# Why doesn't the sun pull the moon away from earth?

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If the suns gravitational pull is strong enough to hold much larger masses in place (all the planets) and at much greater distances (all planets further away from the sun then earth) why does it not pull the moon away from earth?

The Moon is in orbit about the Sun, much as the Earth is. Although this is not the usual perspective from the Earth, a plot of the Moon's trajectory shows the Moon in an elliptic orbit about the Sun. Essentially the Earth, Moon, Sun system is (meta) stable, like that of other planets orbiting the Sun.

I agree with Adrian's answer. If you look at the moons orbit, in a very real sense it orbits the sun maybe more than it orbits the earth. The Earth/Moon system orbits the sun at 30 KM/s, the Moon orbits the earth at about 1 KM per second. Both orbits are reasonably eliptical.

The entire solarsystem orbits around the center of the Milky-way, so orbiting more than one center of mass isn't unusual. Orbits can exist within other orbits, within limits. The orbital limit is sometimes referred to as the Sphere of Influence http://en.wikipedia.org/wiki/Sphere_of_influence_%28astrodynamics%29

If the moon was a bit more than twice as far as it is from the Earth as it is now, the Earth might lose it.

Why doesn't the sun pull the moon away from earth?

Short answer: Because the Moon is much closer to the Earth than it is to the Sun. This means the gravitational acceleration of the Earth toward the Sun is almost the same as is the gravitational acceleration of the Moon toward the Sun.

The Moon's acceleration toward the Sun, $-GM_odotfrac{oldsymbol R+oldsymbol r}{||oldsymbol R+oldsymbol r||^3}$ is indeed about twice that of the Moon toward the Earth, $-GM_oplusfrac{oldsymbol r}{||oldsymbol r||^3}$. This is irrelevant. What is relevant is the Moon's earthward acceleration due to gravitation compared to the difference between the Moon's and Earth's sunward gravitational acceleration, $$oldsymbol a_{odot, ext{rel}} = -GM_{odot}left(frac{oldsymbol R + oldsymbol r}{||oldsymbol R + oldsymbol r||^3} - frac{oldsymbol R}{||oldsymbol R||^3} ight)$$ This relative acceleration toward the Sun is a small perturbation (less than 1/87th in magnitude) on the Moon's gravitational acceleration toward the Earth. Given the current circumstances, the Sun can't pull the Moon away from the Earth.

The gravitational force exerted by the Sun on the Moon is more twice that exerted by the Earth on the Moon. So why do we say the Moon orbits the Earth? This has two answers. One is that "orbit" is not a mutually exclusive term. Just because Moon orbits the Earth (and it does) does not mean that it doesn't also orbit the Sun (or the Milky Way, for that matter). It does.

The other answer is that gravitational force as-is is not a good metric. The gravitational force from the Sun and Earth are equal at a distance of about 260000 km from the Earth. The short-term and long-term behaviors of an object orbiting the Earth at 270000 km are essentially the same as those of an object orbiting the Earth at 250000 km. That 260000 km where the gravitational forces from the Sun and Earth are equal in magnitude is effectively meaningless.

A better metric is the distance at which an orbit remain stable for a long, long, long time. In the two body problem, orbits at any distance are stable so long as the total mechanical energy is negative. This is no longer the case in the multi-body problem. The Hill sphere is a somewhat reasonable metric in the three body problem.

The Hill sphere is an approximation of a much more complex shape, and this complex shape doesn't capture long-term dynamics. An object that is orbiting circularly at (for example) 2/3 of the Hill sphere radius won't remain in a circular orbit for long. Its orbit will instead become rather convoluted, sometimes dipping as close to 1/3 of the Hill sphere radius from the planet, other times moving slightly outside the Hill sphere. The object escapes the gravitational clutches of the planet if one of those excursions beyond the Hill sphere occurs near the L1 or L2 Lagrange point.

In the N-body problem (for example, the Sun plus the Earth plus Venus, Jupiter, and all of the other planets), the Hill sphere remains a reasonably good metric, but it needs to be scaled down a bit. For an object in a prograde orbit such as the Moon, the object's orbit remains stable for a very long period of time so long as the orbital radius is less than 1/2 (and maybe 1/3) of the Hill sphere radius.

