Tuesday, September 30, 2014

Make your own Solar Calendar

I've long been interested, in a general way, in the movements of the sun through the seasons.  The sun reaches its highest point each day at "solar noon."  And this high point gets higher and higher in the sky each winter and spring until it reaches its highest point at the summer solstice (usually June 21-22), then descends gradually towards its lowest point at the winter solstice around December 21-22. 

Since the sun's axis is tilted 23½ degrees, here in Massachusetts (latitude 42 degrees) the sun reaches a high of 71½ degrees above the horizon at the beginning of summer, but only 24½ degrees at the beginning of winter.  At the equinoxes (beginning of spring or fall), the noonday sun would be at 90-42=48 degrees. 
Along with this comes the lengthening and then shortening of the daylight hours.  The solstices, and the equinoxes midway between them, provide the defined beginnings of the four seasons; while the daylengths, together with the changing angle of the noonday sun, give our hemisphere more or less solar heat energy, providing us our seasonal weather.

There are lots of solar calendars in the world.  This is one of the better-known ones.

Ancient solar calendars like Stonehenge give these movements of the sun a mystical air that excites many people into spiritual experiences.  I used to impute to these New Age types a longing for intense spirituality that they never found in the staid worship of their parents, and which traditional worship they'd rebelled from anyway.  Such stuff.  At the same time, I felt condescension toward those ancients who worshiped thus.

But in the second episode of the new Cosmos,Neill Degrasse Tyson explained it in a way that made perfect sense to me.  Ancient people depended for their survival on knowing the time of year when prey animals migrated and food plants came into season, and, with the advent of agriculture, even more the dates when the killing frost would be safely past.  These calendars were intensely practical--even crucial to their survival.  And the idea that the stars governed their fates an entirely logical extension of the same practical experience.

For my part, my interest is also sometimes just as practical: my little vegetable garden is strategically placed for the longest full sun in late spring and early summer.   My tomatoes, cucumbers, zucchini, sage and rosemary enjoy over five hours of full sun at the summer solstice, but now at the autumnal equinox (September 23) receive less than four hours, since much of the day high trees block the lowering sun.  Lover of tomato sandwiches (on toast with mayo, salt & pepper, and a liberal sprinkling of dill weed) that I am, I check the times of sun and shadow whenever the sky is clear and I am home at the right moments.  (I count about two more tomato sandwich lunches for my wife and I from the last tomatoes that will ripen before the light fails.)

A few weeks ago I finally decided to act on the impulse to make a solar calendar of my own.  Such an enterprise takes a fair amount of thinking.  What to observe?  Several things change through the seasons: the greatest height of the sun, the times of sunrise and sunset, and the direction of sunrise and sunset.  (The sun rises directly east and sets directly west only on the equinoxes; toward summer it rises and sets northerly, and towards winter it rises and sets more southerly.)  Even the time of local noon varies through the year for a combination of reasons.  

I first thought to try to fix the direction of sunrise and sunset at the solstices and equinoxes, but the trees of my neighborhood make that impossible even from an upstairs window.  In fact, there is not a single broad, straight, east-west street I could use to even approach witnessing sunrise or sunset at the equinoxes.  Next I thought of a straight, vertical rod to made a gnomon for a sort of seasonal sundial.  I like the idea of having a sundial or calendar large enough that you can actually watch the motion of the shadow in real time.  The trouble is, the taller the gnomon, the less distinct its shadow.  (I once had my 7th grade science classes make a sundial using the school's flagpole for the gnomon; it was less than satisfactory, since the top of the flagpole made almost no shadow at all.)  I still think the idea of a moderate-sized gnomon a good one, but it would take a yard with a lot of open sky, and any simple pole would very easily go out of adjustment and become useless.

I also failed miserably at finding local noon experimentally: theoretically, it is the time that the sun is highest in the sky, and therefore casts the shortest shadow on a level surface, but in practice the shadow is too indistinct to measure and the differences in its length too slight.  If I had a clear horizon, and had a sextant and were good at using it, I could probably nail the time of solar noon to less than a minute (this is routine in traditional navigation aboard ship), but I have none of these.  Fortunately, NOAA's solar calculator comes to the rescue.

Last weekend I hit on the perfect plan for a practical solar calendar of my own: looking around outdoors near solar noon, I found a place where the edge of a gutter cast a pretty reasonable shadow on the side of the house a little distance away.  I waited with ruler and Sharpie in hand until it was exactly solar noon, and marked the fuzzy shadow as best I could on the siding.  If I make marks regularly--especially at the solstices and equinoxes--I will be able at a glance to know two things: first, whether it is before or after solar noon on that particular day; second, where we are in the march of seasons.  Since the shadow falls about six feet  up the wall, there will be plenty of room for the shadow to go higher (as the noonday sun drops towards the winter solstice), and lower (sun rising towards summer solstice).  It should work perfectly--at least as long as the gutters don't go out of alignment.  --and as long as my long-suffering wife doesn't object too strenuously to Sharpie on the white siding!

Gnomon: the corner of the gutter.  It's shadow falls at noon on the side of another part of the house.
  The red mark is labeled "AE +4," i.e. Autumnal Equinox +4 days.  (Not exactly Stonehenge, but it's mine!)

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