# How can we calculate velocity of an object given the distance from an object in parsecs?

If you had the distance from a celestial object, in parsecs, how would you calculate the velocity of the said object?

Edit: I'm sorry I don't have more context, the specific questions is: "A galaxy that is 10 million parsecs away will be moving at what velocity?"

• Welcome! Your question is a little short on detail. Distance alone does not control the velocity at which an object is moving. A space probe, and the planet Pluto, and an incoming comet can all be at the same distance but one will be moving away from us, one towards us, one neither towards or away. Are you perhaps asking about the expansion of the universe, rather than the movements of individual objects? Jun 7 '19 at 7:29
• There's Hubble velocity and peculiar velocity. You can roughly calculate Hubble velocity, but not peculiar velocity. For that, you need redshift: en.wikipedia.org/wiki/Peculiar_velocity Jun 7 '19 at 16:07

## 2 Answers

You can't, not very well anyway. That's simply because the velocity of an object isn't calculable from the Hubble constant until the so-called Hubble flow starts to dominate (which in turn only happens at cosmological distances, i.e. very far away).

That said, if you are in the regime where Hubble flow dominates, you can do this easily. The Hubble constant is approximately $$70 km/s/Mpc$$. Just convert your distance into megaparsecs, and multiply. You end up with a velocity, which is the one you're looking for (this neglects peculiar velocity, which will affect everything in the sky).

If the object is moving directly towards or away from us & we can get a spectrum,the Doppler shift will give us its velocity. Blue shift means it's approaching us,reshift means it's receding. If the object is orbiting some larger body with a known mass,the period of its orbit will indicate velocity (if the orbit is elliptical,its velocity will vary,speeding up as it gets nearer to the larger body). If its distance is neither increasing nor decreasing,we can track its motion within a certain timescale to calculate its velocity. Sometimes more than one method may need to be used.

• The original (and revised) question is "What is the correlation between velocity and distance?". I do not see that mentioned in your answer. Jun 7 '19 at 15:57
• That's a good wheeze; if you don't like somebody you can change the question after they've answered it! I must try that myself. Sep 5 '19 at 18:42