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What features of the Milky way's appearance in our sky led scientists to conclude that its wider than thick?
What features of the Milky Way’s appearance in our sky led scientists to conclude that its width is much larger than its thickness? Why did they originally believe that the Sun was near the Milky Way’s center? What key observations forced scientists to change their views about the location of the Sun within the Milky Way?
If the Milky Way was not wider than it's thickness, we would see stars distributed more or less evenly across the sky. The fact that it is a narrow band with many stars in it, while the sky outside the band has much fewer stars, leads to the assumption that it's fairly thin for it's diameter.
The reason for the original assumption of the Sun near the center of the galaxy is that if you do "star counts" in different sections of the Milky Way, you will see basically the same number of stars in equal areas of it. Until the mid 1800's, there was no way to tell exactly how far away the stars were. Once the distance to some stars could be calculated using the star's parallax, we knew how far away some stars were, but we could only measure the close ones, so there was no way to measure the very distant stars in the Milky Way, and see if one side was closer to us.
It wasn't until about the 1920's that astronomer Harlow Shapely studied the distribution of globular clusters in the Milky Way, and found that they were not evenly distributed across the sky. The most logical reason for that was that we were on one side of the galaxy, not in the center.
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