Astronomy Cast

Ep. 113: The Moon, Part 1

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Hey, here’s a topic we haven’t really gotten around to yet… the Moon. Today we look at our closest astronomical companion: the Moon. What impact does the Moon have on our lives, where did it come from, who walked on it, and are we ever going to walk on it again? We’re going to learn about the phases, the tides, and even a little bit about NASA’s plans to send humans back to the Moon.

  • Ep. 113: The Moon, Part 1
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    Shownotes

    Moon Phases

    Eclipses

    Tides

    The Moon Illusion

    The Moon Itself

    Transcript: The Moon, Part 1

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    Fraser Cain: The Moon!

    Dr. Pamela Gay: The Moon – just in time for Halloween!

    Fraser: Exactly you know we’ve been doing this show now for 113 episodes and we haven’t even done ‘The Moon’ yet. We did an episode on where did the Moon come from. Yeah, but people ask are you ever going to do any episodes? Huh!

    Pamela: [Laughter] No, we just going to forget important topics.

    Fraser: Wait until you hear our topic for next week. [Laughter] Let that be a surprise. Okay today we look at our closest Astronomical companion – the Moon. What impact does the Moon have on our lives? Where did it come from? Who walked on it? Are we ever going to walk on it again?

    We’re going to learn about the phases, the tides and even a little bit about NASA’s troubled [Laughter] plans to send humans back to the Moon. Let’s start with the phases. We look at the Moon and sometimes we see it in shadow and other times we see it a full Moon or various crescents. What’s going on?

    Pamela: Well basically it’s just the Moon is illuminated by the Sun. The Sun is on one side of the planet Earth, the Moon keeps switching which side of the Earth that it is located on and as it moves around in the light of the Sun you see different aspects of it lit up.

    Now one of the really common misconceptions about the Moon and the Sun and how all of these crazy phases work is that the reason that we see part of the Moon in shadows is because it’s actually passing into the Earth’s shadow. That has ABSOLUTELY nothing to do with it.

    The Moon in general stays completely out of the Earth’s shadow. The only time the Moon gets involved with Earth’s shadow is during Eclipses which occur about every six months.

    Fraser: So like the Moon is always illuminated, just half of it, right? Just one half whichever side is facing the Sun is being illuminated. If you could like hold the Moon and turn it around you could just see yeah one half of the Moon is illuminated. The other half is in shadow.

    Pamela: With half the Moon always, always, always illuminated there is always half of the Moon illuminated, what’s changing is what part of the Moon we’re able to see. So when we use the phrase ‘dark side of the Moon’ which is a great Pink Floyd CD…

    Fraser: Completely wrong [Laughter]

    Pamela: Completely wrong. The dark side of the Moon gets lit up just as often as the well, non-dark side of the Moon. What’s dark about it is our ability to see it.

    So we’re lacking information. We’ve sent probes all over, we’ve taken maps, and we just generally haven’t seen it with the human eyeball.

    Fraser: Right that’s the far side of the Moon. That’s totally different.

    Pamela: And so the far side of the Moon that’s the side that is never facing the planet Earth and it gets just as much sunlight as the side that we see all the time. Here’s where the phases come from. When you take the Moon and put it between us and the Sun, the side of the Moon that’s getting illuminated is the side of the Moon that’s closest to the Sun. We don’t get to see that side because we’re on the other side of the Moon from the Sun.

    When you have the Moon located probably below or above the Sun in the Sky, they’re basically on a line between us and the Sun, on a sheet of paper between us and the Sun.

    Then you end up with what we call a ‘New Moon’ a Moon where we don’t see any of the surface of the Moon illuminated. As it moves away from the Sun, as it orbits back to the left in the Sky from the Sun, what we end up seeing is a ‘Crescent Moon’.

    So we have, if you’re looking down on the Earth-Moon-Sun System the Moon is going around the Earth in essentially counter-clockwise direction. If you start off with a nice polite line with Earth, Moon, and Sun then the Moon is going to move up in a counter-clockwise direction. It’s going to become what we call a ‘First Quarter Moon’.

    In this situation, we still have half the Moon illuminated but we now have a right angle where you have the Sun off to one side, the Moon straight up from the Earth and the Earth is forming that right angle part of the triangle.

    We’re going to stick lots of pretty diagrams in our show notes. In this right angle situation half the Moon is illuminated but we only see a quarter of that part that’s illuminated.

    This is why we call it a ‘Quarter Moon’. It looks like half the Moon illuminated but you have to remember when we look at the Moon we see half of the Moon and half of a half is a quarter.

