Tonatiuh Day: The Sun’s Movement and the Best Dates to Absorb Solar Energy

The Myth of the Spring Equinox and Solar Energy

The spring equinox does not always occur precisely on March 21. This year, for instance, it will fall on the 20th. As usual, a gathering of Sun worshipers dressed in white will congregate at the archaeological sites of Teotihuacan and Cuicuilco to receive its energy. They ascend the pyramids and extend their arms to the sky in reverence to Tonatiuh, the Aztec solar deity.

While this is a beautiful and symbolic ritual, if the goal is to absorb the Sun’s maximum energy, the spring equinox is not the ideal day for such a celebration.

Understanding Solar Energy Absorption

When basking in the Sun, the angle at which sunlight reaches you determines how much energy you receive. The closer the Sun is to being directly overhead (at the zenith), the greater the solar energy you absorb. However, the Sun does not reach the zenith every day.

North of the Tropic of Cancer and south of the Tropic of Capricorn, the Sun never reaches the zenith at all. Thus, this discussion focuses on what happens in tropical latitudes in the Northern Hemisphere.

The Dance of Tonatiuh: The Sun’s Path Through the Sky

Observing the Sunrise and Sunset over a year, as the Aztec and Mayan astronomers did, reveals that Tonatiuh—the solar disk—does not always rise or set at the same points on the horizon. Around March 21, it rises exactly in the east and sets exactly in the west. In the following days, these points shift northward, reaching their extreme position around June 21, the summer solstice—when the Sun appears to pause before beginning its journey south again.

Ancient Astronomical Observations

Ancient Mesoamerican astronomers took special note of three key solar positions:

The most northern sunrise (Summer Solstice – June 21).
The most southern sunrise (Winter Solstice – December 22).
The midpoint sunrise (Equinoxes – March 21 and September 23).

These points helped ancient civilizations orient buildings, temples, and observatories, demonstrating their sophisticated knowledge of the Sun’s movement.

The Day of Tonatiuh: When the Sun Passes Through the Zenith

Beyond these three important astronomical positions, another crucial event was observed by pre-Columbian astronomers: the sunrise on the day when the Sun reaches the zenith at noon.

This moment occurs between the equinox and the summer solstice, and its exact date depends on latitude. For example:

Mexico City experiences it on May 15 and July 29.
Guadalajara experiences it on May 22 and July 13.
Oaxaca experiences it on May 6 and August 7.

For cities north of the Tropic of Cancer—such as Monterrey, Chihuahua, or Tijuana—the Sun never reaches the zenith.

Why the Zenith Sun Days Are More Significant

Since the zenith sunrise falls between the equinox and the solstice, there are actually two days per year when the Sun is directly overhead at noon:

One when the Sun moves northward in spring.
One when it moves southward after the summer solstice.

These would be the most appropriate dates to celebrate the Sun’s energy, as they mark when we receive the most direct solar radiation.

Observing the Sun’s Path: A Personal Experiment

Years ago, I conducted a simple astronomical experiment from my home in Mexico City. I marked the Sun’s setting positions on my window sill throughout the year:

On December 22, the Sun set at the far left.
The center of the window aligned with February and October sunsets.
The equinoxes occurred near the far right.
The summer solstice was outside my window’s view.

This is a modern version of what ancient Mesoamerican astronomers did, using natural landmarks to identify the Sun’s movements. Unfortunately, when we painted the house, my markings were lost—so no future archaeologists will uncover them!

Calculating the Zenith Sun Dates

Most modern astronomy books overlook the zenith Sun because they are written in countries north of the Tropic of Cancer, where the phenomenon does not occur. To determine the zenith Sun dates, I derived a formula:

D = \frac{T}{360} \cdot arcsin (α °)

Where:

T = Length of the solar year (365.24 days).
α = Earth’s axial tilt (23.5°).
L = Latitude of the observer.
D = Number of days from the spring equinox to the first zenith Sun date.

The Best Days to Absorb Solar Energy

Here are the correct dates for experiencing the maximum solar energy absorption in Mexico:

City	Latitude	First Zenith Sun	Second Zenith Sun
Aguascalientes	21°53′	May 29	July 15
Campeche	19°51′	May 17	July 27
Guadalajara	20°40′	May 22	July 13
Mexico City	19°24′	May 15	July 29
Oaxaca	17°03′	May 6	August 7
Puebla	19°03′	May 14	July 30
Querétaro	20°36′	May 22	July 23

Since the Sun is most powerful on these dates, Mesoamerican Sun celebrations could be better aligned with them rather than the equinox.

What to Wear to Absorb More Solar Energy

An interesting fact: white clothing reflects most of the light, whereas black absorbs it. If you truly want to maximize solar energy absorption, consider wearing black instead of white!