The Moon's orbit about the Earth is currently about 1/4 of the Earth's Hill sphere radius. That's well within even the most conservative bound. The Moon has been orbiting the Earth for 4.5 billion years, and will continue to do so for a few more billions of years into the future.

If we "hold" the Earth and "move" the Sun away, the Moon wouldn't stay with the Earth, but would follow the Sun. It is the only satellite in the Solar System that is attracted to the Sun stronger than to its own host planet:

our Moon is unique among all the satellites of the planets, is so far as it is the only planetary satellite whose orbital radius exceeds the threshold value, which means it is the one satellite on which the Sun's gravitational acceleration exceeds the host planet's gravitational acceleration. Consequently, it is the only moon in the solar system that is always falling toward the Sun.

The Moon Always Veers Toward the Sun

Now, if the Moon needs to escape the Earth and go for Sun, it needs more speed to do so. It cannot escape Earth until it's speed is enough for escaping. It needs more velocity.

The orbit of the Moon around the Sun is essentially a circle with a radius of 150million km. Its orbit around the Earth has only a 400 000 km radius, thus the effect of the Earth is only a minor perturbation of it.

Looking from the Sun, the Moon has a circular orbit around it, just like Earth, and their effect to eachother is nearly negligible.

Newton law: https://en.m.wikipedia.org/wiki/Newton%27s_law_of_universal_gravitation

F=G*(m1*m2)/d² is the gravitationnal force between 2 things of mass m1 and m2, separated by a distance d. G is the gravitational constant (I don't remember the value).
--> F_earth/moon=F_moon/earth=G*(m_moon*m_earth)/d²
Same thing for F_sun/moon

You'll notice that F_earth/moon is greater than the other force, so the Moon is more attracted by Earth than Sun.

## Why doesn't the sun pull the moon away from earth? - Astronomy

That's right! One lunar month, or 29.5 Earth days.

It's relatively easy to understand why the lunar month and the "Earth" month are the same length. When the moon is full (as seen from Earth) it's on the side of the Earth away from the sun. This means that the Earth is between the moon and the sun. As seen from the moon, the Earth is "new"—only its night side is visible. Conversely, when the moon is "new," it's between the Earth and the sun, and the Earth is "full."

It is quite obvious that the moon orbits the Earth, right? Well, did you know that the sun pulls more than twice as hard on the moon as the Earth does? So why doesn't the sun yank the moon from Earth's pull?

The answer is because the moon is effectively orbiting the sun! The Earth is really just causing a perturbation (in other words, a variation, or a deflection) of the moon's path as the moon travels around the sun. This perturbation is a very regular orbit around the Earth.

By the way, the footage of the Earth rising over the moon's horizon was taken by an astronaut while he was orbiting the moon and was not taken from the moon's surface.

## The Moon, the Earth, and gravity

What would happen if an aspiring villain, a scientist who’s a little too old and deaf, and a group of clumsy and spiteful assistants ventured into the construction of rockets and anti-gravitational weapons? You can find out in Despicable Me, Universal Pictures’ 3D animated feature coming out in cinemas on 15 October. In the film, our odd heroes struggle with the Moon, its mass, and the force of gravity.

So let’s just have a little talk about these key concepts in order to understand better what happens in the film. Have you ever seen a fruit or object falling from a tree? And have you ever tried to throw a stone and watch it fall? This force that ‘brings down’ things is called the force of gravity.

We are constantly attracted to the Earth by its gravitational force , That is the reason why we always keep our feet on the ground. We don’t need to be in direct contact with Earth to be attracted by it however not being too far away is just enough for the same forces to act. This is why our own planet orbits around the Sun, and the Moon orbits around the Earth.

The gravitational force is determined by the mass of an object. The gravitational force among two objects is proportional to the mass of the objects, and it decreases very fast the moment we separate them. In fact we also attract objects with ‘our’ force of gravity, but we’re too lightweight to see the effects! The Sun, however, is so big that it’s able to hold us close even when it’s so far away. The Moon also exerts its force of gravity since it’s smaller and lighter than Earth, if we could weigh ourselves on it we would discover we weigh around a sixth of our weight on Earth.

