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.
圭表是一种既简单又重要的测天仪器，它由垂直的表(一般高八尺)和水平的圭组成。圭表的主要功能是测定冬至日所在，并进而确定回归年长度，此外，通过观测表影的变化可确定方向和节气。
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，人们发现房屋、树木等物在太阳光照射下会投出影子，这些影子的变化有一定的规律。于是便在平地上直立一根竿子或石柱来观察影子的变化，这根立竿或立柱就叫做“表”;用一把尺子测量表影的长度和方向，则可知道时辰(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，发现正午时的表影总是投向正北方向，就把石板制成的尺子平铺在地面上，与立表垂直，尺子的一头连着表基，另一头则伸向正北方向。正午时，表影投在石板上，古人就能直接读出表影的长度值。
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.
经过长期观测，古人不仅了解到一天中表影在正午最短，而且得出一年内夏至日的正午，表影最短;冬至日的正午，表影则最长。于是，古人就以正午时的表影长度来确定节气和一年的长度。