Saturday, June 20, 2015

Mark the solstice--literally

I've given up--for now, at least--the idea of making a shadow calendar that would mark the passage of the seasons as a shadow moves from one solar noon to the next.  But simply marking solstices or  equinoxes is simplicity itself.  Try it yourself, tomorrow!

Theoretically, you could find the position of the sun at solar noon on any particular day, choose an object to cast the shadow, and measure the angle to see where that shadow would fall.  But it's far, far easier to simply wait for the event in question, find a convenient shadow, and mark where it falls.  

You could do such a mark at sunrise, sunset, or solar noon.  Sunsets and sunrises change direction as the seasons pass (really!*), while solar noon marks the greatest height the sun reaches on a given day.  (All three of these things also vary in time.)  Since trees and buildings block our view of sunrises and sunsets around here, solar noon makes the best target, and it's what I marked. 

Your first job is to find out the time of solar noon tomorrow--the day of the summer solstice.  Go to NOAA's solar calculatorEnter enter your latitude and longitude, OR) move the nearest orange pin on the map to your location.  I like to put the pin right on my house, but for practical purposes that's overkill: a few miles either way won't matter.  (The sun moves [okay, the earth turns] one degree of longitude every four minutes.  At my latitude of 42 degrees north, a degree of longitude is forty-five or so miles.)  Change the date to June 21.  Click the box for daylight savings time, then click Save.  You now know the times of sunrise and sunset, and the time of solar noon.  (Other check boxes will make lines on the map showing directions of sunrise and set, and the current position of the sun.)  WRITE DOWN THE TIME OF SOLAR NOON.  

As I write, the NOAA solar calculator page keeps timing out--probably swamped with interest so close to the solstice.  Keep trying!

Notice that, not only is it not at noon, it's also not at one o'clock, as you'd expect with daylight savings time.  Here, solar noon comes at 12:45pm, EDT right now.  Part of the reason is that time zones are about fifteen degrees of longitude (one hour of time) wide.  Only those in the east-west center of the time zone could reasonably expect noon to be on the hour.  (Since we in eastern Massachusetts are east of the center of the eastern time zone, solar noon always falls before clock noon.)  The rest of the difference is more subtle, and also causes the time of solar noon to vary by a minute or more from day to day, and tens of minutes from season to season. 

On Sunday go outdoors half-an-hour or so before the time of solar noon.  You need to scope out a good shadow.  The shadow ought best be pointed, and should be cast by a something permanent at some distance away (say, ten or twenty feet away).  I used a corner of the roof, whose gutter casts an angled shadow.  As the sun continues to move westward, that shadow will move eastward, so you need to make sure it will still fall in a convenient place at solar noon.  (In the event it doesn't, you may want to find a back-up shadow.)  

Marked yesterday, when it was sunny.  (Two days before the solstice is close enough
in my book.)  The shadow of the roof corner will just cover this rock tomorrow.

Added 6/22: A cloud blocked the sun right at solar noon on 6/22, but a minute 
later the shadow appeared.  In that minute, the shadow had already advanced
an inch to the right.  (The roof casting the shadow is about 20 feet away.)

That's pretty much it.  Just figure out how you can mark the location of this shadow when solar noon arrives.  This shadow falls on the ground right now, which I marked by pushing a little stone into the ground at the point of the shadow with my foot.  At the equinoxes, on the other hand, that shadow falls on the side of the house, and I marked it in indelible marker.  

Henceforward, every summer solstice (usually about June 21st) that shadow will appear in exactly the same place.  If you're lucky enough that that same shadow will fall on the same surface at the equinoxes and winter solstice (a very tall order), you will be able to mark those the same way.  Then you could watch the sun's noontime progress between those marks, as the earth makes its stately, tilted way around the sun season year after year.  

This was a few days after the fall equinox.  (The shadow has passed, 
since solar noon was ten or fifteen minutes before.

To summarize:
1. Enter your location in the NOAA solar calculator site (not forgetting daylight savings) and find the time of solar noon for June 21.
2. Go out safely before solar noon on the 21st and find a convenient shadow.
3. At solar noon, mark the location of that shadow. If it's cloudy, do it tomorrow!  (Close enough!)

* You can see from the figure below that the sun rises and sets south of east and west in the fall and winter, but north of east and west in the spring and summer.  Curiously, the paths of the sun across the sky are always parallel to each other.  The figure is approximately correct for mid-northern latitudes like those here in New England.

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