A Triangular Shadow of a Large Volcano

As seen on Astronomy Picture of the Day in the moments of sunrise the Full Moon sets over Tenerife in the Canary Islands of Spain as seen from the top of Mount Teide; a magnificent volcano that reaches 3718 m above sea level, and about 7500 m above the floor of the Atlantic Ocean which makes Teide the highest point on this Ocean. But why the shadow of this volcano look like a triangle? Mt. Teide itself does not have the strictly pyramidal shape that its geometric shadow might suggest. The triangle shadow phenomena is not unique to the Mt. Teide, though, and is commonly seen from the tops of other large mountains and volcanoes. A key reason for the strange dark shape is that the observer is looking down the long corridor of a sunset (or sunrise) shadow that extends to the horizon. Even if the huge volcano was a perfect cube and the resulting shadow was a long rectangular box, that box would appear to taper off at its top as its shadow extended far into the distance, just as parallel train tracks do. The above spectacular image shows part of the World Heritage Teide National Park and Pico Viejo crater in the foreground. The nearly full moon is seen just shortly after its total lunar eclipse of 15-16 June 2011.