There’s something I’ve wondered about for years. Astronomers always say that the Andromeda Galaxy (also known as M31) is going to collide with our own. After all, we measure the galaxy’s velocity via the Doppler effect, and we find it’s moving towards us. But the Doppler effect only lets you measure the radial component of the galaxy’s velocity — that is, how fast it’s moving towards or away from us, but not how fast it’s moving laterally. How do we know that Andromeda isn’t moving sideways at a high enough speed to miss us?
Well, it turns out not to be moving sideways very fast at all, so it is going to hit us. Good to know I guess. I’m just glad to see that this question I’ve been wondering about for so long is a legitimate question: I’ve seen so many mentions of the impending collision, with no reference at all to the lateral-velocity question, that I was wondering if I was missing something obvious.