Hitching post of the sun.
"One of Machu Picchu's primary functions was that of astronomical observatory. The Intihuatana stone (meaning 'Hitching Post of the Sun') has been shown to be a precise indicator of the date of the two equinoxes and other significant celestial periods. The Intihuatana (also called the Saywa or Sukhanka stone) is designed to hitch the sun at the two equinoxes, not at the solstice (as is stated in some tourist literature and new-age books). At midday on March 21st and September 21st, the sun stands almost directly above the pillar, creating no shadow at all. At this precise moment the sun "sits with all his might upon the pillar" and is for a moment "tied" to the rock. At these periods, the Incas held ceremonies at the stone in which they "tied the sun" to halt its northward movement in the sky. There is also an Intihuatana alignment with the December solstice (the summer solstice of the southern hemisphere), when at sunset the sun sinks behind Pumasillo (the Puma's claw), the most sacred mountain of the western Vilcabamba range, but the shrine itself is primarily equinoctial."
see:
www.sacredsites.com/americas/peru/machu_picchu.html 