A、Half a year. B、Half a circle. C、A bicycle. D、Quarter of a circle. BWhat is a hemicycle?

Not until somewhat recently(that is, in terms of human history) did people find a need for knowing the time of day. As best we know, 5,000 to 6,000 years ago great civilizations in the Middle East and North Africa initiated clock-making. With their bureaucracies and formal religions, these cultures found a need to organize their time mom efficiently.
    The Egyptians were 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 B.C. Their moving shadows formed a kind of sundial, enabling citizens to partition the day into two parts by indicating noon. They also showed the year’s longest and shortest days when the shadow at noon was the shortest or longest of the year. Later, markers added around the base of the monument would indicate further time subdivisions.
    Another Egyptian shadow clock or sundial, possibly the first portable timepiece, came into use around 1500 B.C. 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 5 variably spaced marks was oriented east and west in the morning, an elevated crossbar on the east end east a moving shadow over the marks. At noon, the device was turned in the opposite direction to measure the afternoon "hours".
    In the quest for more 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 B.C., removed the useless haft of the hemisphere to give an appearance of a half-bowl cut into the edge of a squared block.