    This is one of those things that gets really muddied to think about but ‘First Quarter Moon’ is when you go from ‘New Moon’ to being able to see half of half the Moon illuminated in the Sky.

    Fraser: Right and when the Moon is increasing in illumination we call that ‘Waxing’.

    Pamela: So, we’re gonna have wax on wax off. In the case of wax off it’s a ‘Waning Moon’ – the fancy word we use for it. We go from ‘New Moon’ to ‘First Quarter’. The Moon keeps going in this counter-clockwise direction around the Earth and eventually gets itself lined up so that it’s either above or below the Earth on the Sky when you draw a straight line between the Sun the Earth and the Moon.

    In this case, it’s the Earth that’s between the Moon and the Sun. So, in this case we’re able to look at the Moon and see it fully illuminated and this is what we call a ‘Full Moon’. You’re actually seeing two quarters or half the Moon and we call it a ‘Full Moon’.

    One way to think of this is to imagine you’re on stage with another actor. When you’re facing the spotlight and the actor that you’re talking to is standing in front of you with his or her back to the audience, their back is illuminated by the spotlights and the side of them that you see is in darkness.

    The audience sees you completely illuminated. Now if you reverse positions so that they’re facing the audience and your back is facing the audience, you see them fully illuminated by the spotlight and they see you in darkness.

    A ‘Full Moon’ is straight overhead at midnight when we’re fully having our back to the Sun and the Moon is facing the Sun – our audience in this case.

    Fraser: Right and then this I guess leads to the question which is that if the Sun and the Moon are on opposite sides of the Earth, why doesn’t the Moon go into the Earth’s shadow? You’d think that if it was perfectly lined up it would be in shadow.

    Pamela: And this is why I said it’s above or below. The Moon’s orbit is tilted in relationship to the Earth’s equator. The Earth itself is also tilted. So when you get all of these crazy angles together what you end up with is the Moon is generally in the Sky above the Earth so that you could be standing on the Moon and look over the top of the Earth or under the bottom of the Earth at the Sun off in the distance.

    It’s this tilt where you’re going in a loop-d-loop around the Earth that crosses the Equator once when it’s going toward the Sun and once when it’s coming back away from the Sun. It is generally getting carried above or below the Earth so that you can always see the Sun above or below.

    Fraser: But they DO line up in the shadow sometimes.

    Pamela: Twice a year and that’s where it becomes important that yeah the Moon does cross the Earth’s equator twice on every orbit – once going up and once coming back down.

    Twice a year it lines up typically so that you get the Moon planting itself in Earth’s nice large shadow and often you also get twice a year the Moon putting itself between us and the Sun so that the Moon’s very small shadow is able to get cast somewhere on the surface of the Earth as it blocks out the Sun.

    This precise lining of what we call the Nodes – this precise lining up of where the Moon’s orbit crosses the Earth’s equator typically only happens twice a year.

    Just to make this clear, it’s not the crossing of the Equator that necessarily causes the Eclipse – although that can happen if you precisely have one at a Solstice. It’s actually the crossing of the Ecliptic which is the line that the Sun is on at the Sky that causes the Eclipses.

    So it crosses the Equator twice per orbit and it crosses the Ecliptic twice per orbit. It’s this crossing of the Ecliptic that leads to Solar and Lunar Eclipses.

    Fraser: Right and we have plans to do a whole show about Eclipses down the road but that’s sort of the geometry that’s involved with the Moon and the Sun and the Earth. That’s why we see Eclipses and that’s why we see the Phases. Now, let’s talk about more of the Moon’s influence which is the tides.

    Pamela: Tides are caused by – well water slushes – and in fact rock slushes too, we just don’t usually think of it this way. When the Moon is straight overhead, it’s able to exert an extra pull on whatever is directly below it, the ocean the rock the mountains, the earth and it tries to pull this stuff up towards it.

    Now the Earth is rotating so stuff is getting pulled up and carried away at the exact same time so ‘High Tide’ is always actually a little bit ahead of where the Moon is.

    If you’re looking down again from some mythical location in Space at the Moon going counter-clockwise around the planet Earth and the Earth is also rotating counter-clockwise, if you can look down on the System you’ll see the Earth rotating and carrying ‘High Tide’ in front of where the Moon is located.

    Fraser: But is it actually like the Moon’s gravity reaching down and just pulling the ocean towards it? I’ve seen pictures and it looks like there’s bulges on both sides of the Planet.

    Pamela: That’s the kinda cool part. On the other side you actually have less Force. So since you have less Force things aren’t getting squished as much. It’s the way the Forces add up everywhere.

