The Autumn Equinox in Mexico: The Serpent That Crawls Down the Pyramid at Chichén Itzá
A colleague in Mexico City sent me a picture yesterday: on the steps of a pyramid, seven bright triangles were laid out at a slant, and the lowest one lined up exactly with the carved stone serpent head on the ground, so that it looked like a glowing snake crawling down from the top of the tower.
The caption under the picture read: autumn equinox.
On the Day of the Autumn Equinox, Mexico Is Still on 22 September
The 2026 autumn equinox occurs at 00:05 UTC on 23 September (08:04:56 Beijing time; 18:05 Central Time in Mexico on 22 September) — right around sunset. So while we here write "autumn equinox, 23 September", the people on the Yucatán Peninsula are living through the evening of 22 September.
The Kukulcán Pyramid at Chichén Itzá (also called the Temple of the Feathered Serpent) was built for precisely these few days: the pyramid body has nine tiers in all, and under the setting sun the corners of those tiers cast shadows that fall on the side face of the northern staircase and are cut into seven inverted triangles. With each step the sunlight retreats, the patch of light moves down one notch, and the whole "snake" crawls from top to bottom until it sinks entirely into the serpent head at the base.
The phenomenon appears every year around the spring and autumn equinoxes. As for the exact window, local accounts differ: the park's official blog writes "at sunset", while media reports say it can be seen at any point between 3 p.m. and 4:30 p.m. — go by the park's arrangements for the day. In other words, the building itself writes the answer to the date: when the shadow starts moving, and from which notch, tells you at a glance that you have reached the midpoint of the season.
What Locals Are Doing These Days
According to information published on the Mexican side, Chichén Itzá receives a large number of visitors around the autumn equinox. The local heritage authorities put in place a special operating plan for the spring and autumn equinoxes every year — for the 2026 spring equinox, for instance, operations covered the three days of 20–22 March; and according to Infobae, the 2025 spring equinox event received 32,872 people. What everyone is doing is in fact a very simple thing: standing in front of the staircase, waiting to see when the shadow starts walking down.
I rather like this form of festival: not reading an almanac, but standing in front of a building watching the light. The calendar here is not printed out, it is built.
The "Shadow" on Our Side
Eastern systems have a similar line of thinking: the ancients used the gnomon to measure the length of the sun's shadow in order to fix the solar terms — longest at the winter solstice, shortest at the summer solstice, and those few inches on the ruler are the position of the season. This method and the staircase of Chichén Itzá come from the same craft: writing astronomy into something visible, one written into stone steps, one written into a measuring rod.
If you want to look at this system from the angle of "casting a chart for the sky", you could read two pieces: Qi Zheng Si Yu: A Chinese Star Chart of Sun, Moon and Planets explains how a Chinese-style star chart is drawn from the sun, moon, five planets and the four remnants; Western Astrology Chart: Planets, Signs, Houses and Aspects explains a different set of coordinates. Both systems accept the same thing — that positions in the sky carry meaning — and they differ in how they are read.
Closing
My colleague said they stood below the pyramid until it grew dark, and only when the serpent's shadow disappeared did the crowd disperse. She asked me: why insist on seeing this one thing? I said it is probably because these are the two most typical days of the year for seeing it, and people are animals who like to "confirm the time": only once you have seen it does the season really turn.
If you also have something you want to confirm, on 占卜迷指 it works like this: you describe your trouble in natural language, the system recommends a suitable method, casts the chart and gives a reading, and you can export it to keep and compare against slowly.
Information current as of 23 September 2026 (China Standard Time).
Sources
- Official Chichén Itzá visitor website blog (2026-09-02): the Chichén Itzá autumn equinox in September (00:06 UTC on 23 September, 18:05 Central Time in Mexico on 22 September; nine tiers of shadow forming seven inverted triangles)
- El Informador (2026-09-19): Autumn equinox, Chichén Itzá prepares to welcome Kukulcán (locally expected to be visible on 20 and 21 September, between 3:00 and 4:30 p.m.)
- Infobae (2026-03-12): INAH runs a special operation for the 2026 spring equinox (operations 20–22 March; the 2025 spring equinox event received 32,872 people)
- The moment of the autumn equinox (00:05 UTC / 08:04:56 Beijing time): see the other article in this batch, 《南半球在过春天》 ("Spring Is Underway in the Southern Hemisphere"), which cites the 2026 autumn equinox time lookup and the Brazilian National Observatory
(Note: the park's official blog gives 00:06 UTC and El Informador gives 18:06 local, which differ by less than a minute from the 00:05 derived from 08:04:56 Beijing time — a rounding difference between sources; the main text uses 00:05 / 08:04:56 throughout.)
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