A telescope, a pair of binoculars, or the human eye collects light that was either emitted from an object in space (such as a star, a galaxy, or the Sun) or reflected off of the surface of an object (such as the Moon or planets within our own Solar System).
This light takes 300,000 km per second to travel to us. Therefore, light reflected off the surface of the Moon takes about 1.3 seconds to reach the Earth, and light from the Sun takes 8.3 minutes to get here. This means that when we look at the Moon, we are seeing it as it was 1.3 seconds ago, and the Sun’s image is 8.3 minutes out of date by the time it reaches us.
The farther an object is from the Earth, the longer it takes the light to arrive, and thus we are seeing the object not as it appears to an observer near it today, but as it appeared in the past. By looking far out into the distant Universe, we are able to see galaxies, as they were billions of years ago.
For example, in 1987, a stellar explosion (about 50 times the mass of our Sun) was detected in one of our closest galactic neighbors, the Large Magellanic Cloud (LMC). However, since the LMC is about 170,000 light years (the distance light travels in a year) away from us, the star itself actually exploded 170,000 years ago. It took us this long to get the message.