When we look up at the night sky and gaze at the wonderful array of shining stars most of us probably just admire the pure beauty of it all. The coolest part however, is that when we look up in the night sky, we're actually looking into the past.
Since the universe is massive, it takes a long time for anything to make its way around. We know that there is nothing faster than the speed of light, photons are the fastest moving objects in the universe. So when discussing distance from one point to another within interstellar space, we use light-years to measure it. Its abstract from the way we would measure distance here on Earth and within our own solar system. We know that from the East Coast to the West Coast the distance is around 2,000 miles and that the Earth is around 93 million miles from the sun. However when we get to the vast interstellar distances between our Solar System and Alpha Centauri, for example, measuring in miles becomes ridiculous. This is why we use light years as a distance; instead of figuring out how many miles away Alpha Centauri is from us, we can figure out how many years it takes light from that star system to reach us. It takes about four light years for light to travel from Alpha Centauri to us, so we are technically observing the past whenever we look at that star system.
It's interesting to be able to look up and know we are looking at the past. From a different perspective we are also being observed from the past as well, if a living organism were to look up in their night sky at us. If that organism were living on one of the beautiful spirals within the Messier-100 galaxy, they would currently be observing the flash of an asteroid hitting Earth which caused the last mass extinction. This is because it is tens of millions of light years away from here. We even look at our sun in the past every day. If someone were to flick the off switch on our Sun, it would take about eight minutes for us to know about it.
So I think that what I'm getting at is this; light travels really fast, it just doesn't travel fast enough for us to get real time images whenever we look up at the sky.
