Wednesday, August 15, 2012

Summer Solstice Revised

This is why it always pays to take lots of photos, and review them later.  Here is a photo I took at Miantonomi's Cave on summer solstice at 7:26 PM. The cave was dark at 7:37, and  sunset was at 8:22.  I was expecting to see some solstice indicator closer to sunset.  Evidently, the final light pattern occurs about 55 minutes before sunset on summer solstice.
 This may be due to the structure of the site, since the windows are behind a boulder on a wide platform.  As the sun sets, the edges of the platform and boulder cut off light to the windows. This photo was taken at 7:20 PM on summer solstice.
The top of the platform is about 8 feet above ground level, and this structure is partway up a hill. The boulder with the slanted edge is on top, and the cave roof is barely visible behind the boulder.
Here are the two windows.  The notch is visible in the bottom of the large window, but the small window is barely visible. Below is the  large window, photographed August 13, showing light coming in through the notch. The bearing of the notch is about 305 degrees, so it may be designed to create a sun dagger on August 13.
The light that forms the small dagger to the right on the cave wall comes through this small window, with the large slanted  boulder visible outside.
When I look more closely at the summer solstice photo, I see three slanted lines on the cave wall.  The small sun dagger to the right is at the top of a faint diagonal line, presumably the one for summer solstice. The center line is the one the sun dagger followed on Monday, August 13th.   I am not sure of the significance of the line to the left. I also notice that the edge of the sunlit area to the left touches the tops of the dimples on the wall.


 The angles of these lines look similar to that of the slanted edge of the boulder.  Apparently, as the sun sets further to the south, the sun dagger moves further to the left on the cave wall. The patterns of light admitted by the  two windows and altered by the projecting stone determine the astronomical reading.  It will be interesting to see the patterns of light at equinox and winter solstice.

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