Why The Milky Way Looks Cockeyed In The Sky

Wide angle view of the Milky Way photographed late on July 26. The band stretches from Perseus in the northeastern sky (upper left) to Sagittarius in the south (lower right). The ripples at upper left are caused by natural airglow. Bob King

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 sky. 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.

The Milky Way intersects the ecliptic — the path of the planets and plane of the solar system — at a steep angle because our solar system is tipped on its side relative to the plane of the galaxy. Stellarium

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° with respect to the plane of the galaxy.

I think we instinctively assume 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.

Two views of the Milky Way galaxy showing the sun’s approximate position. At left we see it face on, as if hovering over the galaxy. At right we see it edge-on, viewing it from the side. The white oval represents the orbits of the planets around the sun. Notice how steeply the plane of the solar system is tilted with respect to the galaxy. That’s why the Milky Way band is tipped at a steep angle to the path the sun, moon and planets take through our earthly skies. The sun and the solar system are enlarged for clarity. NASA with annotations by the author

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°, 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 (left side of screen) 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 visualize the cockeyed angle of our solar system with regard to the galactic plane.