For centuries, human beings have not only been simple spectators of the movement of celestial objects over the sky. All over the world, different cultures – regardless of the moment of history – have tried to understand and use the sky for their benefit.
For central American cultures such as the ancient Maya Civilization, the Sun was very important. They realized that each year, in the intertropical region they lived, there was a useful correlation between astronomical events and the weather: the Zenith Passage of the Sun.
This happens when the Sun is at the zenith, vertical over the observer’s head. It occurs twice a year, first, when the apparent movement of the Sun from south to north ends, after the spring equinox and the second from north to south after the summer solstice. This happens typically at the end of April and September, respectively, with substantial variations depending on the latitude. For example, in México, D.F., the first passage is around May 16th and July 17th for the second one, with an approximate one day of variation; for Tegucigalpa in Honduras these occur on April 27th and August 14th; and for San José (Costa Rica) the Zenith Passage occur on April 15th and August 26th.
The dates of the Zenith Passage of the Sun vary strongly with the latitude. This event has two consequences: the first one is the increase of heat because of the perpendicular incidence of solar rays and the second one is that vertical objects cast no shadow as can be seen in the figure above.
This astronomical event was really important in Mesoamerica as the first Zenith Passage of the Sun coincides with the beginning of the rainy season. Maya people used the disappearance of stela’s shadow as marker of the beginning of the rainy season.
It is known that Maya rulers had two main functions: main priest and king maintaining the order both in cosmos and in society. In order to maintain the livelihood of the population through harvest, especially of maize, instructions were given to the population living in the suburbs of large state-cities that started preparing farmland for the imminent arrival of the rains. Likewise people gathered for harvest after observing the loss of shade of vertical elements during the second passage of the Sun across the zenith. In Copan, Honduras, we have a beautiful example of a rainy season marker, the Stela D. As a sundial, the shadow move along several monuments until the First Zenith Passage of the Sun, when its shadow disappears. These are the so called horizontal observatories.
As priests, the rulers had also to ensure the cycle of live with several rituals that represented how the dry lands become fertile with the rains. The “P” structure of Monte Albán and Cave of the Sun in Xochicalco, both in Mexico, are magnificent examples of the Zenith Passage of the Sun for ritual purposes. In this type of structures – called zenithal observatories – an aperture on the top was designed in order to project a light column inside the monument when the position of the Sun is perpendicular to the building corresponding with the exact moment when the Sun is at zenith.
Therefore, the sunlight was used by Mayan elites both as a survival tool and as way of showing their knowledge, which was only available for a few of them. This is also an example of how they used nature and deities – many of them with astronomical origin – as a tool to justify their power and privileges.
Javier Mejuto (@JavierMejuto) is Head of Department of Archaeoastronomy and Cultural Astronomy Department in Universidad Nacional Autónoma de Honduras. He is a member of SEAC (European Society of Cultural Astronomy) and SIAC (Interamerican Society of Cultural Astronomy). Besides several Cultural Astronomy research projects he is currently involved in projects in History, Archaeology and Astronomy, mainly related to American indigenous people.
Ricardo Pastrana is a teacher of astronomy at the National Autonomous University of Honduras, developing teaching activities, entailment university-society and research. Taking place in the latter works on light pollution. He is also ambassador NASE (Network for Astronomy School Education) IAU program that has as main objective to educate the new generations of teachers and retrain the current on the teaching of astronomy.
David Espinoza is the coordinator of the Central American regional academic master’s degree in astronomy and astrophysics from the Universidad Nacional Autónoma de Honduras. He earned a bachelor’s degree in Physics at the UNAH and his master’s degree in physics and mathematics at the University of Granada in Spain.