Like all of the best gods, Kukulkan is not visible most of the time. His head, a gaping maw of serpent badassery, is fixed in place at the base of his temple in southern Mexico. Most days, his body is absent. Twice a year, a one-two punch of astronomical reality and Mayan insight come together, creating the massive, undulating body of a serpent out of shadow and stone. Equinoxes are the great astronomical equalizers. Halfway between the solstices, the Earth’s tilt is neither towards nor away from the Sun. All of us, regardless of where we are on our shared planet, experience similar lengths of day and night. It is not quite true to say that day and night are equal on the equinox, however, for reasons both trivial and profound. With the help of an atmosphere, you see, light spreads into darkness, but darkness does not spread into light. At the solstices, we are at astronomical extremes, the position of the Earth such that half of the planet is tilted most directly towards the sun, the …