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When we read that, for instance, M31 is 2.54 ± 0.11 Mly (778 ± 33 kpc) away from us, does this distance estimate take into account the travel time of the light we observe? In other words, what is the "timestamp" on the distance estimate? Does it actually reflect how far away M31 is now?

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  • $\begingroup$ The light we see today was emitted 2.54 Myr ago. So, no it does not reflect the distance "now." $\endgroup$ – Kornpob Bhirombhakdi Dec 18 '18 at 21:56
  • $\begingroup$ Theret is not such an easy timestamp. Look for cosmological calculator on line. Usually you can imput the various parameters and find out what the distance was in term of cosmological time, I.e. the time elapsed since the big bang. Look at my comment to Rob Jeffries's A, too. $\endgroup$ – Alchimista Dec 20 '18 at 10:32
  • $\begingroup$ The issue of cosmological time or space expansion has no consequence for the distance to M31. $\endgroup$ – Rob Jeffries Dec 20 '18 at 17:39
  • $\begingroup$ A comoving distance and a physical or proper distance are different things. For M31 etc you are correct but you could have made it clear. M31 is an example. One could ask for an object distant 12 billions ly..... $\endgroup$ – Alchimista Dec 21 '18 at 10:49
  • $\begingroup$ About distance at cosmological scale. Let me rephrase everything. The answer by @Chappo is correct, and more so after editing, for objects of interest for a backyard astronomer. They are gravitationally bound and space expansion does nothing. Moreover they propermotion happens at tiny fraction of c.*But in general a light time distance only tells you how long that light has travelled, not about the physical distance of the source now or when the light was emitted*. A galaxy seen at 12 billions ly might be now further away and was closer before. It is all I wanted to say. $\endgroup$ – Alchimista Dec 22 '18 at 8:23
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It represents the distance when light measured was emitted. That is the conventional way that the distance to anything is defined isn't it.

To do anything else requires a very accurate knowledge of what will happen to an object in its future. You could in principle do that, if you have very good measurements of an object's velocity and know all the forces acting upon it) but it doesn't make much difference unless something is travelling at a fair fraction of the speed of light (and M31 isn't).

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  • $\begingroup$ Good to know. It's moving at 0.001003 ly/yr. Not really enough to affect things. $\endgroup$ – Alphecca Dec 18 '18 at 23:23
  • $\begingroup$ @Alphecca exactly: and much smaller than the uncertainty in the distance. $\endgroup$ – Rob Jeffries Dec 18 '18 at 23:51
  • $\begingroup$ It isn't even the distance when it was emitted considering that space expands! It can be reasonable for objects within the galaxy or anyway gravitationally bound. $\endgroup$ – Alchimista Dec 20 '18 at 10:27
  • $\begingroup$ @Alchemista The space IS NOT expanding between M31 and our Galaxy or in any gravitationally bound system like the Local Group. $\endgroup$ – Rob Jeffries Dec 20 '18 at 17:36
  • $\begingroup$ M31 was given as example. What you have said holds for close enough objects and of course for those gravitationally bound to us. $\endgroup$ – Alchimista Dec 21 '18 at 10:51

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