One could ask why the Moon doesn’t fall on Earth as an apple from the tree. The reason is that the Moon is never still. It constantly moves around us. Without the force of gravity from the Earth, it would just float away into space. This mix of velocity and distance from the Earth allows the Moon to always be in balance between fall and escape. If it was faster, it would escape any slower and it would fall!

We said the force of gravity depends on distance too. If we were to distance ourselves enough, we could escape its hold. That’s what we try to do with spacecraft. We need to reach and exceed the so-called ‘escape velocity’, that is about 11.2 km/s (at such velocity, we would be able to move from London to New York in just ten minutes!). Once a shuttle reaches this velocity, it is free to travel in the Solar System.

Inside an orbiting shuttle we do not feel the gravitational force of the Earth. Objects don’t fall, they float if you jump up, you don’t come back down. A similar thing also happens to astronauts when they are in space stations orbiting the Earth.

In Despicable Me you will see rockets flying and the effects of gravity on the Moon. Gru has a group of assistants called Minions who will help him in his quest to steal the Moon and become the most famous villain in the world. We hope you enjoy the film!

## Is the Sun causing global warming?

The above graph compares global surface temperature changes (red line) and the Sun's energy received by the Earth (yellow line) in watts (units of energy) per square meter since 1880. The lighter/thinner lines show the yearly levels while the heavier/thicker lines show the 11-year average trends. Eleven-year averages are used to reduce the year-to-year natural noise in the data, making the underlying trends more obvious.

The amount of solar energy received by the Earth has followed the Sun&rsquos natural 11-year cycle of small ups and downs with no net increase since the 1950s. Over the same period, global temperature has risen markedly. It is therefore extremely unlikely that the Sun has caused the observed global temperature warming trend over the past half-century.

No. The Sun can influence the Earth&rsquos climate, but it isn&rsquot responsible for the warming trend we&rsquove seen over the past few decades. The Sun is a giver of life it helps keep the planet warm enough for us to survive. We know subtle changes in the Earth&rsquos orbit around the Sun are responsible for the comings and goings of the ice ages. But the warming we&rsquove seen over the last few decades is too rapid to be linked to changes in Earth&rsquos orbit, and too large to be caused by solar activity.

One of the &ldquosmoking guns&rdquo that tells us the Sun is not causing global warming comes from looking at the amount of the Sun&rsquos energy that hits the top of the atmosphere. Since 1978, scientists have been tracking this using sensors on satellites and what they tell us is that there has been no upward trend in the amount of the Sun&rsquos energy reaching Earth.

A second smoking gun is that if the Sun were responsible for global warming, we would expect to see warming throughout all layers of the atmosphere, from the surface all the way up to the upper atmosphere (stratosphere). But what we actually see is warming at the surface and cooling in the stratosphere. This is consistent with the warming being caused by a build-up of heat-trapping gases near the surface of the Earth, and not by the Sun getting &ldquohotter.&rdquo

## Q: If the Sun pulls things directly toward it, then why does everything move in circles around it?

Physicist: Newton’s laws of motion say:

Where MP and AP are the mass and acceleration of a planet, MS is the mass of the Sun, R is the distance between them, and G is a universal constant. What this rather bold statement says is “if you exist near the Sun, then you are accelerating toward it”. Each of the planets, moons, grains of dust, etc. all say the same thing (“Hey! Accelerate toward me!”), it’s just that with 99.86% of the mass in the solar system, the Sun says it loudest.

A force, like gravity, accelerates the object it acts on. So to understand what a force does it’s important to understand acceleration. Velocity describes how fast your position is changing, while acceleration describes how fast your velocity is changing.

“Velocity” is different from “speed” because velocity is a description of how fast you’re going and in which direction 󈫺 mph north” is a velocity, while 󈫺 mph” is a speed. So you can have an acceleration that changes your velocity by changing your speed and/or by changing your direction.

