Who can introduce the latitude and longitude of the celestial sphere in detail?

We stood on the earth and looked up at the starry sky, and saw that the stars in the sky seemed as far away as us. Stars are like gems embedded in a round sky.

In fact, the distance between the stars and us is far or near. What we see is their projection on this huge sphere. This imaginary sphere is called the celestial sphere, and its radius is infinite. And the earth hangs in the center of this celestial sphere.

The direction of the stars in air movement is not chaotic, and the shape of the constellation will not change. The stars climb from the eastern horizon to the highest point (meridian) and then sink to the west. It looks as if the whole celestial sphere is going around the earth. I believe everyone knows that the earth is not the center of the universe, and stars do not revolve around the earth. The stars revolve around us in the sky because of the illusion caused by the rotation of the earth, and the celestial sphere itself will not move. When we are on the earth, we don't feel ourselves spinning, just like when we take a train, we see the scenery outside the window retrogressing, but we don't feel ourselves moving.

diurnal motion

The stars rotate once a day in the sky, which is called Sunday exercise. Extending the earth's rotation axis to the position on the celestial sphere is the north pole and south pole of the celestial sphere. The position where the earth equator extends to the celestial sphere is the celestial equator.

There is a second-class star very close to the north pole of the celestial sphere, so it seems to never move, and other stars seem to revolve around it. We call this star Polaris. Because Polaris seems to stay in the north forever and never go down the mountain, we can use Polaris to tell the direction, just like people living in the northern hemisphere. Unfortunately, there are no light stars near the south pole of the celestial sphere, so there is no "south" polar star to guide the residents of the southern hemisphere.

The height of Polaris relative to the ground depends on the latitude of the observer's position. For example, in Beijing, Polaris will be located due north, 40 degrees above the ground; In the Arctic, Polaris will be overhead (zenith); On the equator, Polaris is just on the horizon; In the southern hemisphere, Polaris will never rise above the horizon, so Polaris will never be seen in the southern hemisphere.

Similarly, some stars will never rise. People living in the northern hemisphere will never see the stars near the south celestial pole, and people living in the southern hemisphere will never see the stars near the north celestial pole.

The above three simulation legends show the movement of stars that can be seen in the northern hemisphere. The first one points to the north pole of the celestial sphere (you will find that the Polaris is not completely fixed), the second one points to the south and the third one points to the east respectively.

celestial coordinate system

In order to accurately describe the position of stars in the sky, astronomers have developed a coordinate system to represent the position of stars on the celestial sphere. This coordinate system is very similar to the usual latitude and longitude coordinates on the earth.

This coordinate system divides the celestial sphere into declination and right ascension. The algorithm of declination is from the celestial equator to the pole. The celestial equator is 0 degrees, the celestial north pole is +90 degrees, and the celestial south pole is -90 degrees. The right ascension algorithm is quite special, which is different from the earth longitude algorithm (from-180 degrees to+180 degrees). Right ascension is at the equator of the celestial sphere from west to east for 0 to 24 hours. Right ascension, like time, can be divided into 60 minutes, and each minute can be subdivided into 60 seconds (note: the minutes and seconds of right ascension are not equal to the angular minutes and seconds). The starting point of right ascension calculation is the vernal equinox, which is one of the two intersections (the other is the equinox) between the celestial equator and the ecliptic.

Like a spinning top, the earth's axis of rotation is not fixed in space, but rotates in a period of 26,000 years. This movement is called precession, so the position of the vernal equinox and the north pole of the celestial sphere will also move slowly. Therefore, when we use celestial coordinates to represent the position of celestial bodies, we should also point out which year the coordinates are, such as the year 2000.0.