A simple and important astronomic instrument, the gnomon (an instrument used to calculate the time, season, and so on) is composed of a vertical gnomon (about eight chi, 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 ruler has lines to indicate the months of the year. Every day at noon, the shadow of the sun passes the horizontal line, and shows the season.

A long time ago, the ancient people found that things, like houses and woods under the sunshine, would cast shadows on the earth, the changes of which followed a certain rule. In order to observe the shadow's 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).

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.

After a long-time observation, the ancient people not only figured out when the length of the gnomon's shadow was the shortest within a day, but also the time for the shortest and longest shadow within a year. Therefore, they used the midday length of the shadow to measure the solar terms and the length of a year.

Author: Jessie