The sundial, an ancient Chinese timekeeping instrument consisting of a bronze needle and a stone dial, measures the time by taking advantage of the shadow of the sun. The needle crosses the center of the dial vertically, just like the pole of the gnomon, and the stone dial is placed onto a stone platform, higher in the south and lower in the north. The sundial surface is therefore parallel to the equatorial plane.

The upper end of the sundial points right to the North Pole, and the lower end to the south. Both sides of the dial are divided into 12 parts, each part representing two hours. When the sunlight beams down on the sundial, the shadow of the needle falls on the surface and moves as the sun moves. Therefore, the needle's shadow resembles the needle of a modern watch or clock, and the stone dial is like the watch or clock's dial plate.

From the vernal equinox (about March 20) to the autumn equinox (about September 22), the sun moves in the northern side of the equator, casting the needle's shadow on the upper part of the stone dial. Likewise, since the sun moves in the southern side of the equator from the autumn equinox to the vernal equinox, the shadow is cast on the lower part. Therefore, when observing the sundial, it is important to first find out the projection position in the two different periods.

Author: Jessie