How is the Summer Solstice calculated?

The Summer Solstice is the longest day of the year. On this day, the sun reaches its northernmost point on the earth, almost directly over the Tropic of Cancer (23°26' North). The Northern Hemisphere experiences the longest day, and the further north you go, the longer the day becomes. For example, the daylight in Haikou, Hainan, is just over 13 hours, 14 hours in Hangzhou, about 15 hours in Beijing, and more than 17 hours in Mohe, Heilongjiang.
Summer Solstice Date Calculation Formula: Date = [Y × D + C] - L
Formula Explanation: Y = last two digits of the year; D = 0.2422; C = 21.37 for the 21st century; L = number of leap years.

When is the Summer Solstice usually held?

The Summer Solstice is one of the twenty-four solar terms. It occurs on June 20th, 21st, or 22nd each year. On this day, the sun reaches 90 degrees on the ecliptic. For regions north of the Tropic of Cancer, the Summer Solstice is the day with the highest noon sun altitude. On this day, the Northern Hemisphere receives the most solar radiation, nearly double that of the Southern Hemisphere. Astronomers say that the Summer Solstice is the turning point of the sun. After this day, it begins to "turn back," and the direct sunlight point starts to move south from the Tropic of Cancer, causing the days in the Northern Hemisphere to gradually shorten.