Excerpts from Jim Conrad's
from the May 26, 2007 Newsletter issued from Sierra Gorda Biosphere Reserve, QUERÉTARO, MÉXICO
THE SUN OVERHEAD
One day this week a bit after noon I saw that my solar oven wasn't cooking as effectively as it usually does. I glanced upward, expecting maybe to see haze between us and the sun, and I was surprised by what I saw: The sun, in a cloudless sky, was almost exactly above me. My oven's reflectors are designed to focus sunrays slanting in at an angle, so when the sun is directly overhead the system doesn't work...
So, of course! This summer I'm south of the Tropic of Cancer, so naturally as the sun's daily path, its ecliptic, migrates northward toward the Tropic of Cancer -- exactly above which it'll stand on the date of the Summer Solstice, June 21 -- at some point the ecliptic has to pass directly above us.
But, it wasn't exactly overhead that day. It still lay a little south of overhead. Therefore, the question: On what date will the sun, at midday, be as close to possible directly overhead at Jalpan?
I found my answer in the online "Table for the Declination of the Sun," located at http://www.wsanford.com/~wsanford/exo/sundials/DEC_Sun.html.
That table shows that today -- Saturday, May 26th -- at midday, the Sun will be directly overhead at latitude North 21°01'.
Since Jalpan's latitude is North 21°23', we're still a tiny bit north of today's declination.
The chart also shows that here in Jalpan, at North 21°23', the Sun will be most directly overhead at midday next Monday, May 28th. It won't be exactly, exactly overhead, for the ecliptic at midday will actually be one minute south of us, at North 21°22', but it'll be closest to our latitude on that date.
After next Monday on each consecutive day the ecliptic will continue drifting a little farther northward until the Summer Solstice on June 21st, when it'll stand exactly overhead at North 23°26' about 160 miles (260 kms) north of here, at the Tropic of Cancer.
After the Summer Solstice the ecliptic will begin migrating back southward on its journey to being overhead at the Tropic of Capricorn at South 23°26', on the Winter Solstice, December 22nd. The chart shows that on that return trip back south it'll again appear most exactly overhead at Jalpan on July 16th and 17th.