<p>As the comments to your question say, the short answer is "yes". A lightyear is, by definition, the distance traveled by a photon in one year.</p>

<p>Note, however, that at large distances, the answer becomes more complicated. Due to the expansion of the Universe, the light received from a galaxy far, far away was emitted when that galaxy was closer to us. That means that in the beginning, the photons traverse a larger "fraction" of the Universe per time than at a later time.</p>

<p>Thus, to calculate the total pathlength you need to integrate its distance in an expanding coordinate system.</p>

<p>For instance, if a galaxy was a 4 billion lightyears away from the Milky Way when the Universe was only 1 billion years old, it is now 28 billion lightyears away. But as the Universe is 13.8 billion years old, the light received today from that galaxy has been traveling for "only" 12.8 billion years.</p>

<p>Objects in our local neighborhood, however, such as stars in our own galaxy and the nearest galaxies (e.g. Andromeda) are gravitationally bound and thus "resist" the expansion of space, so here your assumption is correct.</p>

CC BY-SA 3.0
2015-01-08T13:24:00.390Z
2015-01-08T13:24:00.390Z
pela
<p>So, I hear people say blah blah blah is 3 light years away, and here's a photo. So does that mean the photo that we see are actually at least 3 years ago since that's how long it takes that light to travel to Earth for us to take the picture?</p>

CC BY-SA 3.0
2014-12-24T18:05:00.420Z
Well, yes, it is three light-years away. Although that's not even to the nearest star, so I'm curious as to what's in the photo in question. I'm guessing it's an arbitrary number, though, right?
HDE 226868
CC BY-SA 3.0
2014-12-24T19:32:50.793Z
Yes. And I'll have to add some words to make it a legitimate comment!
ProfRob
CC BY-SA 3.0
2014-12-25T15:31:56.163Z
Looking into space is looking back into time. With light moving at only 29.98 centimeters (11.8") per nanosecond, your view of the far wall of whatever room you're currently sitting in is multiple nanoseconds older than your view of the LCD display directly in front of you.
Wayfaring Stranger
CC BY-SA 3.0
2014-12-25T19:59:31.467Z
The exact time you are looking back will depend on your relative motion w.r.t. the distant object, which gives rise to the famous [Andromeda Paradox](https://en.wikipedia.org/wiki/Rietdijk–Putnam_argument).
Count Iblis
<star><distances>
Photos from galaxy
TomZilla
# Photos from galaxy

(Asked by: username_0 on 2014-12-24)

So, I hear people say blah blah blah is 3 light years away, and here's a photo. So does that mean the photo that we see are actually at least 3 years ago since that's how long it takes that light to travel to Earth for us to take the picture?

**Tags:** star, distances

> **Comments:**
> **username_1**: Well, yes, it is three light-years away. Although that's not even to the nearest star, so I'm curious as to what's in the photo in question. I'm guessing it's an arbitrary number, though, right?  
> **username_2**: Yes. And I'll have to add some words to make it a legitimate comment!  
> **username_3**: Looking into space is looking back into time. With light moving at only 29.98 centimeters (11.8") per nanosecond, your view of the far wall of whatever room you're currently sitting in is multiple nanoseconds older than your view of the LCD display directly in front of you.  
> **username_4**: The exact time you are looking back will depend on your relative motion w.r.t. the distant object, which gives rise to the famous [Andromeda Paradox](https://en.wikipedia.org/wiki/Rietdijk–Putnam_argument).  

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## 1 answers

### Answer by username_5 on 2015-01-08. Score: 1

As the comments to your question say, the short answer is "yes". A lightyear is, by definition, the distance traveled by a photon in one year.

Note, however, that at large distances, the answer becomes more complicated. Due to the expansion of the Universe, the light received from a galaxy far, far away was emitted when that galaxy was closer to us. That means that in the beginning, the photons traverse a larger "fraction" of the Universe per time than at a later time.

Thus, to calculate the total pathlength you need to integrate its distance in an expanding coordinate system.

For instance, if a galaxy was a 4 billion lightyears away from the Milky Way when the Universe was only 1 billion years old, it is now 28 billion lightyears away. But as the Universe is 13.8 billion years old, the light received today from that galaxy has been traveling for "only" 12.8 billion years.

Objects in our local neighborhood, however, such as stars in our own galaxy and the nearest galaxies (e.g. Andromeda) are gravitationally bound and thus "resist" the expansion of space, so here your assumption is correct.


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