    You sort of end up with when you’re at a right angle plus the rotation of the Earth thrown in to make things more complicated you are halfway in-between ‘High Tide’. This is where we have the ‘Low Tides’.

    In this case the Forces are at their mid-point when you have the Moon straight overhead plus a little bit for the rotation of the Earth you have the most Force getting exerted on you. You end up with a ‘High Tide’.

    When you are on the opposite side of the Planet – plus a little bit thrown in for the rotation – you have the least Force on you and this also leads to a ‘High Tide’ because things aren’t getting squished as much. It’s kinda weird to think about.

    Fraser: That’s why we get two high tides and two low tides every day. We’re passing through the high tide and then the low tide and then the other high tide and then the low tide and then back to the starting point again.

    Pamela: So if you hang out on the beach notice when you see the Moon straight overhead and then notice when you see the ‘High Tide’.

    Fraser: Cool. Alright and there’s one last thing I want to talk about which is the Moon Illusion. [Laughter] Have you ever seen it?

    It’s totally true you see the Moon down at the horizon and it looks gigantic. Then later on when you see the Moon really high overhead, it’s teeny tiny.

    Pamela: But you can always block it out with the tip of your finger.

    Fraser: Yeah, one of the great experiments – I think Phil told me this – is you hold an aspirin at arm’s length and that’s how big the Moon is. You can see that the Moon if you hold out as you said your pinkie finger, your nail should just cover the end of the Moon. Then try it again when the Moon is way overhead and it’s the same size.

    Pamela: What’s happening is when our brain has trees, cars, and all this other stuff that it can contextualize the size of the Moon with it goes ooh, large pretty Moon – beautiful.

    But then when the Moon is lost in a sea of nothingness up in just hanging out in the middle of the Sky, without anything around it our brain goes, ooh, little tiny thing. It’s just completely an illusion – that’s all it is.

    Fraser: Just completely trick of the brain, wow.

    Pamela: The human mind is a strange and scary place. [Laughter]

    Fraser: Okay so now we mentioned earlier on in the show that there’s a near side of the Moon and a far side of the Moon. What’s going on there? Maybe we can talk a bit about the Moon’s orbit around the Earth.

    Pamela: Once upon a time, long, long ago in the Solar System we live in the Earth was a large blob. We talked about in a former episode that the Moon was formed by something roughly the size of Mars coming along and splashing into what used to be the Earth colliding and flinging the lighter stuff up into Space.

    That lighter stuff re-congealed in the form of the Moon. It was closer, it was rotating and over time this new body that formed out of this collision, this new Moon that formed around the Earth formed with a very strange asymmetry.

    If you were able to take the Moon cut it in half and put an ‘X’ down where the center of mass is and where the center of its shape is, the center of mass is actually off to one side.

    This is sort of like you can imagine having a basketball that has a lead weight off slightly to the side of the center of it. It’s not perfectly centered but off to the side. When you try and spin it, it’s going to wobble in strange ways.

    In this case as it tries to rotate this extra mass, this extra density on the one side is always getting yanked by the Gravity of the Earth.

    This off-center yank had the effect of over time slowing rotation of the Moon. The Moon is trying to rotate and every time that extra mass isn’t pointed directly towards the Earth, the Earth’s gravity yanks on it and says “no, point that extra mass this direction” it’s exerting a torque.

    Over enough millions and millions of years this extra torque, this extra yank on this non-spherical distribution of mass stopped the Moon’s rotation so that the Moon always keeps this extra dense region pointed directly at the planet Earth.

    What’s kinda cool is when you actually map the entire surface of the Moon the two sides look VERY different. This is because it was easier for lunar lava to leak out on the side of the Moon that’s not facing towards the planet Earth.

    We get much more lava and much more of this black stuff – the salts; Lunar Mare is what they call it.

    Fraser: But those are like the Seas, right? The big black blotches on the face of the Moon. So those aren’t on the far side of the Moon as much?

    Pamela: No, we see at a few percent level that on the far side of the Moon, inside the deep craters there is this lava there as well. But on the near side of the Moon, over 30 percent of the Moon is covered in this black lava flow; whereas it’s only a couple percent on the far side of the Moon.

    Fraser: That’s pretty cool. Now if I remember correctly, the orbit of the Earth – because right now the Moon that is tidally locked to the Earth, that’s always showing the same face to the Earth – but the Earth isn’t tidally locked to the Moon.

    We rotate in 24 hours while the Moon takes 27 days to go around the Earth. We’re actually slowing down, right to become tidal locked to the Moon?

    Pamela: Right, our own Planet also isn’t a perfectly symmetric distribution of stuff. If we were, the entire Planet would have the exact same thickness of the ocean everywhere; the exact same distribution of metals everywhere and we don’t.

