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I have a question about the view of the Milky Way that we can see from earth with the naked eye. I believe this to be light from the stars contained in one of the "arms" of the galaxy. I had always assumed that, from our vantage point, I was looking outward through one of the arms. This was based on the appearance of the apparent curvature over the sky above.

However, it occurred to me that this could also be looking in towards the center of the galaxy. And the fact that it appears to curve around us over the night sky is just an illusion.

Or, perhaps at different times in the year we are alternatively looking inward and then later looking outward.

Can any shed some, errr, light on this?

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    $\begingroup$ Inward is towards Sagittarius. $\endgroup$ – Wayfaring Stranger Sep 25 '17 at 14:00
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    $\begingroup$ There is no curvature detectable from our viewpoint, at least not as visible arc. We see, as you already said, the whole panorama of the Milky Way throughout one year. Here the galactic centre is marked with the red box, and here you get an impression of the galactic planes' orientation. $\endgroup$ – Vroomfondel Sep 25 '17 at 14:11
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An image of the whole sky (360 degrees) centred on Sagittarius and rotated so the galaxy is in the middle:

Milky way

This image from the "2MASS infrared survey" actually uses infra-red light, as it penetrates the interstellar dust better than visible light. It shows that any apparent curvature is an illusion. The milky way goes straight across the sky.

It also shows that when we look towards Sagittarius we look towards the galactic centre, and when we look away from it (towards Auriga) we are looking through to stars in the outer spiral arms. The night side of the Earth faces the galactic centre during July, and faces outwards in January.

Also visible in the image are the two Magellanic clouds (small but nearby galaxies outside the milky way), the clear bulge around the centre of the galaxy and the dark bands of dust along the milky way. The central bulge is much brighter in infrared than the spiral arms. Using infrared makes these features more visible.

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  • $\begingroup$ Why there is the aec illusion? $\endgroup$ – Alchimista Nov 10 '17 at 12:07
  • $\begingroup$ If we are able to see the sky as a celestial sphere, then the milky way would be a great circle, but the sky near the horizon appears further than the sky near the zenith (see moon illusion) which might make the great circle be curved. $\endgroup$ – James K Nov 10 '17 at 16:16
  • $\begingroup$ Ok. I couldn't geometrically figure it out so it is nice to see that is a real illusion. $\endgroup$ – Alchimista Nov 10 '17 at 16:19

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