Re: How do astronomers know that stars are located in the same place as we see

Hi Koen,

Your question touches some very deep parts of our understanding of the
universe. Let's look at what we mean by "location", first, and use an example.

Where am I located? As I'm currently typing this, I'm sat in a chair in an
office in Glasgow. So we've got a time and a position. But by the time you
read it, I could well be somewhere else. The information you've inferred
about my location (chair, office, Glasgow, "now") is likely to be
out-of-date due to the time it takes that information to reach you. 

What does this mean? Well, when that information finally reaches you, at
the moment you call "now", you'll be receiving a snapshot of my location at
a point in time that I would then be calling my "past". 

This is caused by a "finite speed of information" which also applies to
visual information conveyed by the speed of light. Everything you
see, from your computer monitor, to the planets, and on to the furthest
stars, is a portrayal of objects in the past.

While your monitor may not have moved much in the couple of nanoseconds it
takes for its light to travel to your eyes, the planets and further stars
may have moved great distances. Jupiter moves over 30000km in the
40-or-so minutes it takes its light to reach us. Arcturus (36.7 light years
away) moves about 140 billion kilometres since that star's light set out.

Now factor in your light bending. For nearby stars and galaxies this effect
is really very small, but it can be measured when light passes near to the
surface of the Sun, and it can be highly noticeable over greater distances
when larger gravitational masses are involved. Here is a 
good example of light from a distant galaxy being bent by the
gravity of a nearer one.

What does this all mean with regards to location? 

In effect, we never know the present location of an object, as there
is no universe-wide sense of "now" as a point of reference, no "infinite
speed of information" to back up such a view. Astronomers are looking into
time when they receive information about the universe, and while the
rough location of objects is easy (Jupiter is orbiting the Sun, Arcturus is
a near neighbour star in this galaxy) our ideas of "location" for distant
objects are relative to our position in time and space as viewers. Can you
see that these "locations" would necessarily be different if our position
was different?

I hope this helps answer your question - and makes you think more about the
effects that relativity can produce!