    As a result of differences in density in different parts of our Planet as we rotate there’s a tidal friction that is slowly trying to torque our Planet as well. As a result of all of this the Moon actually appears to be moving away from the Earth a few centimeters a year.

    What’s happening is the Earth’s rotation is slowing down just a very little bit. This slowing of the Earth’s rotation with conservation of angular momentum requires that the Moon move to a larger distance away from the planet Earth.

    So over time the Earth’s rotation is going to slow and slow and the Moon is going to as a result move further and further away. This means that we actually live at a pretty special time in the history of the planet Earth where the Moon is uniquely located such that most of the time when it passes in front of the Sun it fully blocks the Sun out.

    Over time as the Moon moves further and further away, its size on the Sky is going to get smaller and it will reach the point where Solar Eclipses get such that what you’re actually creating is a donut of Sun instead of a completely blocked out Sun.

    Fraser: You’ll be seeing transits, right? Where they just zip across the face of the Sun but you don’t actually get that big block that we do now.

    Pamela: We already get this some of the time with what we call Annular Eclipses where you’re left with an annulus of Sun. But the size of the annulus and the frequency of Annular Eclipses is going to increase until all we have is Annular Eclipses and as the Moon gets further away, yeah transits may be a better word for it.

    Fraser: The Annular Eclipse that’s because the Moon changes – you know it’s in an elliptical orbit around Earth – and it changes its distance, how close it gets to the Earth and if things time out right the Moon is at its farthest point when it passes in front of the Sun.

    Visually it’s the smallest in the Sky and so you get the black Moon with a ring of sunlight around it. That would be pretty amazing to see I think.

    Pamela: And we’re getting there, just hang out for a few more billion years.

    Fraser: I think it’s 50 billion years when the Earth and the Moon become tidally locked to each other.

    Pamela: But our Sun is going to crispify our System first so I’m not real worried about it.

    Fraser: Yeah, I knew we had an appointment before then. [Laughter] Okay so I think that kind of explains the orbit. What is the Moon made out of?

    Pamela: Swiss cheese.

    Fraser: Don’t say Swiss cheese, aw I knew it! [Laughter] I guess I flubbed up on that one, didn’t I? Fine, apart from vast quantities of Swiss cheese, what is the Moon made out of?

    Pamela: The tactical words we use for it is it’s made out of basically the lava stuff is the salts. It’s mostly I guess, avoiding all the geophysics and a lot of vocabulary words where I have to admit I’ll be in way over my head – I’ve been teased by more than one geophysicist for how badly I pronounce the names of minerals.

    It’s made out of a lot of different minerals that are really lacking in water. That’s one of the things that we keep finding over and over. You take a lunar rock, look at what’s in it and water is not one of the ingredients.

    Fraser: Right it’s like Silicon-Oxide, Titanium-Oxide, lots of Oxygen but no Hydrogen.

    Pamela: It’s lacking in volatiles as well. What gets neat is when you start looking at how the surface was made. The surface is generally composed of two different regions.

    There is the Lunar Mare – this is the section that is made primarily of lava either from volcanoes or from the surface liquefying during an impact event. Then there are also the Terrae, the Lunar Highlands. These are the light areas of the Moon.

    The entire surface of the Moon has just been completely pulverized with craters. We can start to age different parts of the surface by looking at the number of the craters. We’re looking at the impact of large craters that we can generally see and determine a particular section has been hit by so many objects while another section has been hit by a different number of objects.

    The majority of the stuff that’s hitting the Moon is little micro-meteorites. With all the impacts that have occurred over all of the millennia, this has led to the surface of the Moon basically becoming granulated. We talk about the surface of the Moon being what we call Regula which is basically dusty pulverized rock.

    The thickness of the Regula varies depending on what type of surface you’re on with the older surfaces have much thicker regula and the younger surfaces have much thinner regula. We have a surface that has been blasted, is constantly getting impacted – the largest impacts ended up melting the surface, flipping the surface – we end up aging the surface by looking at the craters.

    We look at the density of the craters in different places and looking at the structures that the craters have. Can we still see the rays? Have the craters themselves had craters placed on top of them? This is how we end up aging the surfaces.

    Then what’s cool is we can actually say we know one section is older than another section due to the number of craters. Then we’ve actually sent people to go pick up rocks and use radio-carbon dating to put absolute numbers on the ages of the sections tested.

    Fraser: When the Apollo mission was being planned, scientists weren’t sure that the Lander would be able to sit on top of the Regula. One of the fears was the Lander would land and it would just sink into the Regula like it was a snow bank.

