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Astronomy as a science is pretty tough to define these days. I actually point that out in the first episode of Crash Course Astronomy; if I observe Mars using my backyard telescope, I’m doing astronomy, but what if I’m a professional scientist looking at isotope ratios in samples of atmospheric gas trapped in an air bubble from a Mars meteorite? That’s maybe chemistry, or geology, or just planetary science.
Where does astronomy start? With the Moon? Above the atmosphere? It’s like asking where the sky starts. I’m not sure the question has meaning.
I’m mulling this over because it’s interesting to think of where Earth fits in this scheme. For thousands of years it was a unique place, but around the time of Kepler and Galileo people started to think of it as a planet like the others in the sky. It’s the one we inhabit (for now), so it gets its own flavors of science (geology, geophysics, and so on). But astronomy?
I vote yes. It’s a planet, and when we think of it that way, we see it differently than we might by living on it and looking around. And so, from Planet Earth, here is Episode 11 of Crash Course Astronomy: our home world.
The way we label things can guide our thinking. That is a two-edged sword; it can help us see things in a new way as well as put blinders on us. But I rather like to think of Earth as a planet; it reminds me that we are immersed in astronomy, we literally live on it, and our world is one of countless others in the Universe. That is both humbling and awe-inspiring.
Update (April 3, 2015 at 19:45 UTC): ARG! A couple of people pointed out I made a mistake at the 7:34 mark, saying the pressure of Earth's air is ten tons per cubic meter, when it's per square meter. That's totally my fault (the caption put up as I speak that line is just quoting me, so it's incorrect too). Think of it is a verbal typo. My apologies!