# Thread: Light year and a telescope's 'eyes'

1. ## Light year and a telescope's 'eyes'

If, for example. the Andromeda Galaxy is 2.3 million light-years away, does it mean that it takes a telescope that amount of time to see that galaxy? Yet it is not the case. So how long does the light from a telescope take to reach that galaxy? Or have I misundrestood the concept?

2. I think you're missing the concept...

A telescope collects and focuses the light that hits it. It does not "reach out" to the galaxy. It collects and focuses the light from that galaxy.

So... A telescope is seeing the Andromeda galaxy at the same time that your eyes are - it's just seeing much more because it can collect more light.

3. Yes, to add to what Smilin Bob said, there is no light from the telescope. The 'scope only gathers the light from the galaxy.
However, in the case of the Andromeda galaxy, the light that you see in the 'scope actually left the galaxy 2 million years ago and has been on its way here ever since. That means that what you see tonight is actually the galaxy as it WAS 2 million years ago.
If you want to see it as it is tonight, you'll have to wait 2 million years.
And when you look at our sun, you're actually seeing it as it was 8 minutes ago. And when you look at your friend across the room, you're actually seeing her as she was a few nanoseconds ago.
Light takes time to move from place to place, so looking great distances away is actually looking back in time.

4. Here are a few examples I use to explain the 'light year' thing to people not familiar with the term:

Light travels at approx. 300,000km per SECOND.

That is the distance from the Earth to the Moon.

It is the diameter of the Sun.

Light takes 8 minutes to go from the Sun to us.

It takes 4.5 years to reach the nearest star (other than the Sun).

30,000 years to reach the centre of the Milky Way from where we are.

And 150,000 years to span the whole Milky Way.

Every thing else is just too far away, .

Mental

5. Hello Tom, and welcome to Astronomy Forum. Your question comes up from time to time, and thanks for asking it. If you have any more, fire away.

Hello to you, too, Smilin' Bob, ye be welcome as well.