<p>It represents the distance when light measured was emitted. That is the conventional way that the distance to anything is defined isn't it.</p>

<p>To do anything else requires a very accurate knowledge of what will happen to an object <em>in its future</em>. 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).</p>

CC BY-SA 4.0
2018-12-18T23:23:05.617Z
Good to know. It's moving at 0.001003 ly/yr. Not really enough to affect things.
Alphecca
CC BY-SA 4.0
2018-12-18T23:51:14.350Z
@Alphecca exactly: and much smaller than the uncertainty in the distance.
ProfRob
CC BY-SA 4.0
2018-12-20T10:27:01.587Z
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.
Alchimista
CC BY-SA 4.0
2018-12-20T17:36:36.800Z
@Alchemista The space IS NOT expanding between M31 and our Galaxy or in any gravitationally bound system like the Local Group.
ProfRob
CC BY-SA 4.0
2018-12-21T10:51:10.890Z
M31 was given as example. What you have said holds for close enough objects and of course for those gravitationally bound to us.
Alchimista
CC BY-SA 4.0
2018-12-21T10:56:57.673Z
As it is  is now correct for M31. Almost correct in general as far away galaxies are not really travelling.
Alchimista
CC BY-SA 4.0
2018-12-22T16:23:49.487Z
" It represents the distance when light measured was emitted. That is the conventional way that the distance to anything is defined isn't it.". No it isn't.  It is the distance the light has travelled. Sorry to insist but I see a flaw and moreover in an accepted answer. For M31 it holds but it is not the general, physics answer.
Alchimista
CC BY-SA 4.0
2018-12-22T16:59:35.103Z
@Alchimista You should write an answer. This answer is not "cosmological".
ProfRob
CC BY-SA 4.0
2018-12-23T08:31:42.710Z
@Rob Jeffries Your answer will be totally right with two grammatically light changes. Like remove reference about distance definition and adding one to now and past alongside with *future*. Speaking about lighttime distance is cosmological in nature. I didn't write one yet because I feel I should address z (which is the accessible parameter) but I am afraid it might deviate from the easy level discussion the Q settled down. Perhaps I'll do. Do not take it personal!
Alchimista
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2018-12-24T09:48:21.090Z
New and related question. See comments to that. https://astronomy.stackexchange.com/q/28850/17437
Alchimista
CC BY-SA 4.0
2018-12-18T20:55:18.973Z
2018-12-18T21:27:00.473Z
2018-12-18T21:27:00.473Z
ProfRob
<p>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 <em>now</em>?  </p>

CC BY-SA 4.0
2018-12-18T21:56:06.037Z
The light we see today was emitted 2.54 Myr ago. So, no it does not reflect the distance "now."
Kornpob Bhirombhakdi
CC BY-SA 4.0
2018-12-20T10:32:28.540Z
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.
Alchimista
CC BY-SA 4.0
2018-12-20T17:39:46.327Z
The issue of cosmological time or space expansion has no consequence for the distance to M31.
ProfRob
CC BY-SA 4.0
2018-12-21T10:49:18.820Z
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.....
Alchimista
CC BY-SA 4.0
2018-12-22T08:23:18.200Z
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 *proper*motion 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.
Alchimista
CC BY-SA 4.0
2018-12-22T12:39:01.477Z
Chappo is an error. I wanted to mention @Rob Jeffries.
Alchimista
<astrophysics>
Distance to objects
Alphecca
# Distance to objects

(Asked by: username_0 on 2018-12-18)

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*?

**Tags:** astrophysics

> **Comments:**
> **username_1**: The light we see today was emitted 2.54 Myr ago. So, no it does not reflect the distance "now."  
> **username_2**: 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.  
> **username_3**: The issue of cosmological time or space expansion has no consequence for the distance to M31.  
> **username_2**: 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.....  
> **username_2**: 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 *proper*motion 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.  
> **username_2**: Chappo is an error. I wanted to mention @Rob Jeffries.  

---

## 1 answers

### Answer by username_3 on 2018-12-18. Score: 3 | Accepted answer

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).

> **Comments:**
> **username_0**: Good to know. It's moving at 0.001003 ly/yr. Not really enough to affect things.  
> **username_3**: @username_0 exactly: and much smaller than the uncertainty in the distance.  
> **username_2**: 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.  
> **username_3**: @Alchemista The space IS NOT expanding between M31 and our Galaxy or in any gravitationally bound system like the Local Group.  
> **username_2**: M31 was given as example. What you have said holds for close enough objects and of course for those gravitationally bound to us.  
> **username_2**: As it is  is now correct for M31. Almost correct in general as far away galaxies are not really travelling.  
> **username_2**: " It represents the distance when light measured was emitted. That is the conventional way that the distance to anything is defined isn't it.". No it isn't.  It is the distance the light has travelled. Sorry to insist but I see a flaw and moreover in an accepted answer. For M31 it holds but it is not the general, physics answer.  
> **username_3**: @username_2 You should write an answer. This answer is not "cosmological".  
> **username_2**: @Rob Jeffries Your answer will be totally right with two grammatically light changes. Like remove reference about distance definition and adding one to now and past alongside with *future*. Speaking about lighttime distance is cosmological in nature. I didn't write one yet because I feel I should address z (which is the accessible parameter) but I am afraid it might deviate from the easy level discussion the Q settled down. Perhaps I'll do. Do not take it personal!  
> **username_2**: New and related question. See comments to that. https://astronomy.stackexchange.com/q/28850/17437  

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