Two people talking about the unavoidable ecliptic-on the ecliptic

Talking about things without the sun and the earth. When it comes to the sun and the earth, you can't bypass the ecliptic. So here is a brief explanation of the ecliptic.

When it comes to the ecliptic, many people think of it as "auspicious day", which seems a bit superstitious. In fact, the ecliptic is just the name given by our ancestors to the path that the sun "walks" in the sky. Generally speaking, the ecliptic is the path taken by the sun's apparent movement on the celestial sphere for one year when we observe the sun on the earth. In modern terms, the ecliptic is the projection of the earth's orbital plane around the sun on the celestial sphere.

The so-called celestial sphere is an imaginary big sphere like the sky we see on earth. You can see the sun (and sometimes the moon) on the celestial sphere during the day, and you can see the stars and the moon at night. The position of most stars on the celestial sphere is basically unchanged in a person's life, and they are called stars. Some stars have obvious changes in their positions like the sun and the moon. They are called planets. It is said that there are nearly 7000 stars visible to the naked eye on the celestial sphere. For easy identification, people divide these stars into several groups called constellations. Different ancient civilizations divide and name constellations differently, but they often have some similarities. The legends produced by constellations have also become a part of various civilizations. 1922 the international astronomical union unified the naming of 88 constellations covering the whole celestial sphere. The fixed positions of these stars and constellations on the celestial sphere provide reference for the position changes of the sun, moon and planets.

The sun rises and sets in the morning, and the celestial sphere we see "revolves around us" with the sun every day. We can't directly see the position of the sun in the celestial sphere, because the sunlight blocks the light of all the stars. But there are some ways to estimate the position of the sun in the celestial sphere. For example, it can be inferred from the position where the celestial sphere is blocked by the sun during the day. For another example, you can write down the position in the celestial sphere that you are facing at some time in the evening, and the position opposite to this position, that is, add 180, which is the position of the sun in the celestial sphere after 12 hours.

If you carefully observe and record the position of the sun on the celestial sphere every day, you will find that the position of the sun on the celestial sphere moves a little westward every day. This movement is called the apparent movement of the sun, because this movement of the sun on the celestial sphere is actually the result of our observation of the sun on the moving earth. The earth goes around the sun, and the apparent motion of the sun draws a circle on the celestial sphere. Our ancestors thought that this was the path taken by the sun in the sky for one year, which was called the ecliptic. From a modern point of view, the ecliptic is actually the projection of the earth's orbital plane around the sun on the celestial sphere.

The following figure shows the relationship among celestial sphere, earth, sun, celestial equator, earth orbit and ecliptic:

Sorry, I didn't find such a picture marked in Chinese, so I have to say a few words. )

The big circle in the picture represents the celestial sphere, and the green circle is the celestial equator, which is an extension of the equatorial plane of the earth (the blue ball in the middle). The blue earth revolves around the yellow sun, and its orbital plane (small black circle with arrow) extends to the celestial sphere, which is the ecliptic plane, and the big black circle around the ecliptic plane is the ecliptic.

The figure also shows the north celestial pole (green) and the south ecliptic pole (gray). The north and south poles (celestial poles) of the celestial sphere are the projection of the earth's rotation axis in the celestial sphere, and the north and south poles (ecliptic poles) of the ecliptic are the projection of the perpendicular line perpendicular to the ecliptic plane and passing through the center of the ecliptic. The picture shows an included angle of 23.4 between the celestial sphere line and the ecliptic line. This included angle is the inclination of the earth's rotation (axis). Therefore, the equatorial plane of the celestial sphere and the ecliptic plane present an included angle of 23.4. This included angle is called the yellow-red intersection angle. Two points (red) projected on the celestial sphere by a straight line intersecting two sides are called vernal equinox. The red dot in front is the familiar vernal equinox, and the red dot behind is the familiar vernal equinox.

Just as we can describe the position of a place by the latitude and longitude on the earth, the stars in the sky can also describe their position by the latitude and longitude on the celestial sphere. If the earth is the center, the ecliptic is zero latitude, and the meridian connecting the two ecliptic poles through the vernal equinox is zero longitude, the spherical coordinate system thus formed is called the ecliptic coordinate system. The latitude to the north of the ecliptic is positive (0–90), and the latitude to the south of the ecliptic is negative (0–90), and the longitude varies from 0 to 360 from the vernal equinox to the east (counterclockwise). Figure 2 below shows the position of the star on the celestial sphere in the ecliptic coordinate system. Its longitude is the degree of pink angle and its latitude is the degree of yellow angle.

Babylonian astrologers also observed that there were only 12 constellations near the ecliptic. If the ecliptic is divided into 12 segments, each segment is 30, then there is almost exactly one constellation in each segment. They called the region where the 12 constellation is located on the celestial sphere the zodiac, and called the 12 part of the zodiac as the zodiac, and each house was named after the corresponding constellation. Zodiac 12 Palace is the core concept of ancient astrology. We'll talk later.

Some people say that the concept of the ecliptic was introduced to China in the Sui Dynasty. Although the Sui Dynasty was about 1000 years from the 5th century BC, it was not impossible to introduce it, but there seems to be no connection between the ecliptic and China's ancient description of the ecliptic does not seem to mention the connection with the solar eclipse. Therefore, unless there is obvious evidence, I would like to believe that the ecliptic of China and Babylonia should be independent of each other.