1Hi, it's Doug.
2I've got a plastic sandwich bag
3and I was noticing how if you looked through one plastic sandwich bag, it's clear.
4But if you get a whole bunch of them, check this out.
5Now it's not clear. It looks kind of blue.
6You see that?
7The reason I have these plastic sandwich bags with me is because...
8I heard there's an interesting question this week from someone named Celine.
9Let's give her a call now.
10Hi, Doug.
11Hi, Celine.
12Good to see you.
13Our class has a question for you.
14Why is the sky blue?
15That's a great question.
16Before I say anything, I want you to stop and think about it.
17What do you think?
18Could you come up with any idea for why you think the sky is blue?
19There are a lot of different ideas you could have come up with.
20I can't guess all of the ideas you might have had,
21but I'll bet at least one of you thought:
22maybe the sky is blue because our oceans are blue.
23Maybe the sky reflects the color of our oceans.
24That's a really interesting idea.
25There's actually a way to figure out if that's the reason.
26Think about it.
27If the color of the sky had to do with the color of Earth's surface,
28then if you were in the middle of a continent
29surrounded by thousands of miles of land like this,
30then the sky shouldn't look blue at all, right?
31It should actually look green,
32reflecting the color of all the land.
33But that's not the case.
34Even people who live in the middle of a whole bunch of land still see blue sky.
35Why is the sky blue then?
36This is a hard question to solve.
37Scientists themselves didn't know for a long time.
38Here's an interesting clue I can point out though.
39We've sent spacecraft up to the moon and to other planets
40and it turns out the sky in those places isn't blue.
41Have a look at the sky as seen from the moon.
42Whoa. It's a black sky.
43Okay, now let's check out what the sky looks like from Mars.
44Here it is.
45Weird. It's kind of a yellowish sky.
46So the moon's sky and Mars' sky are somehow different from Earth's sky.
47What's going on here?
48Well, keep in mind, our own sky is made out of air.
49It turns out the moon has no air at all,
50so the sky just looks like outer space even when you're on the surface of the moon.
51Now Mars' sky has air,
52but it's a different kind of air than what's found on earth.
53It's mostly made of a gas called carbon dioxide.
54So the blue color of Earth's sky (this is a view from space)
55it has something to do with the kind of air that Earth has.
56Our air is made of two gases.
57It's got some oxygen.
58That's the part of air that our bodies need to breathe in.
59And Earth's air has a whole bunch of another gas too called nitrogen.
60When the sunlight shines on all that oxygen and nitrogen,
61it looks blue to our eyes.
62And the reason Mars' sky looks yellow
63is because of the gas that its sky is made of.
64When sunlight shines on carbon dioxide,
65it looks yellow.
66Now I have to point something out.
67This is not the full story.
68There's more to this question.
69It's complicated.
70The reason that our planet's air looks blue
71also has something to do with how light works.
72Look around you.
73You're surrounded by Earth's air.
74It's the same stuff that's in our sky,
75but it doesn't look blue, does it?
76No, it's clear.
77You see, it matters how much air you're looking at.
78When you look up at the sky, you're looking at a lot more air than just the air in the room around you.
79Sometimes when you have a lot of something, like this water in a swimming pool,
80it can look different than when you just have a little bit of something,
81like this water in a glass of water.
82It's also kind of like those plastic sandwich bags I was playing with earlier.
83Just one of them looks clear,
84but when you have a whole bunch of them,
85the color looks different doesn't it?
86Hm, it's Interesting.
87Here's another thing to notice.
88Our sky isn't always blue, is it?
89At the time around sunrise or sunset,
90you can see lots of different colors,
91especially reddish orange color.
92All of this is something we can explore in a future episode.
93But as a first step toward understanding why our sky is blue,
94you can just notice that our sky is blue because of the type of air we have in our sky.
95We can see for ourselves by comparing the color of our sky
96with the color of the sky on other planets and the moon.
97The way that light works,
98when you have a whole bunch of our type of air, it looks blue to our eyes.
99Thanks, Celine, for asking that question.