Have you ever had plans to meet someone in Mexico City’s central plaza? If so you probably met at the flagpole in Mexico City's zocalo, a commonly used meeting place. Whichever of the parties gets there first soon looks for shade and will find only the shadow of the flagpole to give some relief from the searing sun. Next time you drive around the zocalo notice the line of people in the shadow of the flagpole. As the sun moves through the sky the shadow moves as does the line of people.
Next Monday there will hardly be any space to stand in the shadow of the flagpole. We're getting close to an event that occurs twice a year everywhere in the tropics, on different days in different places in that band on either side of the Equator between the Tropic of Capricorn in the southern hemisphere and the Tropic of Cancer which runs through northern Mexico. In central Mexico the sun will be directly overhead on May 16th on the sun's trip north to the Tropic of Cancer where it will stop and turn around on or about June 21st. It will be directly overhead us again on July 28th, an equal number of days after the summer solstice as May 16th is before. It is an event that never occurs in latitudes north or south of the tropics. Furthermore, between those dates our shadows will be cast towards the south.
I urge you to make a point of being outside at noon next Monday to search for your shadow. There will hardly be anything there. Your shadow will be pretty close to just your footprints. Noon will be at about 1:40 p.m. on our watches – an adjustment for daylight savings time and our location in the western part of our time zone.
It would be an even grander event if you stand on top of Teotihuacan’s Pyramid of the Sun at 1:40. As you climb notice the shadows of the steps you climb being parallel to the length of the step. While on the top at 1:40 look at the walls in front of the pyramid which are perpendicular to the Avenue of the Dead. They'll be casting no shadow to the north or south. Eerie.
May 16 and July 28 are the days on which the sun is directly overhead Teotihuacan. The Pyramid of the Sun faces the spot on the horizon where the sun sets on the two days it is directly overhead. Since it isn’t at the equator that spot is not due west, it is 15º25' north of west making the Avenue of the Dead -- which is perpendicular to that line -- run 15º 25' east of north. The alignment of the whole city of Teotihuacan is off from true north, south, east, and west by that number of degrees. It was not a mistake but very exact according to Teotihucan's location on the face of the earth. Buildings and roads in Teotihuacan are either parallel or perpendicular to the Avenue of the Dead.
Farmers in central Mexico know that when the sun is directly overhead the rains are coming and fields should be prepared for planting.
Located on a mountaintop in southern Morelos, a wonderful archeological site, Xochicalco, was home to ancient astronomers. One of the surviving observatories is found in what would appear to have been an underground labyrinth created by astronomer-priests to allow study of the movement of the sun. In a large underground room a "chimney" extends 8.7 meters with a hexagonal entry at the top. From outside one sees only a small hole in the ground. The chimney points slightly to the north and for 105 days between April 30 and August 12 the sun shines through the chimney and is projected onto the floor of the underground room. Each day as the sun moves towards the Tropic of Cancer, and again upon it’s return when the sun is at its zenith at noon, a beam of light falls through the chimney projecting directly onto the floor of the underground room. When the aperture of the chimney is further reduced the image of the sun can be focused to allow its sunspots to be seen.
For many years I've taken groups of students to Xochicalco. Much of the year I don’t mind if we ran a little late and students dawdle among the ruins. But for those dates when being in the observatory was most exciting I would tell the group that we had to hurry because we had a “date with the sun.” And as all know, the sun waits for no one.
Sadly the administration at Xochicalco has decided to no longer allow visitors inside the observatory. Years ago when a similar policy was instituted while conservation work was going on, I fashioned a similar observatory on the grounds of the Cemanahuac Educational Community in Cuernavaca. Cemanahuac’s observatory is functional in May and July. Last year while using this observatory I was startled to see sun flares from a solar storm clearly projected on the floor of the observatory. The U.S. National Center for Atmospheric Research announced early this year that strong solar storms are continuing. You know where I will be at noon May 14-18. If you’d like to join me, please email a couple of days ahead; we'll try to accommodate readers.