Why is the summer solstice sometimes 2 1 and sometimes 22?

Because the time of the earth's orbit around the sun is not completely 1 year, and the time when the sun directly hits the Tropic of Cancer is not a certain time every year, but it changes, but the changing interval is between 2 1 and 22.

When the sun moves to the longitude of the Yellow River at 90 degrees, it is the intersection of the summer solstice, usually on June 2 1 ~ 22 of the Gregorian calendar. On the solstice of summer, the sun shines directly on the Tropic of Cancer. At this time, the daytime in all parts of the northern hemisphere reaches the longest in the whole year, and the higher the latitude, the longer the daytime. For the Tropic of Cancer and its northern area, the summer solstice is also the highest day of the year at noon.

Extended data

The summer solstice is the tenth of the twenty-four solar terms. The "Twenty-four solar terms" were originally determined by the bucket handle direction of Beidou, which means noon, that is, the summer solstice. The current "twenty-four solar terms" are divided by the method of "constant gas", that is, each solar term corresponds to a certain position reached by the earth every time it runs 15 on the ecliptic, and the summer solstice is when the sun runs to 90 degrees on the ecliptic.

Seen from the ecliptic plane, the sun is on the background of Taurus, Gemini and Cancer. The summer solstice is the northernmost day of the sun in a year, the limit of the sun's northward movement and the longest day in the northern hemisphere. The higher the latitude, the longer the days. This is because the "long day and short night effect" caused by the tilt of the earth's rotation axis is more obvious as it approaches two levels.

On the day of the summer solstice, except the south pole of the Antarctic circle and the polar night area, the sunrise direction in all parts of the earth starts from the northeast and sets in the northwest. The summer solstice is the northernmost day of the sun in a year and the limit of the sun's northward movement. On this day, the northern hemisphere gets the most solar radiation, almost twice as much as the southern hemisphere. After the summer solstice, the Tropic of Cancer and its northern region, the midday sun altitude angle also began to decrease.