雙語科技百科(天文) 第15期:圭表

The Gnomon

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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.

圭表是一種既簡單又重要的測天儀器，它由垂直的表(一般高八尺)和水平的圭組成。圭表的主要功能是測定冬至日所在，並進而確定迴歸年長度，此外，通過觀測表影的變化可確定方向和節氣。

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，人們發現房屋、樹木等物在太陽光照射下會投出影子，這些影子的變化有一定的規律。於是便在平地上直立一根竿子或石柱來觀察影子的變化，這根立竿或立柱就叫做“表”;用一把尺子測量表影的長度和方向，則可知道時辰(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.

後來，發現正午時的表影總是投向正北方向，就把石板製成的尺子平鋪在地面上，與立表垂直，尺子的一頭連着表基，另一頭則伸向正北方向。正午時，表影投在石板上，古人就能直接讀出表影的長度值。

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.

經過長期觀測，古人不僅瞭解到一天中表影在正午最短，而且得出一年內夏至日的正午，表影最短;冬至日的正午，表影則最長。於是，古人就以正午時的表影長度來確定節氣和一年的長度。