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The Milky Way galaxy is disc-shaped and Earth is in its plane:
(source)
In the following image, the Milky Way appears form an arch shape across the sky:
(source)
This looks like an out of plane view of the Milky Way. How is it possible? Why does it appear as an arch shape and not a thick straight line?
The Earth is a sphere (or is nearly a sphere). So to make a map of the whole Earth you would need to project it onto a flat surface. When you do this you create distortions. For example, on many maps, the straight line between New York and Japan looks like a long curve. It is not actually a curve, but when you stretch the surface of Earth to make it flat, some straight lines become curved.
The sky is also a sphere. If you ignore distance and just think of the direction of each point in the sky it would be a huge sphere that surrounds you. When we take a picture of the sky we must project that sphere onto a flat surface. That causes distortions, and it means that what is a straight line on the sphere of the sky is distorted by the projection into a curve.
If you choose a projection in which the milky way goes straight across the middle, you can see that the milky way goes straight across the sky.
The appearance of an "arch" is a distortion caused by stretching the image to fit on your flat screen. It is not real.
The arc is due to the panoramic shot. If you take masking tape and put it on a straight line on your ceiling and then take a pano, you'll have an arc.
I arrived here because I thought I'd find a milky way pic where you could see the earth surrounded by the whole donut (milky way across the whole horizon in a pano, with the bright center on one side and the doughy exterior on the other), but there don't seem to be any of those photos.
For example, if you then take masking tape and put it at eye level across 360 degrees of your room and take a 360 degree pano, you will have no arc. Yet it seems like this case never exists on the horizon in the world of milky way photography.
It is always curved, in other words, and this can be seen here, where the milky way is close to the horizon in a pano and still curved. In fact, it looks as if we are in a high elliptical orbit looking down in the disc... which I don't think we are.
Without getting into details and because I am getting trolled (see comments) there seems to be reason to believe that there will be extra curvature in some circumstances. We'll call it aberration.
For that reason, pictures like the one above will always be arced and never have the milky way stretching through the whole pano along the horizon, I guess (or not).
There is also this wacky article that significantly muddies the water:
Milky way is MORE warped than astronomers thought (a floppy sombrero)
As a skeptic (or rather cynic), it is unclear where the gravitational lensing starts and the mistakes stop, but there seem to be many contradictions related to how the milky way ought to look as a general matter.
Interestingly, the floppy sombrero milky way article and flat views of the milky way (accepted answer as of 5/2022) contradict themselves, and the contradiction helps illustrate the power of cv2 to warp a milky way image to whatever shape you need.
The appearance of an "arch" is a distortion caused by stretching the image to fit on your flat screen. It is not real.
I have seen a number of explanations of this that claim it is due to lens distortions or projections on the flat plane of the image/screen. I don't think either one is correct. If you are ever in a dark-enough place to actually see the MW with your own eyes, it looks like an arc any time the band is lower than 45 degrees or so above the horizon. If you take an image of the MW at that point with a strictly rectilinear lens, mounted with nodal slide to conrtrol for paralax, and then stitch it together it will still look like an arc. Yes, the arc can be exaggerated by lens distortion, but that is not the primary reason the MW looks like an arc.
A number of apps that predict the position of the MW exist today, and they clearly show the COMPUTED position of the MW in the sky as an arch (see for example PhotoPills or Stellarium). The appearance of an arch is due to the projection of the MW on the curved sky, not on "your flat screen". The solid angle subtended by the MW is very small, and when you view it from a point on the Earth's surface that is substantially out of the projection plane it looks like a curve. As pointed out above, when the MW projection plane directly intersects a large circle of the Earth AT THE POINT WHERE YOU ARE STANDING the band is directly overhead and it looks like a flat plane.
The closest analogy I can come up with is to think of a LARGE hoop. Place your head in the center of the hoop and look at it edge-on. It looks like a straigt line. Now rotate the hoop slowly around an inplane axis while maintaining your head in the center. It starts to look like an arc.
