Ask Ethan: Why are maps of the cosmos always oval-shaped? | by Ethan Siegel | Starts With A Bang!

These two projections show the primordial sky in microwave light as seen by ESA’s Planck mission. At left, a hemispherical projection (encapsulating half of the sky) is shown; at right, a Mollweide projection (encapsulating the full sky) is shown. (Credit: ESA/Planck Collaboration (both); Damien George/thecmb.org (L))

It’s difficult to project a sphere onto a flat, two-dimensional surface. All maps of the Earth have flaws; the same is true for the cosmos.

The Universe is a vast and expansive place. From any location, you have total freedom to look in any direction you like: up or down, left or right, and near or far, to any distance in any direction that you choose. (Well, so long as there isn’t anything nearby in the way of a more distant object that you want to observe.) It’s like you have a buffet, an omnidirectional buffet, of targets to choose from. You can even imagine observing it all: not just the half of the sky you can see by lying down in a field on a clear night, but in all directions all at once, like if you had an array of lenses that looked around in all 360° at once (plus the ability to view 90° up and down from the horizontal), that gathered light from all possible angles simultaneously.

And yet, when we show images of the cosmic microwave background — whether from COBE, WMAP, Planck, or a different mission — they’re almost always shown as oval-shaped. What does that oval shape actually show us, and why do astronomers make that specific visualization choice? That’s what Ed Matzenik wants to know, writing in to ask:

“I don’t understand the projections we see of the CMB. They are usually a circle or an oval. Is that the whole sky or just a section? If I was looking at a sphere from inside I don’t know how I’d represent it on a flat sheet… hope you can clear up this mystery for me.”

Honestly, the first time I encountered them — and remember, I’m a professional cosmologist who first encountered them in graduate school — I suffered from almost exactly the same puzzlement. Let’s begin with something we’re much more familiar with in order to get started: planet Earth.

This view of the Earth comes to us courtesy of NASA’s MESSENGER spacecraft, which had to perform flybys of Earth and Venus in order to lose enough energy to reach its ultimate destination: Mercury. Several hundred images, taken with the wide-angle camera in MESSENGER’s Mercury Dual Imaging System (MDIS), were sequenced into a movie documenting the view from MESSENGER as it departed Earth. Earth, an oblate spheroid, rotates roughly once every 24 hours on its axis and moves through space in an elliptical orbit around our Sun. (Credit: NASA/MESSENGER)

This is going to sound obvious, but the first thing you have to realize about planet Earth is that, to a first approximation, its shape is spherical. The most accurate tool we use to model and represent…

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