mollweide projection

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Ask Ethan: Why are maps of the cosmos oval-shaped?

“Why are the microwave background radiation graphs oval and not symmetrically round?”

We know the Earth is an almost-perfect sphere, yet maps of it always stretch it down to a two-dimensional surface. Outer space, too, when we look at it, offers views that extend deep into the cosmos in all directions, also on a sphere. But if you want to make a map of the Universe, you must stretch it down to a two-dimensional surface, too. These maps are almost always shown as oval-shaped, but there’s a good reason for this! You cannot show, simultaneously, a two-dimensional projection of a sphere that is simultaneously: connected, area-preserving, free of shape-distortion, and where latitude and longitudes appear perpendicular. You can pick three at most, and the projection most frequently chosen for space, the Mollweide projection, is very different from the Mercator projection chosen for Earth.