I pointed a radio telescope in the direction of Perseus and Outer arms (about 80deg on pic related) to look at the 21cm hydrogen line. Got two peaks, as expected. But the second peak is blueshifted.

What I don't understand is: if the galaxy is rotating clockwise (in pic related) shouldn't both the Perseus arm and Outer arm be redshifted? i.e. moving away from the Sun faster than the Sun is approaching them?