This panorama captures an optical phenomenon called Brewster's Dark Patch.  Around sunrise or sunset, light from the sky is vertically polarized in a band that runs from the northern horizon, overhead to the southern horizon.  This image opens looking south and up at the curved glass facade of the Joseph Moakley Courhouse in Boston.

The reflection of the northern sky is missing in a patch at the center of the glass wall but easily seen elsewhere.  Vertically polarized skylight is poorly reflected off the glass when striking it at Brewster's Angle*, and the interior lights of the courthouse are visible.

The effect is more commonly seen when looking down on water at sunrise or sunset.  With no lights under the water to compete with the reflected sky it became known as Brewster's Dark Patch.

*Sir David Brewster (1781-1868)