    Pamela: That was one of the fears put out by a man by the name of Fred Hoyle who has alternately come up with some of the greatest ideas in Astronomy and also some of the most wrong ideas in Astronomy.

    What’s cool is he was consistently trying to think outside of the box and just make people aware of what they might be walking into as they explore new worlds and built new scientific ideas.

    This was one of those things where as we contemplated what’s it going to be like to land on the Moon we had to contemplate what’s it going to be like to land on really thin pulverized dust.

    Is it going to be like landing on powdery snow where you sink straight down or is it going to be more like landing on nice wet soggy snow where you can compact it and stand on the surface?

    Fraser: It’s kind of both, right? The very top few centimeters is this really light powdery stuff almost like talcum powder. Below that it’s actually pretty dense.

    The Astronauts had a little trouble you know they had to use hammers and chisels to actually dig out samples from the Regula.

    Hoyle was thinking that they would sink into the Regula and that was wrong. But this dust is actually pretty nasty stuff.

    Pamela: It is and it’s one of the things that NASA is working the hardest to try and figure out how to cope with as we look to landing on the Moon in the future. One of the more fascinating women that I interviewed when I was at the Lunar and Planetary Sciences conference last March was a Biologist who is working on trying to figure out how to mitigate the effects of dust on human beings.

    You get the dust on your spacesuit, bring your spacesuit in with you and strip it off and no matter how careful you are you end up getting this dust into the atmosphere of the crew area.

    It’s extremely abrasive. As you said filled with different metals and silicates, this is like the finest nastiest glass-based sand that you’ve ever encountered.

    A lot of the lunar surface when it gets liquefied and re-solidifies as an effect of an impact it becomes glass. Imagine living in a low Gravity environment – Gravity on the Moon is one 6th of what it is here on Earth – where this glass-based dust can suspend itself in the air. You’re getting it in your clothing and it is rubbing on your skin between you and your shirt.

    They are actually investing money in trying to figure out what type of apparel will cause Astronauts to get the least damaged by getting dust in their clothing.

    Fraser: Well but they’re going to get it in their lungs. That’s the trouble right? It’s like little pieces of glass going into your lungs.

    Pamela: Being an Astronaut isn’t safe.

    Fraser: No, no but I think this is not what they were anticipating and now when they’ve had a chance to really look at this stuff under the microscope, yikes, it’s really dangerous.

    There was a new announcement this week which we thought we’d report on. We’ve covered this in the past which is: “Is there ice on the Moon? Maybe in some of the craters at the southern and northern poles?” There might be deposits of water ice.

    Pamela: The basic idea is the Moon just like the planet Earth has gotten creamed with Comets now and then. It’s gotten basically hit with watery things that should have deposited their material on the surface of the Moon and there are places on the Moon that never see any daylight.

    There are also places of the Moon that never see darkness because the mountains extend out so they’re always in sunlight. There are craters that go down deep and sunlight is never able to get inside of them at the two poles.

    We’ve thought that maybe one of the craters – Shackelton is the one getting explored lately – maybe in Aitken’s Basin and Shackelton Crater, in one of these polar craters, maybe a Comet hit.

    Maybe it left its ice; maybe that ice is still there and we can land and use that water to help fuel a colony, provide water – we need water, it’s just that simple. However, we can’t find it.

    Fraser: NASA’s Lunar Surveyor found evidence of water. It is more like found chemical evidence of it. It wasn’t actually able to take pictures of water at the southern pole but yeah, the news isn’t good.

    Pamela: No and so right now we can’t completely eliminate the fact that maybe there is water maybe there isn’t water.

    What we can say is if there’s water, it’s not hanging out sitting on the surface where it’s nice and shiny and easy to take pictures of.

    Fraser: Right, yeah the Japanese spaceship Kaguya just took pictures of the bottom of Shackelton crater and nothing.

    Pamela: Nothing – no go.

    Fraser: No go – just dry dusty shadowed Moon just like the rest of it.

    Pamela: If there is water ice on the Moon it’s either covered in dust so that we can’t see it or something else has happened so that it just looks just like the rest of the lunar surface. The Moon is still keeping its secrets or it has no water.

    I think a lot of us are going please, please let there just be hiding the water because otherwise it’s going to be a lot harder to start putting colonies on the Moon.

    Fraser: The plan was that we were going to talk about some missions, but we’re out of time. So I think we’ll stretch this out to next week.

    So, next week we will talk about past and future missions to the Moon.

    Pamela: And we will save our cool show to be two weeks from now. We’ll have a really cool show coming your way.