In this episode, we talk about what happens in the shadows around our planet, from the phases of the Moon to why eclipses can turn our satellite blood red!

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Chris
Question. You know, sometimes people get really excited because there’s a lunar eclipse. On a clear night, you can look up and you can see this lunar eclipse, which is effectively from my knowledge, the shadow of the Earth on the Moon.

Alfredo
Absolutely correct.

Chris
Right. So you know how, at any given time of the year, you look up, and you see the moon and it’s in different phases, you can see the tiny little New Moon, or you can see a half moon or whatever it is, yeah, half of the moon, or whatever percentage of the moon, you cannot see is effectively in shadow. Does that not make every phase of the moon an eclipse?

Alfredo
That’s an interesting question, but I think it’s a semantic question rather than an astronomical one, in the sense of how we define an eclipse.

Chris
Maybe I should start running these questions past you before we record…!

Alfredo
No, no, no! Okay. I think there is an issue of what you actually think of as the shadow.

Chris
So when there’s a new moon… I am afraid I can’t say what that is actually called in English! In Welsh, it’s called ‘ewin’, which means like fingernail. So it’s that sort of very small…

Alfredo
Crescent!

Chris
Crescent! Thank you. There we go. So you’ve got a crescent moon and you can see that part of the moon.

Chris
Yeah.

Chris
The rest. Is that not just the shadow of the earth covering the moon?

Alfredo
No.

Chris
….So tell us more!

Alfredo
Okay, yeah. So the semantic part: an eclipse is when a body is casting a shadow – a celestial body is casting a shadow onto another celestial body. If the shadow you saw on the moon during its phases was actually due to the Earth then yes, it would be a constant Eclipse. But the shadow that you see on the moon during its phases is not the shadow of the earth. What do you think its the shadow of? Chris is making a thinking face..

Chris
Always good when I make a thinking face! I mean, there’s nothing else. If it’s not Earth that’s making it shadow. Is it like when it’s night on earth?

Alfredo
Absolutely correct. So why is it night on earth? Why is half of the planet in shadow?

Chris
Because half the other half of the planet is shielding it from the sun.

Alfredo
Yes, absolutely correct night on earth is simply its shadow. And in the same way, the phases of the moon are simply the shadow of the moon as it goes around the Earth. So when we have a new moon in which the moon doesn’t appear in the sky, it’s simply because it’s in a position that we’re only getting sort of the backside of the moon. While when we have a full moon, the moon is completely behind the earth. And we see the full disk so we are seeing the full illuminated moon and all the other points so we’re just getting a little bit of the moon in shadow.

Chris
Okay, going back to the crescent: so when you can just see maybe a little bit of the moonsometimes you can actually see a very faint outline of the rest of it. It’s not completely in shadow. What’s going on there?

Alfredo
What do you think could create that effect, this Game of Shadows?

Chris
Oh my gosh, I know. I know this. So what I’m picturing is not from memory… When I’ve seen things like this. It isn’t actually during night. You look up into the sky, and sometimes it’s a blue sky and you can still see the moon, but you can see like a crescent, and that’s when you can see like the think bit.

Alfredo
Yeah.

Chris
Is it daylight reflected off the earth?

Alfredo
Absolutely correct.

Chris
Yes.

Alfredo
So I was going to point out what happens when it’s a full moon and you’re outside at night. And what happens is that you’re still casting a shadow. So it’s still sunlight bouncing off the moon. I think it would be fascinating if somebody / a space agency took pictures of the earth during the night side of the Earth during a full moon, and during a new moon, to see the difference the light of the moon would have..

Alfredo
Would it even register? I mean, would that even show up on the photo?

Alfredo
Actually, I don’t know. We definitely have too many artificial lights for it to be very powerful

Chris
But I mean, yeah, hypothetically, if all of the lights were turned off on Earth, then maybe we’d see a slight difference?

Alfredo
The light of the Moon is something tiny compared to the light of the Sun. I think it’s in the millionth or something. It’s a very small number. But the fact that full moon is capable of casting shadows, and they’re quite harsh shadows. So it should be. It should be enough to provide an illumination. I am not sure about that as an answer… we’ll have to find somebody that has access to satellites to take the picture. You know, the source of the light reflected by the Earth onto the moon during its phases, it’s actually called Earthshine.

Chris
Oh that’s a cool name.

Alfredo
Yeah, it’s quite good. Do you know who sort of worked it out? Or was the first one to realize where it was from?

Chris
Galileo!

Alfredo
No, but it is Italian.

Chris
The other one, the heretic, Bruno.

Alfredo
Not Giordano Bruno. But good guessing. I would say this somebody that we don’t associate often with astronomy

Chris
Mussolini

Alfredo
What???

Chris
I don’t associate him with astronomy!

Alfredo
No, this is not a fascist discovery from the 20th century. It’s more Renaissance.

Chris
Oh, Da Vinci

Alfredo
Yes!

Chris
Much better than Mussolini.

Alfredo
Much, much better! Leonardo was the first one to realize that the effect that was due to the Earth, that it was sunlight reflected from the Earth and bouncing off onto the shadow-y side of the Moon

Chris
The more you know! I’m going to jump back to what I was mentioning right at the start about actual eclipses and lunar eclipses: it glows red. Why?

Alfredo
Wait! It doesn’t always glow red!

Chris
Sometimes it glows reddish-orange.

Alfredo
Yeah! red-orange. Do you know when that happens?

Chris
Honestly, I don’t know. I’m not very clear about lunar eclipses.

Alfredo
So there are a lot of eclipses in which the full moon looks like it’s a different phase, simply because it passes through the Earth’s shadow cone, but not completely. In that case you are seeing the shadow of the Earth, covering the full moon. But there is something else. When the moon goes perfectly through the shadow of the Earth, what happened is that the little bit of sunlight that goes through the Earth’s atmosphere reaches the moon. So the shadow of the Earth is not a perfectly stark, dark shadow. It’s not perfectly black, but it’s reddish, simply because there is in the thin atmosphere of our planet enough red enough light to cast this reddish the shadow.

Chris
Is it kind of like a prism effect? like Earth’s atmosphere acts as the prism and the sunlight comes through and sort of refract slightly…

Alfredo
Refract is the right word.

Chris
You’re only seeing the red light.

Alfredo
Why are we only seeing the red light? It is the same reason why you see red light at sunrise and sunset.

Chris
I know! I can never remember… low wave frequency of the color red or something?

Alfredo
So our atmosphere is much better at scattering light that is blue and violet and scatter it than it is at scattering red and yellow light. So when the sunlight goes through more of the atmosphere, you get the color red compared to when the sun is up in the sky and going through a shorter path through the atmosphere. In a lunar Eclipse, you get a deep red because the sunlight goes through a lot of atmosphere. And so a lot of the shorter wavelength gets scattered. And you might be asking, okay, if the shadow of the earth has this reddish hue attach to it, why, when we have a partial lunar eclipse or the beginning of a total lunar eclipse don’t we have sort of reddish shadow on the moon? And that is simply because the rest of the Moon, the part that is not in shadow remains bright enough to just outshine the faint red color.

Chris
That makes sense. Well, I feel much more educated now. And I do feel like it’s something that I should probably already have known.

Alfredo
I’m not sure, I mean you didn’t know how to say crescent in English.

Chris
That’s true. Maybe I knew all of this in Welsh but just not in English. Okay. Well, thank you very much for answering my curious little question today!

Alfredo
You’re very welcome.