2. Side view

3. Orthogonal projection

The vernal equinox is the 90-day vernal equinox of spring. Gregorian calendar is one of the 24 solar terms. On March 20th every year, the sun is at 0 (vernal equinox).

The autumnal equinox is the sixteenth solar term in the twenty-four solar terms of the lunar calendar, usually from September 22nd to September 24th in the Gregorian calendar. The climate in the south began 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. In the northern hemisphere, the days start to get shorter and the nights start to get longer (shorter than the nights). In the southern hemisphere, the days start to get longer and the nights start to get shorter (longer than the nights), so the autumnal equinox is also called the landing point.

The summer solstice is one of the 24 solar terms, and June 20th is 2 1 or 22 in the solar calendar.

During the summer solstice, the sun moves to 90 degrees of the yellow meridian (summer solstice, currently in Gemini). The sun points directly to the northernmost point of the year on the ground, almost directly to the Tropic of Cancer. For the Tropic of Cancer and the Tropic of Cancer, the summer solstice is also the highest day of the year.

After the summer solstice, the solar altitude angle in the tropic of cancer and the northern region began to decrease. At the same time, after the arrival of the summer solstice, the stars in the night sky gradually became the summer sky.