What Is the Celestial Sphere?

The celestial sphere refers to an imaginary sphere centered on the center of mass M of the earth and the radius r is arbitrarily long. The purpose is to project celestial bodies onto the sphere along the observer's line of sight in order to study celestial bodies and their relationships. Sometimes the celestial sphere center is also set at some special points, such as the earth's center and the heliocentric. The corresponding celestial spheres are called the earth's celestial sphere and the heliocentric celestial sphere. Astronomical navigation takes the earth's center as the center of the celestial sphere according to its own needs. ^[1]

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Celestial sphere is research

Distance on earth, angular distance and line distance. But on the celestial sphere, there is only angular distance and no line distance, because the size of the celestial sphere is arbitrary. As for the actual distance between two celestial bodies, for example, 16.4 light-years apart between Cowherd and Weaver, that is the straight-line distance in space. Any point on the celestial sphere only represents one space direction; the arc length between any two points is actually the angle between the two directions. For example, the angular distance between Cowherd and Vega is 35 °

Introduction to celestial sphere

Celestial coordinates

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

This coordinate system divides the celestial sphere into declination and right ascension. The declination algorithm starts from the celestial equator and ends at the poles. The celestial equator is 0 degrees, north to the celestial pole is +90 degrees, and south to the celestial south pole is -90 degrees. The algorithm of the right ascension is special. Unlike the algorithm of the longitude of the earth (from -180 degrees to +180 degrees), the ascension is from 0 hours to 24 hours from the west to the east of the celestial equator, which is to divide the 360 degrees of the week into 24 on average. It can be known that one hour is equal to 15 degrees. Like time, the right ascension can be divided into 60 minutes per hour, and each minute can be further divided into 60 seconds. In addition, the minute and second here refers to the hour, minute, hour, and second, which is different from the traditional angular minute, angular second, and 1 hour = 15 angular minutes, and 1 hour = 15 angular seconds. The starting point of the right ascension is the vernal equinox, which is the position of the sun on the vernal equinox (around March 21).

Like a spinning gyro, the earth's rotation axis is not fixed in space, but rotates at a period of 26,000 years. This movement is called precession. Therefore, the positions of the vernal equinox and the north pole of the celestial sphere will also move very slowly. Therefore, when we use celestial coordinates to indicate the position of a celestial body, we should also indicate which year's coordinates, such as the year 2000.0.

Celestial sphere coordinates

Zodiac coordinate system	Huang Jing, Huang Wei, Zodiac
Equatorial coordinate system	Right ascension, declination, equator, hour angle
Ground coordinate system	Azimuth, Altitude, Horizon, True North, Zenith, Zenith
Galactic coordinate system	Yin Jing · Yin Wei · Yin Pan · Yin Xin
other	Vernal Equinox, Autumn Equinox, Celestial Ball, Celestial Point, Arctic