High Noon Day

August 21st was High Noon Day in Granada.  Actually, 11:46:50 that day was High Noon in Granada.

What is High Noon Day?

I’m glad you asked.  It lets me drag you into my astronomical fascination with living in the tropics.

The short story is that it’s the day and time that the sun passes directly (or a close as possible) through the zenith, the point in the sky directly overhead.  We’ll get to the long story for those interested, but for those about to click out let me say why you should care. If you walk down east/west streets such as Liberdad, Real Xalteva, the Calzada, Santa Lucia, etc. and you like to walk in the shade, today is the day the shadier side moves from the north to the south side of the street.  That’s the best I can come up with for practical applications of this information, but for those interested,  let me get into a technical discussion.

We live in the tropics, technically defined as the area of earth between the tropic of Cancer in the northern hemisphere, and the tropic of Capricorn in the south.  These are not just arbitrary lines on the globe. In earth centric vocabulary, they are the points on the planet directly under the sun on its northern most position (the June solstice) and its southern most position (the December solstice).  Unlike Europe, Canada and the United States (except Hawaii) where the sun is always south of the zenith, locations between these two lines will experience the midday sun both north and south of the zenith at different times of the year.  It follows then, that for tropical locations there is a point in time where the sun crosses from north of the zenith to south and vice versa.  This is what I am calling High Noon.

There are two High Noon days per year.  Today is the day the sun passes from north to south, and April 21 is the day it passes from south to north.  One interesting aspect of High Noon is that is that it falls on different days for different locations.  In Managua this N2S High Noon day is the 20th at 11:48:20 and in León it the 19th at 11:51:05.  It makes it more of a local event, and I think it is worthy of celebration.  I’ll try to remember to drink a toast to the sun at 11:46:50 today.

How can you observe this event?  I happen to have a clothesline pole mounted on the mirador wall slightly above floor level.  The steel tube is vertical as checked by a mason’s level so when the sun is directly overhead it shines down through the tube and on to the floor, casting a circular shadow of the pole.  Last April 20th, the day before the S2N it cast a shadow about 1/2 inch long to the north of the pole as the sun was still south of the zenith.  The next day it shot straight down through the tube.  Another simple observing device would consist a string tied between two uprights (like a hammock.) In the middle of the string hang something long and vertical like a broomstick or another string with two small objects attached in line with the string, one near the top and the other at the bottom.  When the shadow of the broomstick aligns to a circular dot, or when the shadows of the two objects align, then the sun is directly overhead and you set off the fireworks or whatever you do at High Noon.

In case you’re interested, here is the solar position calculator I used to determine these times:
http://www.esrl.noaa.gov/gmd/grad/solcalc/azel.html

Enter the location latitude and longitude and then search for the day/time that has a solar elevation nearest to 90 degrees.  Wikipedia has lat/lng for most major cities.  They had 11°56′N 85°57′W for Granada, 12°26′0″N 86°53′12″W for León.

Have fun!

Craig Winters