Subtitles section Play video Print subtitles now I've been around Sydney and I've asked people what causes the phases of the moon and you know what they say. How do you get the face of the moon Because of the the earth blocks The light that comes from the sun full moon is basically where we're seeing the full. A circle off the moon reflected back to work. But the light is actually coming from the sun in the present, with the presence, of course, by the shadows off the earth, from the sun onto the moon Phases of the moon Is that about the sun and the moon? And And something goes in front of something that's between the moon and the sun. Way won't see the moon. And then it has heroes around. The earth gets out of the way so more of the sun shines on the minute we consider Mitt. I think so. Let's try to resolve it. What really caused the phases of the moon phases of the moon? We need a couple of things. First one is this only one source of light in the solar system and that's the sun at the center of the solar system. It produces all the light. So both the earth and the moon ah, half illuminated by that one source of life As the moon moves around the Earth. Our perspective on it changes because sometimes we see just the unlit face of the moon. When the moon is between us and the sun, that's a new moon. Other times we'll see. The moon has moved around the earth and we see the moon half illuminated and half dark. Would call that first quarter at other times. Still, when the moon has further round the earth, we see the full illuminated face of the moon and we call that full moon. But it's just because the moon is half illuminated by the sun. And it's our perspective on that half eliminated moon, which gives the phases. Why doesn't the earth block the light when we're seeing a full moon? You know, if it's son Earth Moon, what isn't the earth block out the light so we can't see them? You would think that what happened Every full moon, you'd think the earth the moon would go through the earth's shadow at every four minutes. But he felt the moon's orbit is tilted slightly to the orbit of the earth around the sun. How much is it? About five degrees. So virtually every full moon the moon is moving just above or just below the earth's shadow. I'm here with Andrew Jacobs at the Sydney Observatory, and we've just witnessed a total lunar eclipse. So can you tell me what that is? So total lunar eclipse is when normally they will be a full moon. But the full moon happens when the moon is in Earth's shadow, so the moon is passed through Earth's shadow. Um, it's gone through the outer part of the shadow into in a dark apartment turned grades. So why does it look red when it's in the middle of her shadow? 33 effects here going on one is that the Earth's atmosphere is refracting the light into towards the moon. A second, like a prison, is like a prison. Yes, exactly like that. The blue light, however, is being scattered out by the earth's atmosphere, just like a sunset sea of red sunset. Because the blue light is scattered outwards, eh? So that leaves the red light preferentially to get through to the moon. And then, if there's a dust, perhaps volcanic dust in the earth's atmosphere that dims the light so leads to either a brighter or darker red color on the face of the moon. What can we use the lunar eclipse? Four. Why is it important to us? A few years ago would have said it with had no importance at all, not much importance and beautifully to look at, but not much scientific importance. But nowadays we know that there are planets orbiting around other stars. Alien stars expect so planets we call them. And if we can measure on the light, it's going through Earth's atmosphere on reflecting off the moon. It could give us an idea off watch. We might expect to see if the light is coming through the atmosphere of an exoplanet orbiting around another star. So we can use the observations off a lunar eclipse around the Earth to infer what, what the atmosphere of a except plant, maybe like so we can detect things like ozone or coven dog side. You know, it's atmosphere by looking at the spectrum that reflects off the moon, and if we see a similar spectrum when we look at a exoplanet around another star. We could infer there is no ozone or capital. Exciting. That atmosphere. You detected water vapor as well. You could take water vapors. Well, yes. And that would obviously be a pretty exciting sign If we saw that. That would be very exciting. Signed? Yes. Yes. All those things together, absent methane as well. Mind suggests that there's life out there in the universe.