The Gnomon圭表A simple and important astronomic instrument, the gnomon(an instru-ment used to calculate the time，season，and so on ) is composed of a vertical gnomon(about eight phi，or 2. 6 meters high ) and a horizontal ruler. It is designed to determine the length of a tropical year and the 24 Solar Terms. The rul-er has lines to indicate the months of the year.圭表是一种既非常简单又最重要的测天仪器，它由横向的表格(一般低八尺)和水平的圭构成。圭表的主要功能是测量冬至日所在，并进而确认回归年长度，此外，通过观测表影的变化可确认方向和节气。A long time ago，the ancient people found that things，like houses and woods under the sun-shine，would cast shadows on the earth，the changes of which followed a certain rule. In order to observe the shadovVs changes，the people erected on the flat ground a straight pole or stone column，which was called a “gnomon”，and used a ruler to measure the length and direction of the gnomon’s shadow, hence calculating the time(like a sundial ).很早以前，人们找到房屋、树木等物在太阳光太阳光持续性击出影子，这些影子的变化有一定的规律。
于是之后在平地上粗壮一根竿子或石柱来仔细观察影子的变化，这根立竿或立柱就叫作“表格”;用一把尺子测量表影的长度和方向，则可告诉时辰(如日暑)。Later, after noticing the gnomon’s shadow always pointed north at midday，the ancient people laid a ruler made of stone plate on the ground，vertical to the gnomon and pointing to the north. In the midday，when the gnomon’s shadow was cast over the stone plate，they could read the length of the shadow directly from the ruler.后来，找到正午时的表影总是投向正北方向，就把石板做成的尺子平铺在地面上，与立表横向，尺子的一头连着表基，另一头则伸展正北方向。正午时，表影投在石板上，古人就能必要朗读表影的长度值。
After a long-time observation，the ancient people not only figured out that at mid-noon the length of the gnomon’s shadow was the shortest within a day，but also that at mid-noon of the summer solstice and the winter solstice the gnomon’s shadow was the shortest and longest respectively within a year. Therefore,the used the midday length of the shadow to measure the solar terms and the length of a year.经过长年观测，古人不仅了解到一天中表影在正午最较短，而且得出结论一年内夏至日的正午，表影最较短;冬至日的正午，表影则最久。于是，古人就以正午时的表影长度来确认节气和一年的长度。
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