We were having a conversation where I work about the Fomalhaut pictures, and it occurred to me that I've always known that the farther you look out, the further you're looking into the past.

I'm grabbing my info from Wikipedia. Assuming Fomalhaut is about 25 light years away, is it also safe to assume that we're looking at Fomalhaut 25 years ago?

On that note, the Sloan Great Wall (link), is around one billion light years from Earth. Therefore, many of the objects making up the wall may or may not even be there today.

Does this sound right?

Sorry, misposted here. Should probably be in q/a.

That's one way to view it. It makes language difficult though. Many just don't let the lightspeed get in the way and speak of events at great distances just as they do ones at nearby distances, saying, for instance, they occur when the signal arrives.

As the Bad Astronomer said, in BA Blog: Mi Cas A es su Cas A (about an event happening 10000 lightyears away, recorded in 1680):

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I have no problem seeing a supernova tonight and saying that it happened tonight. Just wanted to make sure I was understanding the distances involved.

Cas A exploded in 1680 for the record, but actually occurred about 10,000 years ago. That's what I was looking for. Cheers!

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You are understanding the distances involved correctly.

That's where it gets weird and almost philosophic. And it has to do with what your mean by "today" (and I'm not being a smart mouth ). Unless and until there is some sort of faster-than-light travel, we can really only interpret today, at least with regard to a specific star or object, as what we see today.

But on the other hand, when we talk about generalized things, such as what was the structure of galaxies in the very early Universe, looking at objects 10 billion light years away (and thus 10 billion years ago) is sort of looking at the past.

At least that's my take on it.

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Now I see it a little differently. To me, it gets more complicated when astronomers don't use light-travel time. Cas A exploded 10,000 years ago, but the light only reached Earth in 1680. What could be simpler? If you say we saw Cas A explode in 1680 that is fine, but to say it exploded in 1680 is misleading.

Some might say we cannot consider an event to have happened until we have seen it, but obviously as soon as we see it we know it has happened and if we have an idea of how long the light we see has been travelling for, then we have an idea how far in our past the event happened.

Relatively speaking, it did.

The one seen in 1987 took place about 168,000 years ago.

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I am in agreement with 'speedfreek' We saw this event in 1680. Allows room for the explanation if it's required.

I am so thoroughly accustomed to thinking of light requiring time to
travel from the point of origin to my eye that it seems silly that this
could be a problem for anyone. When I watched the TV coverage of
the Mars Phoenix lander as it landed on Mars, I knew that the events
being being described by the people at JPL had already happened
25 minutes earlier. When I see light from the Sombrero Galaxy in a
telescope, I know the light was emitted more than 50 million years
ago, so what I see is the way it looked 50 million years ago.

What I don't remember is when I first learned about this time delay.
Probably before I was ten years old, but possibly as early as four or
five, when I may have learned that there would be a time delay
communicating between Earth and astronauts on the Moon or Mars,
if that was mentioned in Disney's Space movies made in the 1950s
with the help of Werner von Braun.

-- Jeff, in Minneapolis

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