# What is the Orientation of the “All-Sky” Projection?

The image below is the 9-year WMAP image (2012) of the cosmic microwave background radiation across the universe (from Wiki). As I understand it, the major axis represents the equator and the minor axis the central meridian. But, what is the orientation of the equator and central meridian relative to the earth ? In other words, how do I know where I'm looking ?

• – Farcher Nov 18 '16 at 9:20

That's just a Mollweide projection of the celestial sphere (or to be more accurate, of the surface of last scattering).

The usual convention is to use the equatorial coordinate system, so the major axis of the ellipse represents the projection of the Earth's equator. The minor axis of the ellipse is usually right ascension = 0h or 0°, but I guess RA = 12h or 180° is also a reasonable choice.

## Update

Oops! As Thriveth mentions in the comments, it appears that the usual coordinate system used for CMB is the galactic coordinate system, which is a spherical projection centred on the Sun. The equatorial plane of this projection is approximately the galactic plane of the Milky Way, with the centre of our galaxy (the radio source Sagittarius A*) in the centre of the map.

It's difficult to determine the true galactic plane, since that's dependent on the distribution of all the mass of the galaxy, so the galactic coordinate system is defined in reference to radio observations of galactic neutral hydrogen through the hydrogen line.

• I may be wrong but I believe the convention for the CMB maps is actually to have them arranged along Galactic coordinates, so the "equator" is the Miolky Way disk. See e.g. wmap.gsfc.nasa.gov/media/101082/index.html for comparison. – Thriveth Nov 18 '16 at 22:24
• @Thriveth It looks like you are most likely correct. :oops: Thanks for that info! – PM 2Ring Nov 19 '16 at 2:28
• So does this mean that the center of the CMB projection is in the direction of Sagittarius A*? And which way would "up" on the projection be? Galactic "up"? – Aaron Franke Jan 14 at 3:07
• @Aaron Yes, the centre of that CMB map is in the direction of Sag A*. I can't give you any more info than what's in the linked Wikipedia article on the galactic coordinate system. Click on the images in that article for higher res views. – PM 2Ring Jan 14 at 4:40