AstroBob: Why the Milky Way looks cockeyed in the sky

A reader asked a great question the other day about whether the Andromeda Galaxy is north of the band of the Milky Way or south. Remember that when we look at the band, we’re looking directly into the plane of the galaxy, neither above nor below. From mid-northern latitudes, the galaxy appears high in the sky, well north of the sun’s highest point in the. So you might guess it’s also north of the Milky Way, too. But it isn’t. It’s below the plane, south of the Milky Way band.

Have you ever noticed on a summer night that the Milky Way slices across the sky at a steep angle from Cassiopeia in the northeastern sky to down to Sagittarius Teapot in the south? There’s a reason it doesn’t follow the same path as say, the planets, across the sky. A reason that may just blow your mind. Turns out the plane of the solar system — the flat paths the planets follow around the sun — is tilted 60.2 degrees with respect to the plane of the galaxy.

I think we instinctively assume the solar system orbits flat around the center of the galaxy, when the truth is it’s tipped up on its side. Like the Earth, the Milky Way galaxy has north and south poles. The galactic north pole points in the direction of the constellation Coma Berenices not far from the brilliant orange star Arcturus. The south galactic pole resides in Sculptor, located low in the southern sky on autumn nights as seen from the northern hemisphere.

Anytime you look outside the band of the Milky Way — whether above or below — you’re looking above or below the plane of the galaxy. To know for sure if an object is north or south of the galactic plane, note its position in relation to the Milky Way band. If it’s below the band, you’re looking south. If above, you’re looking north of the band (in the northern hemisphere).

Sometimes it’s not so easy to determine north or south because the band is so strongly tilted. Then you’ll need to know the object’s galactic latitude. Yep, it’s just like latitude on Earth. For instance, the Andromeda galaxy has a galactic latitude of -21.5 degrees, placing it below or south of the plane of the galaxy.

If you download a star plotting program like Stellarium (free at stellarium.org) it will give the galactic latitude and longitude of every object you click on. If you pair your visual observations of the Milky Way and click around in Stellarium, in time, you will be able to visualize the cockeyed angle of our solar system with regard to the galactic plane.