秋分时日的卡斯蒂略金字塔，墨西哥奇琴伊察 Equinox at the Temple of Kukulcan in Chichen Itza, Mexico (© Somatuscani/Getty Images Plus)

秋分时日的卡斯蒂略金字塔，墨西哥奇琴伊察 Equinox at the Temple of Kukulcan in Chichen Itza, Mexico (© Somatuscani/Getty Images Plus)

Stepping into autumn

Look closely and you'll see a snake slithering down the steps of the Temple of Kukulcan (aka El Castillo or The Castle), in Chichen Itza, Mexico. Not a real snake, it's an image created by natural light and shadows only during the spring and fall equinoxes. The equinox (which means equal night in Latin) is either of the two times each year—like today, the first day of fall—when the Earth's orbit and position cause the Sun to pass directly over the equator, creating equal amounts of daylight and darkness. According to Mayan legend, on both equinoxes this pyramid is visited by Kukulcan, the feathered serpent god. Thousands of spectators gather to watch and celebrate as seven triangles of light slide down the pyramid, illustrating Kukulcan's descent.