How to calculate the sunshine time of 20 degrees north latitude at vernal equinox?

At the vernal equinox, the direct point of the sun is on the equator, and the northern and southern hemispheres are equally divided day and night. The sunshine time at 20 degrees north latitude is 12 hours.

Sunshine time =12× 2× arccos (tan α× tan β)/180.

α is the latitude of the direct point of the sun, which is negative when it is on the same side as the equator and positive when it is on different sides; β is the latitude of this place, which is positive.

When the direct point of the sun is on the equator, tanα=0, so the sunshine time is 12 hours.

Extended data

At the vernal equinox, theoretically, the world is as long as day and night (see the dawn). After the vernal equinox, the days in the northern hemisphere are getting longer and shorter, and the nights in the southern hemisphere are getting shorter and shorter. At the vernal equinox, there are no extreme days and nights in the world. After the vernal equinox, it began to be extremely daytime near the North Pole, and its scope gradually expanded. Near the Antarctic, the extreme day ends and the extreme night begins, and the scope gradually expands.

The vernal equinox is an important solar term, which not only has astronomical significance: the northern and southern hemispheres are equally divided day and night, but also has obvious characteristics in climate. During the spring equinox, China has entered a beautiful spring scenery except the Qinghai-Tibet Plateau, Northeast China, Northwest China and North China. On the vast land, willows are green, yingfei grass is long, wheat is jointing and rape is fragrant.