  The pyramids of ancient Egypt are said to have shone brilliantly in the sun.
Surfaces of polished limestone would not only have reflected diffusely in all
directions, but would also likely have produced specular reflections in
particular directions. Reflections toward points on the horizon would have been
visible from large distances. On a particular day and time when the sun was
properly situated, an observer stationed at a distant site would have seen a
momentary flash as the sun's reflection moved across the face of the pyramid.
The positions of the sun that are reflected to the horizon are confined to
narrow arcs in the sky, one arc for each side of the pyramid. We model specular
reflections from the pyramid of Khufu and derive the annual dates and times
when they would have been visible at important ancient sites. Certain of these
events might have coincided with significant dates on the Egyptian calendar, as
well as with solar equinoxes, solstices and cross-quarter days. The celebration
of Wepet-Renpet, which at the time of the pyramid's construction occurred near
the spring cross-quarter day, would have been marked by a specular sweep of
sites on the southern horizon. On the autumn and winter cross-quarter days
reflections would have been directed to Heliopolis. We suggest that on those
days the pyramidion of Khafre might have been visible in specular reflection
over the truncated top of Khufu's pyramid.
