1. The morning and evening line of the winter solstice

2. Side view

3. Rectangular projection

The vernal equinox is the middle of the 90 days in spring. point. The Gregorian calendar is one of the twenty-four solar terms. Around March 20 every year, the sun is located at 0° of ecliptic longitude (the vernal equinox).

The autumnal equinox is the 16th solar term among the 24 solar terms in the lunar calendar, usually from September 22nd to 24th in the Gregorian calendar. The climate in the south begins to decline from this solar term.

After the autumnal equinox, the direct point of the sun continues to move from the equator to the southern hemisphere. The days in the Northern Hemisphere start to get shorter and the nights get longer (the days are shorter than the nights), and the days in the Southern Hemisphere start to get longer and the nights get shorter (the days are longer than the nights), so the autumnal equinox is also called the setting point.

The summer solstice is one of the 24 solar terms on June 20, or the 21st or 22nd of the solar calendar.

On the summer solstice, the sun moves to 90 degrees of yellow longitude (summer solstice, currently in Gemini). The sun points directly to the ground to its northernmost point at the end of the year, almost directly toward the Tropic of Cancer. For the Tropic of Cancer and the Tropic of Cancer, the summer solstice is also the sun's highest day of the year.

After the summer solstice, the solar altitude angle in the Tropic of Cancer and northern regions begins to decrease. At the same time, after the summer solstice arrives, the stars in the night sky gradually turn into the summer sky.