system
Given the following post, respond as if you're commenting on Reddit:
human
How can we create maps of a galaxy/universe if we don't know the relative position of the celestial objects?

Allow me to elaborate; when I see maps of our galaxy or the universe, they are shown to be "complete", as if we knew the relative position of all the celestial objects at a single point of time. Since light takes time to travel from distant points, we can't know for certain where a distant object B was at a point of time when closer object A was at that same point of time. The way I see it is that the map of galaxy/universe would be best represented like an animated sonar, where it begins with our solar system as the starting point (hope that makes sense).

Are the maps of the galaxy/universe accounting for the relative disposition of objects by predicting where distant objects would be after x amount of time? Or are the maps "flattened", so to speak?

I apologise if it's difficult to understand, I'm trying my best to articulate what I'm trying to ask, please let me know if I can clarify the question.
gpt
Almost always, the speed of light is so much faster than the dynamical speed of a system that you do basically get a snapshot of the whole system simultaneously at some moment in time. Predictions for current positions can also be quite accurate because accelerations are fairly small during the light travel time, again because the light is just that fast.

Within the Milky Way, most things orbit at <300 km/s, which is 0.1% of the speed of light. We're about 30,000 light years from the centre of the galaxy, and the disc as a whole is maybe 100,000 light years in radius. At our orbital speed, it takes about 250 million years to orbit the galaxy. But the light takes well under a million years to cross the galaxy. So, even for the most distant object on the far side of the galaxy, it has moved much less than a degree around the galaxy since during the light delay time.

It's similar on extragalactic timescales. The Andromeda galaxy should collide with the Milky Way in about 4-5 billion years. But it's less than 3 *million* light years away. So the error from light delay is like 0.1%. This is typical of galaxy interactions in general - in a cluster or galaxy group, you've generally got distance scales of millions of light years, and time-scales of billions of years.

Now, this is just per *system* - where a system is a star or a galaxy or a group or cluster of galaxies. But we do see distant systems as they were in the past. In that sense, you are correct that we are mapping things as we see them, and not in their present-day position. However, that's okay because we really only use these maps to understand the history and evolution of the universe, and the details of exact present-day positions aren't as interesting as seeing how galaxies have changed and interacted and changed density over time.