There are a few days each year when the shadow stops guiding the people who live near the equator in some parts of the planet.
They are known as “shadowless days” and are found in regions such as the South Mexico, Central America and the Caribbean, as well as northern South America.
And it’s just Between the Tropics of Cancer and Capricorn This is where this phenomenon occurs, according to Dr. Salvador Cuevas Cardona, a physicist at the National Autonomous University of Mexico (UNAM).
In Mexico, it initially takes place between the second half of May and the first half of June each year. And it is repeated between June and August, depending on the location of each place.
What is happening?
As Cuevas Cardona explains, “with the sun overhead, suddenly there’s no shadow at noon.”
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“You can put a stick that day or see a columnar monument and notice that it has no shadow,” says the UNAM researcher.
Like dr. Ismael Arturo Montero García explains on his website: “The zenithal passage of the sun takes place when the position of the star is completely vertical., occupying the highest place in heaven.”
“This only happens two days a year, in which at noon no lateral shadow is cast. The phenomenon is only observable in the regions south of the Tropic of Cancer and north of the Tropic of Capricorn,” explains Montero García.
“Further north and farther south, the sun never reaches its zenith. The date differs according to latitude, which follows the inclination of the Earth; For example, the sun illuminates different parts of the planet on different dates.
For his part, Cuevas Cardona points out: “The fact that the Earth revolves around the Sun and changes its inclination throughout its orbit in the year, causes the Sun to move ‘up’ in relation to the horizon until I get right down to it point above our heads”.
Last week, the residents of Mexico City were able to experience this phenomenon. Some students gathered in a plaza to witness the effect in the company of Cuevas Cardona and other experts.
As they were able to verify, a cylinder placed upright on the ground had no shadow for a few minutes around noon, which is the only time this phenomenon occurs.
In different places in Mexico (and in the countries of the intertropical region), the phenomenon occurs at different times. Even in areas of the same city, it can occur on several consecutive days.
Since ancient times
The phenomenon has been known since ancient times.
The pre-Hispanic peoples, including the Mexicas and Mayas, had great astronomical knowledge and carried out verifications of the accuracy of their calendars by means of the zenithal sun.
In the remains of cities such as Teotihuacán, Monte Albán and Xochicalco, there are buildings or caves with a hole that allowed astronomers of the time to record the moment when the sun was exactly at its zenith.
“There are several days between one zenithal passage and another, what enabled them to calculate their calendar very accurately”, explains Cuevas Cardona.
There is even a theory by astrophysicist Jesús Galindo about the creation of ancient Tenochtitlan, the Mexica city where Mexico City is currently located.
It is known with some certainty that it was founded in 1325. Galindo’s research suggests that it might have occurred on May 17, 1321, when the zenithal sun rose that year. (F)
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