Imagine you’re in a car (your velocity points forward):

If you accelerate forward, you speed up.

If you accelerate backward, you slow down (“decelerate”).

If you accelerate to the right or left, you turn in that direction but maintain the same speed.

Notice that when you talk about acceleration this way, suddenly the push you feel into your seat when you step on the gas is the same as the push you feel into your seat belt when you brake is the same as the centrifugal force pushing you to the left when you turn right.

A planet orbiting the Sun is always accelerating toward it. But rather than changing the planet’s speed, the acceleration changes the planet’s direction.

With planets the same rules apply. A planet moving around the Sun in a circular orbit always has the Sun about 90° to the side of the direction they’re moving. This means that the planet is always turning, but always moving at about the same speed. The planets are moving so fast that by the time they’ve turned a little, they’ve moved far enough that the Sun is in a new position, still 90° to the side.

So that’s how a planet can accelerate toward the Sun forever without getting any closer. The sideways motion of planets is due to the fact that if a planet were not moving sideways, it would find itself in the Sun in short order. In fact, the Sun is nothing more than a massive collection of all the matter from the formation of the solar system that wasn’t moving sideways fast enough (which is nearly all of it).

Why things end up in circular orbits is a more subtle question. The quickest explanation is that things in not-circular orbits run into trouble until either their orbit is sufficiently round or they’re destroyed. It’s not that circular orbits are somehow better, it’s just that other orbits carry more risk of serious impacts or gravitational interactions (e.g., with Jupiter) that may lead to short, unfortunate orbits.

Assuming that an orbit is stable, then it will be an ellipse (there’s a post here on exactly why, but it’s a whole thing.). A circle is the simplest kind of ellipse, but ellipses can be extremely stretched out. For example, comets have very elliptical orbits (like Sedna in the picture below). In these orbits the comet is mostly moving toward and away from the Sun, so for them the Sun’s pull mostly changes their speed and changes their direction less.

There’s nothing special about the orbits the planets are in. The eight (or nine or more) planets we have in the solar system aren’t the only planets that formed, they’re the only planets left. When things are in highly elliptical orbits they tend to “drive all over the road” and smack into things. When things smack into each other one of a few things happen generally they break or they don’t. When we look at our planetary neighbors we see craters indicating impacts right up to the limit of what that planet or moon could handle without shattering. Presumably there should be impacts bigger than a planet can stand, but (not surprisingly) those impacts don’t leave craters for us to find.

Stickney Crater (left side) on Mars’ tiny moon Phobos or “Why Phobos Nearly Wasn’t”.

So objects with extremely elliptical orbits are more likely to get blown up. But even when two objects hit each other and merge, the resulting trajectory is an average of both objects’ original trajectories, and that tends to be more circular. This is a part of accretion, and Saturn’s rings provide a beautiful example of the nearly perfect circular orbits that result from it.

The grains of dust in orbit around Saturn bump into each other and slowly average out until their orbits are almost perfectly circular (which means they bump into each other far less often).

Given a tremendous amount of time, a big blob of material in space tends to condense into a ball (with most of the matter) and a thin disk of left over material traveling in circular orbits around it.

## Ep. 113: The Moon, Part 1

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.

### Shownotes

Moon Phases

— Moon Connection — US Naval Observatory (see what the moon looked like/will look like on any day, past or future — US Naval Observatory — Universe Today’s Guide to Space — Universe Today’s Guide to Space — Yahoo Education (does YE seem like an oxymoron?)

The Moon Illusion

The Moon Itself

— Starry Skies — DigiPro — Cornell U — UTK — Nine Planets — Wiki — UTK (names and locations)– Ames Research Center — Wiki — Wiki — Wiki
• Moon dust problems here and here — Universe Today — Universe Today

### Transcript: The Moon, Part 1

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 6 th 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.

## No, a Planetary Alignment on May 28 Won’t Cause an Earthquake

I’m seeing some buzz on social media that a planetary alignment on May 28 will cause a huge magnitude 9.8 earthquake in California.

