Who knows that there are tall stones in the square in front of the Forbidden City Gan Qing Palace (also in the Temple of Heaven)? What is it and what is its historical significance?

The sundial, an instrument for measuring the apparent time of the sun, originated from standard watches. The table is a vertical rod, and Gui is a ruler lying across the north and south. The two are vertical, and the apparent solar time is measured according to the length of the shadow on the table. The ancients began to use this method in the Spring and Autumn Period. A sundial consists of a watch (dial hands) and a dial engraved with a time line, which can be divided into a horizontal sundial. At present, the earliest reliable records can be found in Sui Shu. In the Yuan Dynasty, Guo Shoujing invented Yang Yi, which also had the function of spherical sundial. Sundials, all kinds of vertical and inclined sundials, were introduced by Jesuits who came to China in the late Ming Dynasty, or created by China scholars after studying Euclid's geometry.

I haven't been to Beijing, just from the description, I don't know if it's right, the attached picture, see if it's this.

/art/twdg/gtwyq/TG 00 13 . htm