Why should the winter sundial look behind?

In a year, from summer to the sun, the shadow of the needle on the front of the sundial is the shortest; On the winter solstice, the shadow of the sundial needle on the back of the sundial is the shortest. At the vernal equinox and autumnal equinox, the shadow of the sundial needle is infinite. On the other day, whether in the morning, at noon or at night, the shadow length projected by the sundial needle on the sundial surface remains unchanged.

The equatorial sundial has two sides, but whether it is morning, noon or night, the sun can only shine on one side of the sundial, either front or back. For half a year, the sun only shines on the front of the sundial, from the vernal equinox in March to the autumnal equinox in September; For half a year, the sun only shines on the back of the sundial, from the autumn equinox in September to the spring equinox in March.

When the sun shines, the shadow of the needle moves with the movement of the sun, so that different positions of the shadow of the needle can reflect different positions of the sun, which also reflects different times of the day. At noon, the shadow is just below the needle.

Precautions:

There are many scale lines on the dial of a sundial. You can read the season, month and time by observing that the shadow of the dial pointer falls on the specific position of the scale on the dial. From "watch" to sundial is a gradual transition process.

If the "watch" is divided with appropriate scales on the ground, it can become a horizontal sundial. The calibration of horizontal sundial needs the knowledge of projection geometry, which was lacking in ancient China. Although the installation of equatorial sundial is complicated, its scale is relatively simple. Therefore, it is natural that the equatorial sundial was popular in ancient China.