Sundials have their origin in shadow clocks , which were the first devices used for measuring the parts of a day.The oldest known shadow clock is from Egypt, and was made from green schist Ancient Egyptian obelisks , constructed about 3500 BC, are also among the earliest shadow clocks. Egyptian shadow clocks divided daytime into 10 parts, with an additional four “twilight hours”—two in the morning, and two in the evening. One type of shadow clock consisted of a long stem with five variable marks and an elevated crossbar which cast a shadow over those marks. It was positioned eastward in the morning, and was turned west at noon. Obelisks functioned in much the same manner: the shadow cast on the markers around it allowed the Egyptians to calculate the time. The obelisk also indicated whether it was morning or afternoon, as well as the summer and winter solstices.