Sunday, October 5, 2014

Vernal Equinox plus Twelve

Finally a sunny weekend day for checking my solar calendar.*  (How quickly forgotten the drought that gave almost nothing but sunny days!)  The NOAA Solar Calculator page gave solar noon today for my address as 12:32:34 EDT.  I checked my watch against my "atomic"** bedside clock: my watch was 20 seconds slow. 

I figured that all this precision was a little over the top, as I got a chair to stand on and a Sharpie and ruler to mark the shadow of the gutter with.  Half-a-minute before the time, I lined up my ruler and began to draw, but found the shadow was moving visibly even as I did.  By the time the second had struck, I had to draw new lines.  I ran for alcohol to take off the original lines and made a discovery: alcohol merely faded and smudged it a little.  (With these marks  permanent, I'm glad I have a patient, understanding wife!)

If you make your own sundial/solar calendar, I wouldn't recommend a greater distance from gnomon to dial than maybe twenty feet--as you can see, the edge of the shadow is already pretty fuzzy and hard to pinpoint.  Greater distance would only make it fuzzier.  (The top of the school flagpole I once tried to use when I taught seventh grade science was so far away it didn't even cast a shadow!)

The corner of a gutter that serves as my gnomon is about twenty-two or twenty three feet away from the side of the house that is my "dial."  Clearly, that is more than distance enough to actually watch the shadow move.  You can see the change in the photos below, taken just eight minutes apart. 


I was also startled that the solar noon shadow today was nearly two feet higher on the siding than the one I made just eight days ago.  At that rate, will the solstice shadows fall on the house at all?  I have about six feet above and below my original mark.  That isn't much, but the shadow also moves farthest nearest the equinoxes, so it might be okay.  When I have time, maybe I'll try to work it out trigonomically.  Yeah, right.

*Another site will print solar noons for an entire year.
**The only thing "atomic" about atomic clocks (besides they're being made of atoms), is that they receive ultra-low-frequency radio signals from the atomic clock in Colorado that keeps the nation's time.  That IS pretty cool, though!

No comments:

Post a Comment