Deciphering the Secrets of Celestial Bodies
By Yongfeng Huang, 1st Aug 2023
Translated by Yifan Wang, 15th Aug 2023
Although you may very rarely observe the night sky, you can be sure that it is never static. Rather, it is ever-changing, shifting between different beautiful and unique appearances. What set of rules does it follow in order to change in such a way?
PART 01
To understand this question, it is necessary to construct an image of the universe as a 3-dimensional area of space. Imagine the sun and the earth at the centre of this space, with countless stars in all directions. We also imagine ourselves always standing in a certain position on Earth. Lastly, for the convenience of describing the locations of stars in the sky, all leads will be placed on a celestial sphere with the earth and the sun at its centre.
Furthermore, we also need to know that the positions of the celestial bodies relative to the sun are virtually constant (at least during a person’s short lifetime). Thus, we see movements in the night sky must be caused by movements of the earth itself and changes in the sun-earth system. Additionally, as the earth itself has a rotation axis with fixed poles, we can use the celestial coordinate system to describe the position of every celestial body in the night sky.
When observing the sky, we can only see the half of the night sky above us as the ground below us blocks the other half. When standing at different positions on the earth, the night sky we can see is also different. (You can picture this by looking at the diagram above, when standing at the North pole, you can only see the north half of the celestial sphere. The same is true for standing at any other point on Earth.) By having the above knowledge, we can make understanding the original question much easier.
PART 02
Firstly, as obvious as it sounds, we can only see the night sky at night, which means being outside of direct sunlight. Therefore, our view of the night sky is greatly dependent on the current season of the location where we are observing from.
The diagram above of the Earth orbiting the sun shows this very clearly. In summer, the night sky is facing the space on the left of the picture, while the space on the right appears overhead during the day, making stars in this region virtually impossible to see.
The same logic can be applied to the second factor, that being the earth’s own rotation. From sunset to sunrise, the position directly above us moves from one end to the other. For example, when you see the movements of the stars during a single night, it is simply caused by the rotation of the Earth shifting our position of observation.
Additionally, as the above description uses the sun as a stationary frame of reference, it can also be said that the celestial sphere rotates around the earth's rotation axis. To conclude, on different days of the year and at different times of the day, the celestial sphere will turn to different positions, and for different locations on the earth, the night sky will also change accordingly.
Star trail photos like the one shown above are exposed for a long time, as a result, the rotation of the celestial sphere becomes clear as day. The only fixed points are the North Celestial Pole and the South Celestial Pole, from the image, we can see quite a bright star at the North Celestial pole called Polaris.
PART 03
The aforementioned seasonal starry sky is also a phenomenon that we usually notice with relative ease. For example, the Milky Way can only be seen in summer, this is as the sun is on the spiral arm of the Milky Way. Thus, when the earth revolves between the centre of the galaxy and the sun in summer, the night sky faces the direction of the centre of the galaxy at night. In addition, each season has its own representative constellations and stars.
The summer triangle seen above is composed of Vega in Lyra, Deneb in Cygnus and Altair in Aquila. As these stars are very bright, they can still be clearly seen even in cities on clear nights.
The winter triangle on the other hand consists of Sirius in Canis Major, Procyon in Canis Minor, and Betelgeuse in Orion. Orion is also the most conspicuous and famous constellation in the northern hemisphere, with the famous M42 Orion Nebula.
The above is an introduction to the changes in the starry sky. We hope that this article has helped everyone gain a deeper understanding of the starry sky!