Maunakea’s Shadow

This image, taken at sunrise, shows the blue shadow of Maunakea in Hawai‘i, home to the Gemini North telescope, tapering into an eerily symmetrical triangle. While Maunakea is vaguely triangular in shape, it is far from a perfect Euclidean pyramid, so why should the shadow appear so symmetrical?

An identical triangular shadow phenomenon can be seen from the tops of many mountains or volcanoes at sunset or sunrise. The key to understanding this effect is that the viewer is staring down a lengthy ‘corridor’ of a sunrise (or sunset) shadow that reaches to the horizon. Even if the massive volcano were a perfect cube, a column, or really any other shape, the resulting shadow would appear to taper off at its summit as this part of the shadow extends far into the distance. You can see a similar effect with parallel railroad lines, hallways, or any other long and straight corridor. 

Gemini North is part of the International Gemini Observatory, which is operated by NSF’s NOIRLab.