The world's first clock was a stone - the obelisk clocks 3,500 years before Christ

The Obelisk Clock 3,500 years before Christ

The Egyptians were among the first to formally divide their days into parts resembling hours. Obelisks—slender, tapered, four-sided monuments—were built as early as 3500 BC.

Their moving shadows formed a kind of sundial, allowing citizens to divide the day into two parts by indicating noon. They also indicated the longest and shortest days of the year, when the noon shadow was the shortest or longest of the year. Later, markers were added around the base of the monument to indicate further time divisions.

Another Egyptian shadow clock, or sundial, was put into use around 1500 BC to measure the passage of "hours." This device divided a sunlit day into 10 parts, plus two "twilight hours" in the morning and evening. When the long stem, with five differently spaced markers, was oriented east and west in the morning, a raised crossbeam at the eastern end cast a moving shadow across the markers. At noon, the device was rotated in the opposite direction to measure the afternoon "hours."

The merkhet, the oldest known astronomical tool, was an Egyptian development around 600 BC. Two merkhets were used to establish a north-south line by aligning them with Polaris. They could then be used to mark the night hours by determining when certain other stars crossed the meridian.