Tuesday, July 23, 2013

Sun directly overhead

Today, when you step outside in central Mexico your shadow is cast towards the south -- an event that never happens in Europe, Canada, or the continental United States.  A week from today your shadow will be cast towards the north.

A largely unnoticed astronomical event will occur in Mexico City on Sunday.  At midday the sun will be directly overhead and most objects will cast no shadow. You will see a shadow if you look directly under something like a car or table. People tend to cast shadows too—at least those of us with waists wider than our feet. But most objects will cast no shadow.

This phenomenon occurs everywhere in the tropics, though not on the same day. The tropics are the part of the world between the Tropic of Cancer which crosses northern Mexico and the Tropic of Capricorn in the southern hemisphere.

The sun makes a yearly journey from the Tropic of Capricorn to the Tropic of Cancer and back again. It departs the Tropic of Capricorn on or about December 21, which is the shortest day in the northern hemisphere.  The Sun crosses the Equator on the spring equinox, March 21, and is directly over Mexico City on May 16.  It reaches the Tropic of Cancer on June 21, the longest day in the northern hemisphere.  Then it starts its return trip.  The sun is again directly over Mexico City at noon on July 28, over the equator on September 21 which is the fall equinox, and then back to the Tropic of Capricorn on December 21.

If you are in Mexico City on Sunday I suggest you go to one of the three hotels with a rooftop restaurant facing the zocalo. Plan to be there from 11:50 a.m. to 1:50 p.m.  From the restaurant you'll be able to watch a show in which none of the actors will realize they are playing a part. But they will each fulfill their role with chronometric accuracy.

You may have noticed that the flagpole in the Zocalo is a common meeting place. What tends to happen is that the party who arrives first waits in the only shade the zocalo offers, the thin shadow the flagpole makes. The line of people arcs right along with the sun moving through the sky.  Next Sunday morning the shadow will get shorter and shorter until it disappears at midday.  The line of people will have to disband.

You'll also witness the mistake the clergy in the Metropolitan Cathedral make when they ring the bells to announce the Angelus midday prayers.  They’ll start ringing the bells at 11:50, thinking that is ten minutes before midday.  If that were true during Standard Time that would mean that now, while we are on Daylight Savings Time (DST), midday would be at 1 o'clock.  However, in Mexico City we are at the westernmost part of our time zone and midday is at about 1:40 p.m.  You'll be able to confirm that by watching the shadow of the flagpole. 

Despite the Cathedral's bells being rung at the wrong time (11:50 to 12 o'clock DST), it is quite a sight to watch.  They are rung by hand by people as high up in the church's towers as the bells.  Look closely and you'll see them pulling on ropes tied to the bells.  The larger bells require two people to haul the clapper back and forth.

At least I think it's the wrong time.  Shouldn't Midday prayers be at midday?  I don’t think we should be playing around with God's timing based on a 20th century attempt to save electricity through daylight savings. Or the 19th century idea that everyone around the world should share the same minute of the hour regardless of where we live, which is what Greenwich Mean Time imposes.

While enjoying the theater unfolding before you, place your order for lunch and await the astronomical event at 1:40.  While eating -- and waiting -- keep an eye on the status of the flagpole's shadow as if it is a giant sundial. You’ll be part of a long tradition of people in Mesoamerica noticing and studying the position of the sun in the sky by looking at the ground.

Ancient Mesoamericans were great students of astronomy and mathematics and developed a very precise calendar. Calendar origins probably had a practical use. Though Mesoamerica has a dependable rainy season, inhabitants needed to know when the rains would begin in order to burn their fields at their driest.  The longer they waited the drier the cuttings would be, hence the better they would burn.  However if they waited too long and it began to rain the wet cuttings wouldn't burn well and they'd have a reduced area in which to plant.

Watching the sun's progress on its northerly route would guide farmers on when to burn their fields to be ready for planting.   

If you enjoy watching this type of natural theater, one of the grandest performances of all is going on daily until early August in the underground observatory at Xochicalco, a 45 minute drive south of Cuernavaca. For my column about that fascinating place go to http://tinyurl.com/mk3uaed. 

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