We spent the afternoon of January 10, 2012, visiting the ruins at Machu Picchu. Located 7,970 feet above sea level, Machu Picchu overlooks the Urubamba Valley. The Temple of the Sun (Templo del Sol) or Torreón (Sentry Tower or Semicircular Temple) can be seen in the lower center of this photograph. It is the only building in Machu Picchu that has curved walls. This structure was designed as a precise instrument for fixing the date of the winter (June) solstice as well as defining a period about the zenith passage date, and for observing certain constellations. In other words, it is thought to have been used as an observatory. The Torreón is built on a large rock standing above the room that Bingham called the Royal Mausoleum. The stones used to form its walls are regularly shaped, finished with a polished surface, and are fitted together with Incan stonemason precision. As the afternoon wore on, skies cleared, and temperatures rose into the low 70's with increasing humidity.