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Is it actually relevant to study objects that are light years far? Because in theories we are actually seeing the past as the light takes years to reach us.

Lets say we are studying a asteroid that is going to hit as after several hundred years how can we confirm we are truly studying the actually position and everything of that object as we are seeing object from the past.

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    $\begingroup$ An asteroid which is going to hit the Earth is likely to be very close (astronomically speaking) – mere light-minutes away instead of light-years. $\endgroup$ – Glorfindel Apr 17 '18 at 7:12
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    $\begingroup$ I'm fairly certain when it comes to estimating paths of asteroids etc., the light-travel delay can be accounted for whether it's seconds or days. $\endgroup$ – user10106 Apr 17 '18 at 7:27
  • $\begingroup$ Asteroid is just an example, lets take Andromeda - milky way collision. As Andromeda galaxy is about 2.5 million light-years far so actually the scientists concluded it is going to happen by studying things that are probably taken place years back. Right $\endgroup$ – Unnikrishnan R Apr 17 '18 at 7:29
  • $\begingroup$ Well everything can be accounted for, at least for objects moving "around" us and for which a model considering expansion of the universe isn't necessary and won't be affected by change of cosmological model. Obviously the colliding Andromeda will have morphed into something different as compared to what we see. But knowing its "current" position and motion let us predict where it will be. $\endgroup$ – Alchimista Apr 17 '18 at 8:37
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Most things in space move very predictably. For reasonably dense objects, like stars, planets or asteroids, gravity far outweighs all of the other forces acting on them, and it's not too hard to take account of gravity and predict their movements (if you have a powerful enough computer. For things like clouds of very thin gas it's a bit harder. So from the positions and movements of all the distant objects that we can see (at the time when the light we see them by left them), we can pretty accurately predict their movements a considerable way into the future, for instance up to possible collisions a few centuries out (for an asteroid) or a few billion years out (for Andromeda).

Of course there is a possibility of something unexpected happening -- between the light we see now being emitted and a possible future collision, a supermassive black hole travelling at close to the speed of light could pass through the Andromeda galaxy and scatter it, for instance, but we don't see things like that happening anywhere else in the universe, so it seems pretty unlikely to happen here.

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how can we confirm we are truly studying the actually position and everything of that object as we are seeing object from the past.

We can't.

All astronomical observations are of things as they were at a time in the past that depends how far away the object is. We understand that. If we need to calculate the future behaviour of an object, then our extrapolation begins from that past point in time to work out a trajectory through time and space.

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