Always guess a solar term.

Always guess a solar term. Answer: summer solstice.

Because on the day of summer solstice, the direct point of the sun reached the tropic of cancer. At this time, the days in the northern hemisphere are the longest, which means that the days from summer solstice are the longest in a year. Therefore, "everlasting" refers to the summer solstice in the 24 solar terms.

Historical evolution of summer solstice;

"Twenty-four solar terms" is the product of ancient farming civilization. At that time, people discovered the changes of the four seasons in nature, and gradually realized that the changes of climate and phenology in a year are also regular.

Summer solstice is the fourth solar term in summer. At this time, the sun moves to 90 degrees of the yellow meridian, and the time is generally on June 2 1-22 of the Gregorian calendar. It is not only one of the 24 solar terms, but also one of the ancient folk festivals, also known as "Summer Festival" and "Summer Solstice Festival", with the custom of offering sacrifices to gods and ancestors.

The summer solstice was established very early. According to some data, in the seventh century BC, the sun shadow was measured by civil soil gauges, and it was found that the day was the longest and the night was the shortest, so it was designated as the summer solstice. There is also such a record in Zunxian Manuscript: "The day reaches the north, the day is long, and the shadow is short, so it is called the summer solstice. At best, it is excellent. " On the day of summer solstice, the sun shines almost directly on the tropic of cancer, which is the longest day of the year in the northern hemisphere.