The Sumerian culture was lost without passing on its knowledge, but the Egyptians were apparently the next to formally divide their day into parts something like our hours. Obelisks (slender, tapering, four-sided monuments) were built as early as 3500 BC. Their moving shadows formed a kind of sundial, enabling people to partition the day into morning and afternoon. Obelisks also showed the year's longest and shortest days when the shadow at noon was the shortest or longest of the year. Later, additional markers around the base of the monument would indicate further subdivisions of time.
Another Egyptian shadow clock or sundial, possibly the first portable timepiece, came into use around 1500 BC. This device divided a sunlit day into 10 parts plus two "twilight hours" in the morning and evening. When the long stem with 5 variably spaced marks was oriented east and west in the morning, an elevated crossbar on the east end cast a moving shadow over the marks. At noon, the device was turned in the opposite direction to measure the afternoon "hours."
The Merkhet, the oldest known astronomical tool, was an Egyptian development of around 600 BC. A pair of merkhets was used to establish a north-south line (or meridian) by aligning them with the Pole Star. They could then be used to mark off nighttime hours by determining when certain other stars crossed the meridian. The merkhet ("instrument of knowing") was an ancient timekeeping instrument. It involved the use of a bar with a plumb line, attached to a wooden handle. It was used to track the alignment of certain stars, if they were visible, in order to approximate the time at night (10 stars for the 10 hours of the night, with a total of 24 hours including 12 hours for the day, 1 hour for sunset, 1 hour for sunrise). In this way, it was more efficient than other contemporary devices, such as sundials, which were rendered useless during the dark.
In the quest for better year-round accuracy, sundials evolved from flat horizontal or vertical plates to more elaborate forms. One version was the hemispherical dial, a bowl-shaped depression cut into a block of stone, carrying a central vertical gnomon (pointer) and scribed with sets of hour lines for different seasons. The hemicycle, said to have been invented about 300 BC, removed the useless half of the hemisphere to give an appearance of a half-bowl cut into the edge of a squared block. By 30 BC, Vitruvius could describe 13 different sundial styles in use in Greece, Asia Minor, and Italy.