The length of par is from eight to sixteen.

The standard watch is an astronomical instrument invented by ancient Han scientists to measure the length of the sun shadow, which consists of two parts: the standard watch and the watch. Like the sundial, the sundial is an ancient astronomical instrument, which uses the shadow of the sun to measure. As early as the 20th century BC, during the period of Taosi ruins, China began to use it. It is said that the sundial was developed on its basis. The standard table is to determine the length of the noon shadow to determine the solar terms and the tropic year or solar calendar year. For a long historical period, the accuracy of the tropical year determined by China ranks first in the world. Through further research and calculation, ancient Han scholars also mastered the length of the sun shadow in the standard table of 24 solar terms. In this way, the standard table can not only be used to formulate festivals, but also be used to discharge the dates of the future solar calendar year and the twenty-four solar terms in the almanac, as an important basis for guiding the agricultural activities of the working people of the Han nationality.

The standard watch stands upright on the flat ground to measure the shadow of the sun, which is called a watch; The rigid plate used to measure the length of the surface shadow in the direction of due south and due north is called Gui. When the sun shines on the watch, the shadow of the watch appears on Guiyu. According to the direction and length of the shadow, the time can be read.

Guibiao is an instrument for observing astronomical phenomena in ancient China. In different seasons, the direction of the sun and the height at noon are different, with periodic changes. Put Guiping in the open air to the north of the table, measure, compare and calibrate the Sunday and anniversary changes of the sun shadow according to the shadow on Guiping, so as to determine the direction, measure the time, find out the anniversary constant, divide the seasons and make a calendar. Therefore, mapping is one of the main observation methods in ancient astronomy in China.