There are a couple of days a year when the shadow stops accompanying the people who live in some parts of the planet near the equator.
They are known as “shadowless days” and occur in regions such as southern Mexico, Central America and the Caribbean, as well as in 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 occurs initially between the second half of May and the first half of June of 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, at noon, suddenly there is no shadow.”
“That day you can place a cane or see a column-shaped monument and notice that it has no shadow,” says the UNAM researcher.
As Dr. Ismael Arturo Montero García explains on his website, “the zenithal passage of the Sun occurs when the position of the star is completely verticaloccupying the highest place in heaven.”
“This happens only two days a year, during which no lateral shadow is cast at noon. The phenomenon is only perceptible in the regions located to the south of the Tropic of Cancer and to the north of the Tropic of Capricorn”, explains Montero García.
“Further north and further south, the Sun never reaches its zenith. The date differs according to the latitude, which obeys the inclination of the Earth; Thus, the Sun illuminates plumb different areas of the planet on different dates”.
For his part, Cuevas Cardona points out: “The fact that the Earth is revolving around the Sun, and the inclination changes throughout its orbit in the year, makes the Sun go ‘up’ with respect to the horizon until I get exactly to point over our heads“.
Last week the inhabitants of Mexico City were able to experience this phenomenon. Some students gathered in a square 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 did not have a shadow for a couple of 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 different contiguous days.
Since ancient times
The phenomenon has been known since ancient times.
The pre-Hispanic peoples, among them the Mexicas and the Mayas, enjoyed great astronomical knowledge and carried out verifications of the accuracy of their calendars by means of the zenithal sun.
In the vestiges of cities such as Teotihuacán, Monte Albán and Xochicalco there are buildings or caverns with a hole that allowed astronomers of the time to record the moment in which the sun was located exactly at the zenith.
“There are a number of days between one zenithal passage and another, which allowed them to calculate their calendar very precisely“, explains Cuevas Cardona.
There is even a theory by astrophysicist Jesús Galindo about the founding of ancient Tenochtitlan, the Mexica city where Mexico City currently sits.
It is known with some certainty that it was founded in 1325. Galindo’s research suggests that it could have occurred on May 17, 1321, when the zenithal sun occurred that year.
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See original article on BBC