How to calculate the vernal equinox and autumn equinox?

The sun shines directly at the equator at the vernal equinox and autumnal equinox, and the sun height at the equator reaches the maximum of 90 degrees in a year at noon.

In summer, the solstice sun shines directly on the Tropic of Cancer. At noon, the sun height of the Tropic of Cancer reaches the highest 90 degrees in a year.

In winter, the solstice sun shines directly on Capricorn, and the sun height in Capricorn reaches the highest 90 degrees in a year at noon.

Variation law of solar altitude angle;

1. Latitude variation law: it decreases from the latitude and longitude where the direct point of the sun is located to the north and south sides. It can be inferred that if the latitude differs from the direct point of the sun by one degree, the altitude angle of the sun at noon will be reduced by one degree. (Further, it can be concluded that there are generally two latitudes equal to this degree when the solar altitude angle is known at noon).

For example, when the sun shines directly at 20 N, the global noon sun height angle will gradually decrease from 20 N to the north and south sides, and the noon sun height angle of19 N is equal to 89. That is, the noon sun height 19 N = 90- (the direct point of the sun-the latitude of the place) = 90-(20- 19) = 89.

2. Seasonal variation law: the direct sunlight point gradually increases when moving in and decreases when moving out (the direct sunlight point is relative to the latitude of a place).

For example, in the area of 31n, during the period from 12.22 (winter solstice) to June 22 (summer solstice), the midday sun height angle gradually increases, and during the period from June 22 (summer solstice) to 12.22 (winter solstice), the midday sun height angle gradually increases.