How far away from the Milky Way disk can we see the entire Milky Way?

A friend asked: If the solar system moves slightly away from the plane of the Milky Way, would we be able to see a huge Milky Way hanging in the night sky?

This depends on how much the "slightly" and "somewhat" in the questioner's "slightly away from the plane of the Milky Way" are. Some friends have analyzed this problem from a geometric perspective. I will not argue it from a mathematical theory here, but only analyze it from an intuitive and perceptual perspective based on Earth observation data.

We know that scientists now understand the Milky Way as a huge spiral barred galaxy, a bit like a discus, a disk galaxy that is thick in the middle and thin at the edges. The disk has a diameter of about 200,000 light-years, a thickness in the center of about 12,000 light-years, and an edge thickness of only about 100 light-years, with an average thickness of about 1,000 light-years.

Now we know clearly that if the Sun is now at the center of the Milky Way's disk (actually 20 light years away, close to the center), in order to be able to see the entire disk of the Milky Way clearly, it must at least jump out of the Milky Way's disk itself by 1,000 light years. The thickness of the Milky Way's core reaches 12,000 light years. To see the entire disk, you must be at least 12,000 light years away from the center of the disk's section.

In fact, even if you are 12,000 light years away from the Milky Way, it is like a person standing on the top of Mount Everest looking at the Earth, and you still cannot see the entire Earth. According to NASA tests, the Earth's arc, or the boundary of space, can be clearly seen at about 100 kilometers above the ground; manned spacecraft can see the Earth's surface curvature in an orbit 400 kilometers above the ground; and to see the entire Earth, you have to be 3,400 kilometers away from the ground.

This field of view is calculated based on the human eye's horizontal viewing angle of 124 degrees, which is the most comfortable field of view. According to this viewing distance, the diameter of the earth is about 12,742 kilometers, which is about 127:1 compared to the height of 100 kilometers where the earth's arc can be seen; the ratio is about 32:1 compared to the height of 400 kilometers where the earth's surface curvature can be seen; and the ratio is about 3.75:1 compared to the height of 3,400 kilometers where the whole picture can be seen.

Looking at the Milky Way according to the ratio of how humans view the Earth, if you want to see the arc of the Milky Way, you have to be at an altitude of 1,574 light-years from the surface of the Milky Way disk; if you want to see the curvature of the Milky Way, you have to be at an altitude of 6,250 light-years from the surface of the Milky Way disk; if you can comfortably see the entire panoramic view of the Milky Way, you have to be at an altitude of about 53,333 light-years from the Milky Way disk.

A light year is the distance that photons travel in a vacuum in one year. Photons travel about 300,000 kilometers per second in a vacuum, and a light year is about 9.46 trillion kilometers. Therefore, the diameter of the Milky Way is about 189.2 trillion kilometers, and the average thickness is about 964 billion kilometers. To see the whole Milky Way, you need to be 50 trillion kilometers away from the surface of the Milky Way and observe it at a 124-degree angle of the human eye.

According to the question, in order to see a huge Milky Way disk hanging in the night sky, the actual distance is much farther, so that you can see the huge Milky Way with a certain amount of space left. So, is this distance "slightly away from the Milky Way plane" as the questioner thinks? Only he knows.

So far, humans have not stepped out of the Earth's gravitational circle, and unmanned spacecraft are still far from flying out of the solar system. So the question is, how are the countless pictures of the Milky Way circulating on the Internet obtained?

In fact, all the pictures of the appearance of the Milky Way are not what humans actually see or what are photographed by cameras. Instead, they are obtained by scientists through observations of many similar galaxies and the integration of massive data obtained from observations of the distribution of stars in the Milky Way on Earth over the years, and then through computer modeling.

Most of the images of the Milky Way that we actually see on Earth are only partial images, which are obtained by synthesizing many photos taken by large telescopes or satellites toward the center of the Milky Way from our location. The only one that claims to be a 360-degree panoramic view of the Milky Way was taken by NASA's Spitzer Space Telescope over 10 years, using more than 2 million photos.

This 360-degree panoramic view of the Milky Way contains a lot of information, including 20 billion pixels. If it is printed, it will take a stadium-sized space to display it. Now NASA has released a digital version of this picture to scientists and astronomy enthusiasts around the world for research and inquiry (see the picture above). However, this picture is quite professional and difficult for ordinary people to understand.

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