I asked a certain physicist to help me understand what happened in the sky near sunset in Copley Square. We discussed the interplay of light and shadow. How the sun below the horizon was illuminating the clouds from below. How the sharp line in the sky might result from shadows cast by something on the horizon. Then of course there were the clouds at different altitudes. Shorter wavelength blue light scattered out. Longer wavelength red light left behind. In the end all we could agree upon was that it was beautiful.
The sun set today in Boston at 4:27 pm.
Words + Images 
As lovely as the actual sky! I love to witness art. Thanks for these!