Clearing the air (or, Mea Culpa Part 1)

Bad AstronomyBy Phil PlaitDec 21, 2010 6:02 PM


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Like most of you, I'm human. I try to be as accurate as possible when I write, but sometimes I make mistakes. A lot of these errors are small and I just fix 'em. Some of them are bigger, and I generally strike them through and correct them up front -- leaving them public keeps me honest. Also, part of science is learning from your mistakes. If we don't, we get ossified and dogmatic, and that's the very antithesis of science! Also, sometimes, these mistakes deserve more airtime. They deserve their own post, and it so happens I have a couple on which I'd like to elaborate. I want to clear the air, so to speak, and what better way to start than to talk about clear air? In the past, when giving talks (as well as in my first book) I say that the Earth's air is very transparent. In fact, based on the memory of a paper I read in grad school and which I can no longer find, I've said specifically that 98% of the visible light that enters our atmosphere from space will make it to the ground (barring clouds and so on). This is usually in response to people asking if they'd see more stars from space due to the lack of air, and my answer has always been "not really", most of the light from even faint stars makes it to your eye. That turns out not to be correct. To illustrate this, here's a graph showing an example of how much light from the Sun actually gets to the ground:

The details of this are complicated, but it illustrates the point. This graph is from the American Society for Testing and Materials, who created it to aid the solar power community. The vertical axis is brightness (specifically, the amount of energy hitting an area every second), and the horizontal axis is wavelength (think of it as color). Visible light has a wavelength of very roughly 300 - 700 nm (nanometers, a billionth of a meter) on that scale. The black line is a model of the Sun's light, and the blue line is the amount of that which hits an area tilted up to face the Sun (details are at the link above). According to this, in visible light quite a bit of power from the Sun is lost through the air, roughly 20-25%. This is a pretty clear (haha!) indication that the atmosphere does rob us of some light. What happens to that light? Well, some of it is simply absorbed by stuff in the air. That light is essentially gone, sucked up by atoms and molecules and turned into heat. Other light gets scattered; imagine a photon of light is like a ball in a pinball game and the atoms in the air are the bumpers, to get the picture. As the light comes down, it bounces off the atoms and goes every which way. This tends to diffuse light a bit, making stars fainter. Also, blue light gets scattered more than red, which is why the sky is blue: those photons from the Sun get scattered all over the sky, so you see blue light coming from every direction. Tadaa! Blue sky. There's more, too. Sky and Telescope has a good explanation of all this. One big factor is where in the sky you're looking. The Earth's atmosphere is a thin shell above us, and the Earth is a ball underfoot. That means that when you look straight up, you're looking through less air than you do toward the horizon. Here's a diagram:

The inner circle is the surface of the Earth, and the outer one represents the top of the atmosphere. The vertical red line is how much air you look through when you look straight up; the horizontal one is when you look toward the horizon. See how much longer the path length is for light coming from a star on the horizon? That's why, on a dark night, you see more stars overhead than lower down. It's also why astronomers like to wait for their targets to rise up high before they observe them! [If this looks familiar, it's similar in principal to what I was describing in a recent post about seeing ring-shaped nebulae in the sky.] We even have a term for this: air mass. When you look straight up, you're seeing through one air mass. The lower you look, the higher the air mass, and the worse your object gets. As you might expect if you've gotten this far, I'm simplifying things a lot here! There are tons of details dealing with all manners of rotten air quality, but the point here is that our air does mess up our view, and starlight from space doesn't all get to your eye. I'll note, though, that our air really is remarkably transparent; given that it's many kilometers thick it's pretty cool that any light gets through at all. As you can see from the top graph, a lot of ultraviolet (shorter wavelength) and infrared (longer wavelength) light never gets to the ground. But then, there's a really good reason we call the light we can see visible light. It's no coincidence; it makes sense from evolution that we see best in the kind of light that makes it down here from the Sun. And of course, all of this is why we launch telescopes into space. No air means more light gets to the telescope (including wavelengths that otherwise never reach the ground, again like IR and UV), as well as losing the annoying twinkling caused by our roiling, moving atmosphere. Twinkling stars are fine for poetry, or for lovers holding hands and stargazing, but it's a nuisance when you're trying to see two objects close together that get blurred into one blob! And finally, what does this mean for seeing stars in space? Well, air removes 25% of the light from a star. In astronomical terms, that's equivalent to about 0.25 magnitudes. The faintest star you can see from the ground is about magnitude 6 or a bit fainter, so pushing it I'd say you could see stars as faint as magnitude 6.5. That means there are a few thousand more stars that would be visible. Under ideal circumstances, the number of stars you could theoretically see would jump from about 6000 to about 8000. I'm being pretty rough here, but the point is the number of stars would increase, but not hugely. And that would be on the fainter end, making them harder to see anyway. So if you were in space, you might expect to see more stars, but not be overwhelmed by them! So that's it. Thanks for indulging me. I'll always strive for accuracy, but when things go wrong, I'll post another "mea culpa"... and in fact, another one will be cometing -- I'm sorry, coming pretty soon. Hint hint.

My sincere thanks to amateur astronomer George Cooper, who sent me emails about this and other interesting topics over the years.

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