Let me be clear: No, it won’t. It can’t. Worse, there’s not even really an alignment on that date, at least not with the Earth. It’s all baloney.

This all stems from a video by someone who I believe is sincere but also profoundly wrong on essentially every level. It’s been picked up by various credulous places online, then spread around by people who haven’t been properly skeptical about it.

While this story hasn’t gone as viral as the usual astronomically impaired tabloid doomsday BS, it’s popular enough to debunk and hopefully can serve as a template for future such claims of doom and gloom that are actually smoke and mirrors.

First, here’s the video. It’s from YouTuber Ditrianum Media.

There’s a whole lot of nonsense in there that I won’t even bother with, including claims of spirits and (seriously) Nostradamus.

But then the narrator starts talking about alignments. Several things struck me while watching this.

First, there is simply no way an alignment of planets can cause an earthquake on Earth. It’s literally impossible. I’ve done the math on this before the maximum combined gravity of all the planets under ideal conditions is still far less than the gravitational influence of the Moon on the Earth, and the Moon at very best has an extremely weak influence on earthquakes.

To put a number on it, because the Moon is so close to us its gravitational pull is 50 times stronger than all the planets in the solar system combined. Remember too that the Moon orbits the Earth on an ellipse, so it gets closer and farther from us every two weeks. The change in its gravity over that time is still more than all the planets combined, yet we don’t see catastrophic earthquakes twice a month, let alone aligning with the Moon’s phases or physical location in its orbit.

I’ll note that in the video the narrator talks about the planets being “energized,” but doesn’t talk about what this truly means … but it doesn’t matter, because it’s meaningless. It’s the usual sort of New Age word salad when they talk about “energy” they never define what that truly means (unlike in science where it has a strict definition) so it means everything and nothing.

Also, if you watch the video, like for example at 7:27 and 8:11, these “alignments” don’t even align with the Earth! One is just two planets that appear to line up with the Sun when the Earth is far off to the side, and in another they actually form a perpendicular line with the Earth. This is beyond silly it doesn’t even make any sense.

Photo by NASA/JPL-Caltech/Space Science Institute NASA,ESA, and M. Buie (Southwest Research Institute) NASA Phil Plait

At 8:40 he shows another “alignment” that apparently goes between the Earth and Moon … but note that the Moon’s distance to Earth isn’t shown to scale! The sizes of the planets and Moon aren’t to scale either. Look at the width of the Earth’s orbit in the display that’s 300 million kilometers in real distance. The Moon’s distance from Earth is about 380,000 km, or a bit more than 0.1 percent of the size of Earth’s orbit.

To scale, the Earth and Moon would be less than a pixel apart on his display! Now imagine how small the Earth itself would be on that scale.

That “alignment” doesn’t come anywhere near splitting the two. I’m not sure I’d take doomsday advice from someone who doesn’t seem to understand the software being used to predict it.

Again, I’m sure the narrator is sincere and honestly wants to help people and warn them of an event he thinks may be real. This puts him a comfortable step up over the various and repulsive scam artists you can find all over the Web.

But it doesn’t make him within a glancing blow of reality. Alignments of the planets have no effect on us at all. They can’t make you float, they don’t cause earthquakes (the “supermoon” doesn’t either), and don’t even get me started about astrology.

There is something very human about being scared of the unknown, and when we don’t understand something, it’s all too easy to supply any number of threatening boogeymen to stand in the nebulous shadows.

Understanding reality makes a lot of those boogeymen evaporate. Poof. This is absolutely one of those times.

And yes, understanding reality also introduces us to real things that are scary. But there’s the beauty of science: We can separate the real things that scare us from the things that shouldn’t. If something isn’t real, you don’t have to worry about it. You can focus instead on the circumstances you can affect.

I think that many people who turn to pseudo- (and outright anti-) science may do so because they feel that things are out of their control. That’s too bad, because—even though it may not seem like it at first—when you begin down the path of studying science, of becoming a critical thinker, these tools actually help you be more in control of your life, not less.

Take control. Think critically. And that goes doubly so when you’re reading stuff on social media.

