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Astrosophers were serious, intelligent people doing what they saw as important work. It tugs the heart to watch how they strained and strained and kept fallling short. In pre-dynastic China, the earth was seen as a gigantic square, not smooth and not quite level (it tilted to the southeast, which is why all rivers flowed in that direction). The sky was a giant rotating disk, angled to the north. During the Han dynasty, the disk model began to be replaced by a spherical sky that surrounded the flat square earth like an eggshell, since a rotating sphere fit their observations better than a rotating disk. They had all the facts available to the ancient Greeks, including the form and timing of eclipses. By the Sui dynasty they had even noticed the precession of the equinoxes (which they call the shifting of the winter solstice). The step to seeing the earth as a sphere would have been such a short one—but ancient wisdom said the earth was square. End of discussion.
There is a term 漏刻 meaning “a few minutes”. These essays contain the explanation of its origin. In ancient China accurate time was kept by clepsydra. It divided 24 hours into one hundred marks, or notches, on the vessel. Thus one 刻 meant not fifteen minutes, as in modern Chinese, but 14.4 minutes. That’s not recondite.
The description of the apparatus is clear:
昔黃帝創觀漏水,制器取則以分晝夜 In antiquity, Huang Di initiated the observing of draining water. He specified apparatus for obtaining a standard, and by that he divided the day and the night.
其後因以命官.周禮挈壺氏則其職也 His successors went on to use it to give orders to offices. This is precisely the duty of the clepsydra-fillers in the Zhou Li.
其法:總以百刻分于晝夜 The method: divide a total of one hundred notches between day and night.
冬至晝漏四十刻,夜漏六十刻 At the winter solstice forty notches drain during day and sixty during night.
夏至晝漏六十刻,夜漏四十刻 At the summer solstice sixty notches drain during day and forty during night.
春秋二分晝夜各五十刻 At the equinoxes each get fifty notches.
日未出前二刻半而明.既沒後二刻半乃昏 Two and a half notches before the sun rises, it gets light. Two and a half notches after the sun sets, it gets dark.
Now here comes the technical term:
冬夏二至之間,晝夜長短凡差二十刻 Between the winter and summer solstices, the day and night grow and shrink, the total difference being twenty notches.
每差一刻爲一箭 Each occasion on which it differs by one notch constitutes one arrow.
冬至互起其首,凡有四十一箭 At the winter solstice it crosses itself and begins again from its head, making altogether 41 arrows.
每箭各有其數,皆所以分時代守更其作役 Each arrow has a number associated with it, all used to divide the hours for changing on-duty times of shift workers.
That is, “one arrow” yijian is a length of time, averaging roughly 9 days, during which the imperial palace astronomers of ancient China considered it unnecessary to adjust the official times of sunset and sunrise so as to regulate the starting and ending times of night shifts, guard tours, etc.