The ancients had a clever trick to determine the date of the winter solstice: measuring the sun shadow with a straight rod.

In ancient times, China determined the winter solstice and summer solstice by measuring the sun's shadow. The specific means is to erect a straight pole, observe the projected shadow of the sun, and determine the solar terms according to the movement and length of the shadow.

Of course, this pole is exquisite, called Tugui, which is a kind of jade. It is said that as early as 4,000 years ago, during the Yao-Shun period, Tugui was used, and it was recorded in Zhou Li. Is it the scientific research achievement of Yao and Shun period? It is difficult to infer, but the Zhou Dynasty generally followed the method of Shang Dynasty, which at least proved that China had a method to confirm the winter solstice and summer solstice more than 3,000 years ago.

Network mapping

So how long is the shadow of the winter solstice? "Zhou Li" records that "the winter solstice day, the sun is in the morning glory, and the scenery is ten feet long."

It is also recorded in the Weekly Parallel Calculation that an eight-foot-high pole is erected on the flat ground, "so the winter solstice sundial is three feet and five inches long, and the summer solstice sundial is six inches long." "Han Shitian Wenzhi" records: "The table is eight feet long, while the scene is three feet one inch and four minutes long."

Of course, the dimensions here are different from those in modern times.