Earth-based (or earth-orbit-based) exploration of the universe using telescopes is the oldest and farthest-reaching of mankind’s attempts to learn about our universe. I also find it one of the most fascinating. This is why.
Any observation of the universe that relies on light or any other form of electromagnetic radiation, as telescopes do, is exploration of time as well as space. Light and all other forms of electromagnetic radiation travel at a finite speed — approximately 300,000 kilometers/second (or 186,000 miles/second) in a vacuum. This speed varies when traveling through an atmosphere or other substance, but when light travels across space, it rarely encounters anything that can slow it down much or for long. In my comments below, I assume that the speed of light is constant.
Distances between any two places on earth are small enough that light, or radio, transmission appears instantaneous. It isn’t, of course — there is always a time interval, even between when you switch on a light at home and the light from the bulb reaches your eyes. We don’t perceive a time gap, though, because it is so tiny. On earth we don’t usually need to consider this time gap — we assume that what we see is what is happening at that time.
When you start looking at other planets within our solar system, though, you must allow for the time it takes light to travel from its origin to you on earth. It takes light about eight and a half minutes to travel the 150 million kilometers (or 93 million miles) from the sun to the Earth. It takes that same light approximately another four and a quarter minutes to travel the 78 million kilometers (or 49 million miles) from Earth to Mars, and another half hour or so to travel the 550 million kilometers (or 340 million miles) from Mars to Jupiter. Even within our solar system, we cannot see what is happening on the sun or another planet at a particular time. We can only see what happened a few minutes earlier.
Most of what earth-based telescopes examine is much, much farther away than the sun or Mars, though. The nearest star to earth, Alpha Proxima, is approximately 4 trillion kilometers (or 2.5 trillion miles) from us. It takes light over four years to travel from Alpha Proxima to the earth, which means that when we look at Alpha Proxima through a telescope, what we see is what was happening there over four years ago.
Even this is a tiny distance compared to the distances to most of what we see in the night sky. It takes the light from stars at the center of our galaxy, the Milky Way, about twenty eight thousand years to reach us. In other words, what we see through our telescopes is what was happening during the Ice Age on earth. The human race had not yet learned to smelt metal. There were no pyramids — they would not be built for another twenty thousand years. The earliest forms of written language would not be developed for another twenty thousand years. The ancient cave paintings in Europe and northern Africa, those astoundingly beautiful earliest works of art, would not be painted for another fifteen thousand years.
And even this vast distance is tiny compared to the distances between galaxies. Light from Andromeda, the nearest galaxy to our own Milky Way, must travel over two million years to reach us. In other words, when we look at Andromeda through our telescopes, we see what was happening during the later Pliocene epoch on earth. The earliest ancestors of the human race, Homo Australopithecus, lived in Africa and the Middle East. They walked on two legs, but had only 1/3 of the brain capacity of modern humans. They used only the most primitive of tools. Descendents of theirs would finally learn how to use, and then start, fires only after another million and a half years passed.
And even this distance, in space and time, pales compared to that between us and the most distant galaxies and astronomical objects we’ve been able to see so far. Some of them are so distant that it takes light over 10 billion years to reach us. When we look at these distant galaxies, we see what was happening long before earth formed, when our solar system was still an undifferentiated mass of hot gas that would not cool and start forming planets for another several billion years.
Light from the most distant of these most distant astronomical objects, huge and energetic masses we call quasars, has been travelling to us for around 12 to 13 billion years. That is approximately how old astronomers believe the universe is — most estimate that the big bang took place aroun 13.7 billion years ago. So when we look at the light from these most distant objects, we are seeing part of our own universe shortly after its birth.
*This* is what the wonderful earth- and space-based telescopes such as the Hubble space telescope, the Chandra x-ray telescope, the Arecibo radio telescope, and the other telescopes that we occasionally see mentioned in the news are letting us see. They’ve got some practical uses, but their real justification for existence is that we human beings want to know about the universe that we live in. Nobel Prize laureate physicist Richard Feynmann titled one of his books, “The Pleasure of Finding Things Out.” That also describes the real purpose and value of astronomy: we can find out about our universe.