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Why is the WMAP picture of the CMB an ellipse?

I do not understand how the WMAP (and earlier, less resolved COBE) picture of the CMB and the early universe can be portrayed as a two dimensional ellipse. If the WMAP satellite scanned the entire universe in all directions, would it not have to be portrayed truly like an image on the inside of a globe? Or like the projection of stars on the inside of a planetarium dome? Has such a three dimensional image been flattened for purposes of display on a page, similar to a Mercator projection map?

You're probably referring to a picture just like this one:

WMAP picture
A map of the sky made by the WMAP satellite. Credit: WMAP,NASA

You're absolutely right that the "true" projection of the sky as seen from Earth is akin to the inside of a globe. Astronomers refer to this as the celestial sphere.

But unfortunately, computer screens are only 2-dimensional. So there's no good way to display a 3-dimensional image. You can see 2-dimensional pictures of the WMAP projected onto a 3-d sphere on the WMAP homepage. But of course you need views from multiple angles to see the whole sky.

The ellipse that you describe is called an Aitoff projection and it's the most commonly used projection by astronomers when we want to look at maps of the whole sky. It's also of course used for many world maps. The Aitoff projection is a compromise between the shape distortions and scale distortions which are inevitable whenever you project a 3-d globe onto a 2-d surface.

January 2004, Christopher Springob (more by Christopher Springob) (Like this Answer)

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