The ancient Greek astronomer Ptolemy's geocentric model believed that the sky was composed of layers of "celestial spheres," with stars fixed on the outermost celestial sphere.
Is the sky really spherical? Why do people all have such similar feelings? Actually, this is an illusion. The reason for this illusion is that the stars are too far away. We cannot judge which stars are close and which stars are far away. As a result, we lose our sense of distance and mistakenly believe that every star is the same distance away from us. Therefore, the sky appears to us as a sphere.
Although this perception of a "celestial sphere" is an illusion, in astronomical observation, we can use the concept of a "celestial sphere" to determine the positions of celestial bodies in the sky. In astronomical observation, the celestial sphere is an imaginary giant sphere centered on the observer and with a radius of infinity. Since this sphere is infinitely far, we don't need to consider which stars are far from us or which are close; instead, we treat them all as celestial bodies at infinity. Therefore, we only need to draw a grid coordinate system on the celestial sphere to...
Longitude and latitude are used to record the position and changes of celestial bodies. When astronomers observe through telescopes, they can use these coordinates to accurately locate celestial objects.
To visually represent the celestial sphere, people created celestial globes. The armillary sphere of the Han Dynasty in China and the Pharnesian celestial sphere of ancient Rome are representative of early celestial globes. Celestial globes are similar in appearance to globes, also having north and south poles and an equator. However, the Earth is a real sphere, while the celestial sphere is an imaginary one. Instead of depicting rivers and mountains on the ground, the celestial sphere shows the sun, moon, and stars. Compared to globes, the coordinate system on a celestial globe is much more complex. Besides the commonly used equatorial coordinate system, there are ecliptic coordinate systems based on the plane of the sun's orbit, galactic coordinate systems based on the disk of the Milky Way, and horizontal coordinate systems based on the horizon of the observation site, all of which can be used to record the positions of various celestial bodies.
Three-dimensional celestial globes are designed from the perspective of an observer outside the celestial sphere, thus showing the opposite orientation as seen from Earth, making them inconvenient to use. Two-dimensional star charts projected from the sky are more commonly used. The Southern Song Dynasty stone-carved astronomical chart in Suzhou is an example of this.