How did the ancients make calendars? How to determine the ecliptic plane?

In China's calendar, the earth revolves around the sun for one year, and the moon's waning period is January. Yang is the "sun" and Yin is the "lunar calendar", that is, the moon, so the traditional lunar calendar in China is actually a combination of Yin and Yang. The earth rotates 360 degrees around the sun, and the average 24 parts are divided into 24 solar terms, starting from the "beginning of spring" solar term and ending in the "Great Cold" atmosphere. Therefore, the definition of solar terms is based on the earth's movement, and the annual "beginning of spring" is fixed on February 4th or 5th of the Gregorian calendar.

It takes 8766 hours 13 minutes and 48 seconds for the earth to go around the sun once, which is 1 year. The moon orbits the earth once (that is, the profit and loss period) for 708 hours, 44 minutes and 2.8 seconds, which is 1 month. The difference between the solar calendar and the lunar calendar lies in the month: a month in the solar calendar is roughly divided by the annual average12; A month in the lunar calendar is represented by a moon's profit and loss cycle. Therefore, the Gregorian calendar sets a leap every four years (1 many days) to supplement the actual number of suns; The lunar calendar stipulates that the lunar month with "festival" and no "qi" is a leap month. The years with lunar leap months are 13 lunar month, 13 festival and 12 qi. There will be two beginning of spring festivals this year.

From this point of view, the Gregorian calendar only considers the solar cycle and completely ignores the moon cycle; However, the traditional lunar calendar in China considers both the solar cycle and the moon cycle. Lunar calendar is more scientific than solar calendar.

The ecliptic plane refers to the orbital plane of the earth revolving around the sun, and the intersection angle with the equatorial plane of the earth is 23 26'. Because the gravity of the moon and other planets affects the revolution of the earth, the position of the ecliptic plane in space is always changing irregularly. But in change, this plane always passes through the center of the sun at any time. The great circle where the ecliptic plane intersects the celestial sphere is called the ecliptic. The ecliptic plane is the great circle of the sun's apparent path through the sky; It can also be said that the earth's orbit around the sun is projected on the celestial sphere. The zodiac signs are arranged along the ecliptic, and the intersection angle between the ecliptic and the celestial equator is 23.4; The two intersections of the ecliptic and the celestial equator are the vernal equinox and the autumnal equinox. In the ecliptic coordinate system, the longitude of celestial bodies is measured eastward along the ecliptic from the vernal equinox, with positive north latitude and negative south latitude. The distance between the south and north poles and the corresponding celestial pole is 23.4. Because the earth's revolution is perturbed by the moon and other planets, the orbit of the earth's revolution is not a strict plane, that is, it produces irregular continuous changes in space, which includes many short-term and a slow long-term movements. Short-period motion can be averagely eliminated in a certain time, and the orbital plane with periodic motion eliminated is called instantaneous average orbital plane. The strict definition of the ecliptic is: the great circle where the instantaneous average orbital plane of the center of mass of the Earth-Moon system revolves around the sun and intersects the celestial sphere. The ecliptic is the base circle of the ecliptic coordinate system on the celestial sphere.