How far can we see into space? A very long way indeed. But not only are we seeing objects that are far away we are also seeing them as they existed (past tense). Why? Because as much as we would like to think light is instantaneous it actually travels at a measured or finite speed.
How fast is it? Light travels at 186,282 miles per second or roughly 300 million meters per second. Pretty darn fast but when you start talking astronomical distances the speed of light becomes apparent. Once you know the distance to an object you can then calculate how long it takes light to reach it.
The light reflected from the Moon takes 1.255 seconds to reach us here on Earth. Light emitted from the Sun takes over 8 minutes to get to us. And so on. The most distant galaxies in the Hubble Ultra Deep Field image are 13 billion light-years away.
What’s a light year? A light year is the distance light travels in a year or almost 6 trillion miles. So to get back to what I was saying about seeing these distant objects as the existed, it should now make sense to you that the farther we look into the universe, the farther we also look back in time.
Take the Andromeda Galaxy pictured above. Under a dark sky you can actually see this galaxy with the unaided eye. The galaxy is about 2.5 million light years from us. So when we look at that galaxy we see it as it existed 2.5 million years ago. Why? because it’s taken that long for the light we are seeing to make it to our telescope mirrors and eyes.
So the next time you look up at the night sky remember you are not only an observer of the stars but also a time traveler and that telescope of yours is your time machine.