This Day in World History
Marking the autumnal equinox in the ancient world
Sometime around September 23 each year, Earth reaches the autumnal equinox, the point when the sun stands directly above the Equator and daylight and dark are roughly equal. (The day, of course, marks the autumnal equinox in the Northern Hemisphere. South of the Equator, it is the vernal, or spring, equinox. March 23 is the spring equinox in the Northern Hemisphere and the autumnal equinox in the Southern.)
These astronomical events did not go unnoticed by ancient peoples. The people of ancient Mesopotamia, or modern Iraq, divided the year into two six-month seasons, each beginning on an equinox. The Chinese celebrated the Mid-Autumn Festival around the time of the autumnal equinox three millennia ago.
Temples and observatories offer some of our best evidence for the importance societies in the ancient world placed on time reckoning. An astronomical observatory in Kenya, carbon-dated to 300 BCE, is the earliest known example of the so-called Borana lunar calendar of 354 days. It is still used today by the Kushite herders of East Africa. Another notable example is the Mayan temple complex known as El Castillo (“The Castle”) at, in the Yucatán peninsula of Mexico that is nearly 1,500 years old. On the autumnal and vernal equinoxes, the shadows cast by the stones that flank the west-facing staircase seem to form a diamond-back snake shape that appears to undulate down the stairs as the sun sets. While the phenomenon might be a happy accident, the pyramid is also adorned with stone sculptures showing the Mayan serpent god, possibly suggesting that the effect is deliberate. Today, thousands flock to the site each year at the fall equinox to see the effect.
The observatory is another important building at Chichén Itzá. This domed building was aligned with the northern extreme of the path of Venus. Here, astronomers made the lengthy and systematic observations necessary to coordinate the three different calendars that were in use in Mesoamerica from about 500 BCE, one of which is still used today by the inhabitants of southern Mexico.