Video Transcript
Which of the following would be the best location for a telescope used to study objects very far from Earth? A) At sea level. B) In Earth’s orbit. C) On a mountain top. D) At the bottom of a deep hole.
Okay, in this question, we’re setting up a telescope. And we want to use it to study objects that are very far from Earth out in deep space. The question is, which one of these four locations would be best for this telescope? As we figure this out, let’s make a sketch of these four different spots.
Our first option says that it would be best to put the telescope at sea level. Let’s say that sea level is in line with this altitude right here. So, we could put our telescope there. Or option C says, what about on a mountain top? And let’s say that this here is our mountain top. And a third option is that it could be at the bottom of a deep hole. So, there’s our telescope down there. And the last option is that it could be in Earth’s orbit. So, let’s say that this is the Earth, and that our telescope is put in orbit around it, much like the Hubble space telescope.
As we consider these four options, it’s important to keep in mind just what we’re looking for with our telescope. We’re looking for some kind of light, electromagnetic radiation, that will come from space and reach our telescope. In order for our understanding of these faraway objects to be as good as it can possibly be, we want to receive as much light from these objects as we possibly can. The better we can see these objects, the more we’ll know about them.
So, we can evaluate these four possible telescope positions based on which ones will allow most light from a faraway object to reach the telescope. Thinking along these lines, there’s an important addition we’ll want to make to this sketch of our telescopes at different positions on the surface of the Earth.
We know that above the solid or liquid surface of the Earth, there’s a gas layer called the atmosphere. This mixture of gases made mostly of nitrogen then oxygen then argon and so on. And one effect of the atmosphere is to both absorb as well as emit certain types of electromagnetic radiation. For the purposes of understanding objects faraway from Earth, neither one of these things is something that we want.
For example, say that light from this faraway object is coming into our telescope, but then some of that light is absorbed by the atmosphere. In that case, only, a relatively weak signal would reach our telescope. Or, on the other hand, imagine that the atmosphere created light at some particular wavelength, and this light was received by the telescope. Looking at the image produced by the telescope, we couldn’t be sure whether that light was coming from the object we’re interested in or from something in-between that object and our scope.
So, neither one of these conditions, the atmosphere absorbing radiation or creating it, is desirable. We could say that the less atmosphere our telescope has to look through, the better the image it collects will be. So, as we consider our possible telescope locations on the surface of the Earth, the best one is the one on the top of the mountain. That’s because this telescope has the thinnest layer of atmosphere that it needs to look through.
The next best would be the telescope at sea level. And the worst off one would be the one that’s in a deep hole. That’s because a telescope here would have to look through an especially thick section of atmosphere. And the atmosphere, as we’ve seen, distorts the signal. If these were our only three options for the telescope location, we would choose the one on the top of the mountain. But there is one other option, which is that our telescope could be in Earth’s orbit.
When the telescope is here, completely outside of Earth’s atmosphere, there’s nothing to distort the signal coming from these faraway objects. The light from them just needs to travel through space; no gases get in the way. This is the biggest reason why the Hubble telescope is in orbit around the Earth rather than on Earth’s surface. It’s so that the images it collects will be as clear as possible.
Just as a side note, if we were restricted to putting our telescope on the surface of the Earth, then the higher up we can get an elevation, the better. This is why astronomic observatories often are on the tops of mountains. But since we do have an option of putting our telescope in orbit completely above the atmosphere, we choose that as our answer. The best location for a telescope used to study objects far from Earth is in Earth’s orbit.
