that particular prophecy is probably a silly (but profitable) hoax, there is no question that the fantastic cities rising from the Yucatán Peninsula use sacred architecture to reflect the astounding astronomical alignments of Mayan cosmology.

The most famous of these remarkable planned structures can be found at Chichen Itza, the Temple of Kukulkán. Convenient to popular tourist destinations along the Yucatán Coast, the site attracts crowds of curiosity seekers on March 20 and September 23, the spring and fall equinoxes. Like many cultures throughout the world, the equinoxes, when day and night are exactly the same length, were sacred to the Mayans.

Beginning at around 3:30 PM on either equinox, the setting sun illuminates El Castillo with a serpentine glow. The serrated staircase begins casting shadows that undulate toward the base of the pyramid, where the sculpted stone heads of Kukulkán, the Feathered Serpent (called Quetzalcoatl in Aztec cosmology), seem to lead this ephemeral illusion into the heart of the ruins.

The scene was supposedly designed by the original architects to depict the descent of Kukulkán from the heavens. The effect was first noted in the late 1940s by photographer Laura Gilpin, and archeologists have since theorized that the deity would have continued its path past the “Platform of Venus, along the Sacred Way, and to the small temple on the brink of the sinkhole known as the Sacred Cenote.” There, scientists have found the remains of gold, jade, and even human sacrifices, perhaps made as part of this biannual celebration.

Visiting Chichen Itza on the equinox itself can be something of a circus—crowds of up to 5,000 people are expected, so don’t count on a great view. The phenomenon, however, is visible for about a week on either side of the equinoxes, when you’ll enjoy smaller crowds and better vistas. You can book a trip through Viator from either Cancún or any hotel along the Riviera Maya. Or stay overnight in the nearby town of Valladolid —but whatever your plans, be sure to make reservations well in advance during the equinox festivities.