Time taken by light and the way we observe stars

[achaleus]

Hello every one, it is known that time taken by light from sun to reach earth is 8 min.. that means what we are observing is (sun) 8 min prior that it was(sun) right.

similarly as stars are so many light years away from us, so, what we are observing is so many light years back that which currently may not be in the same position...

please clarify my doubt.. I think you can understand my question

 

[cameo_2007]

your assumption is True. Sometimes the star would have already dead also.

 

[achaleus]

So, all statements like new star was born... observed supernova... by NASA and others all these happend in past, so we can see the thing which were all ready happen but not the present..

how they calculate that the asteroid may hit the earth so and so time and year.. as they are not observing the current position of that

thanks in advance

 

[jeeudr]

So, all statements like new star was born... observed supernova... by NASA and others all these happend in past, so we can see the thing which were all ready happen but not the present..

how they calculate that the asteroid may hit the earth so and so time and year.. as they are not observing the current position of that

thanks in advance

The frame of reference is the key !

Lets take the example of a star exploding in a supernova.

In the frame of reference of the observer, the fact of the supernova is realised only after the light has reached the observer. There is NO other way to know this information at any instant before this time has passed.

Suppose an event has occurred at a distance 'd' away, then the earliest that you can 'know' about it is after time t = d/c . c = speed of light (theoretical speed limit for any information).

It is does not matter that you miss out on this time interval. Because, in physics, all motion and exchange of forces also follow this rule.

Even if the sun were to disappear right now, we will continue to experience its light and gravity all the way until ~8 minutes later. For our frame of reference, the sun is still there until ~8 minutes later. So it is still fine.

 

[BradtheRad]

how they calculate that the asteroid may hit the earth so and so time and year.. as they are not observing the current position of that

thanks in advance

They can only calculate orbits for asteroids (and comets) they know about. They observe an asteroid's position, and adjust their figures to match.

Sometimes an asteroid is disturbed in its orbit, causing it to go on a trajectory that might bring it close to Earth. Astronomers would not know about it, until they look for the asteroid and find it isn't where it's supposed to be.

There are a great many asteroids we don't know about. Sometimes one approaches Earth, and we have no more than a day's notice.

As to the time lag of observation when light travels between planets...
The story is that after Jupiter's moons were discovered, astronomers calculated tables to predict their locations from night to night. However the moons were found to be sometimes 6 hours before, sometimes 6 hours after the predicted location.

Until one astronomer realized we need to factor the speed of light into the calculations.

https://en.wikipedia.org/wiki/Ole_Rømer

 

[godfreyl]

Until one astronomer realized we need to factor the speed of light into the calculations.

https://en.wikipedia.org/wiki/Ole_Rømer

OT, but I enjoyed this:

In 1705, Rømer was made the second Chief of the Copenhagen Police, a position he kept until his death in 1710. As one of his first acts, he fired the entire force, being convinced that the morale was alarmingly low.

:lol: