The sun at the zenith
The days without shadow, the solstices and one open question
What it covers
The days on which the sun casts no shadow at the latitude of Tenochtitlan, what the sources say about them, and a question this project leaves open.
The full entry is in Spanish, and here is why
What you have read above is this project's own summary of the sun at the zenith. The full entry — the part that rests on the sixteenth-century sources — has not been translated, and that is a decision rather than an omission.
We chose not to touch the primary text. Those sections quote Sahagún, Durán and the codices in their own words, often in sixteenth-century Castilian. A quotation carried across stops being a quotation: it loses the register, the ambiguity and the exact wording someone might want to check against a facsimile.
In Spanish the entry carries:
- El día sin sombra
- Dos días al año
- Una hipótesis sobre el 260
- Y aquí no cuadra
- Los solsticios
- El equinoccio, con reservas
- Sin resolver, a propósito
- Lo que quedó construido
Read the full entry in the original Spanish → Nahuatl names and terms are identical in both versions.
The day without shadow
At noon, a stick set in the ground always throws some shadow. Except on two days a year, and only between the tropics: the days the Sun passes exactly through the zenith, directly overhead.
On that day the shadow disappears beneath the object. No instrument and no counting are needed to notice it: you see it. That is why it is the solar phenomenon first observed anywhere in the tropics.
Two days a year
Over Tenochtitlan it happens around 17 May and 26 July. The dates shift by a day from one year to the next; this app computes them for whichever year is running.
Seventy days pass between the two transits; to come back from the second to the first, two hundred and ninety-five. The first falls as the rains begin, and that ties it to the farming year.
A hypothesis about the 260
Further south, near fourteen and three quarter degrees, the long interval between the two transits measures exactly 260 days. That is the latitude of Izapa and of Copán.
From there came the hypothesis that the count of days was born from watching the Sun in that band. It is a hypothesis, not a datum, but it would explain why the 260 travelled up from the south.
And a qualification the hypothesis carries itself: there the stretch between transits is fixed, and the tonalpohualli runs without stopping. The first abundant evidence of the count, moreover, does not come from that latitude but from the Valley of Oaxaca.
And here it does not work out
In the Valley of Mexico the long interval is 295 days, not 260. The zenithal explanation of the count, true or not, does not work in Tenochtitlan.
It is worth saying, because it is easy to carry a pretty idea up from the south without checking it. Here it was checked: it does not come out.
The solstices
What is firm is the solstices: the longest day and the shortest, when the Sun stops sliding along the horizon and retraces its steps. Many measured orientations point there.
With the zenith transits they are the four moments anyone can observe without counting and without writing. That is the skeleton.
Left unresolved, on purpose
The source says one thing and the stone another. This project does not resolve that tension: it presents it. Perhaps the equinox mattered and we have measured it badly; perhaps it matters to us and we have lent it to them.
What was built
At Xochicalco and at Monte Albán there are tubes cut into the rock rising from a chamber to the ceiling. They let light in only when the Sun is directly overhead.
They are not ornament and not chance: they are instruments, made to see one thing on two days a year. That is as close to proof as one can get.
How to cite this page
Alma Mexica, «The sun at the zenith», The documents. Revised 2026-09-17. https://almamexica.org/en/document/cuentas-cenit/