## How does the sun earth and moon system affect tides on earth?

Gravity from the Moon and Sun work to pull the Earth apart, creating bulges in the oceans on either side of the Earth, which are experienced as tides.

#### Explanation:

The Moon is the closest massive body to the Earth, and just as the Earth's gravity is strong enough to hold the Moon in orbit, the Moon's gravity is able to effect things here on Earth.

The Moon's gravity is enough to pull the water in the Earth's oceans slightly toward itself. Not enough to break orbit, just enough to cause them to bulge. This tidal effect causes the oceans to "pinch" around the middle and bulge on the near and far side of the Earth.

To understand why this happens, lets look at Newton's law of universal gravitation.

Here, #F_g# is the force of gravity, #G=6.67*10^-11"N""m"^2/"kg"^2# is Newton's gravitational constant, the #m# 's are masses, and #r# is the distance between the two masses. Notice that gravity is inversely proportional to #r^2# , so as #r# gets bigger, or the masses are farther apart, the force of gravity diminishes. That means that the Moon's gravity is stronger on one side of the Earth and weaker on the other. We call that difference in gravity tidal force.

The Moon isn't the only massive object that effects the Earth, though. The Sun also influences tides, but on a smaller scale. Whenever the Sun and Moon are inline with the Earth, i.e. during a full or new moon, the tides will be at their most extreme, or spring tides.

During a quarter moon, when the Sun and Moon are not aligned, their tides will work to cancel each other out, and we have less dramatic tides, or neap tides.

Luckily, the tidal forces her on Earth only effect the oceans. Jupiter's moon Io is actually heated by tidal forces stretching the rock of the moon itself, and black holes form accretion disks where tidal forces shred anything that gets near enough.

## Why is there a tidal bulge on the side of the Earth facing away from the moon?

There are two high tides per day, but we face the moon only once. Even when there is a new moon and the sun and moon are aligned in the sky, there are still two tides per day. How?

This problem is best thought of by looking at different reference frames. If we look at the moon’s reference frame, every piece of matter on earth (including water) is being attracted to the moon. The further away you are from the moon, the less gravitational effect there is.

So a piece of matter on the near side is being pulled the most, the center is being pulled a smaller amount, and the opposite side of the moon is being pulled the least.

Now let’s put that into the frame of reference of the earth, which we reduced to “the center” above. We know the center of the earth is not moving with respect to itself, so it is stationary. The moon-side is being pulled more than the earth center, so the water is pulled slightly towards the moon. The non-moon side is not being pulled as much as the center, so in the frame of reference of the center of the earth, it is being pushed away!

Inertia does not really play a large role in terms of two rides per day, as the water’s inertia cannot really be transmitted over landmass as the tides go around the earth.

A very astute response, thank you, which is why I read this over and over (I’m slow) :-). I think the paragraph:

So a piece of matter on the near side is being pulled the most, the center >is being pulled a smaller amount, and the opposite side of the moon is >being pulled the least.

Would the following be more correct “So a piece of matter on the near side of the earth is being pulled the most, the center of the earth is being pulled a smaller amount, and the opposite side of the earth is being pulled the least.”?

This is what I've always heard it explained as, but is it really fair to think of the earth as a single rigid body in all this? On these large scales and with such massive forces youɽ think the earth would react with some flexibility and simply thinking about the net force on the centre of the earth wouldn't work.

But I never knew that anywhere had 2 tides per day until I moved to the PNW. On the gulf coast there is only one tide per day (when not a neap tide).

Could this not be said more simply by stating that the center of rotation of the Earth is not in the center of the Earth? The moon causes the center of rotation to be nearer to the moon than the center of the earth.

By having the Center of rotation not in the Center of the Earth, one side of the earth is moving faster than the other causing a bulge in the water. This Bulge would be on the opposite side of the Earth from the Moon.

The Tidal effects on the near side to the moon are due to direct gravitational attraction

For good reasons, physics teachers avoid referring to "centrifugal force", leading to good answers like /u/AustinHiggs 's. But it can help simplify things, so long as you remember that it's actually the acceleration term in "F=ma" and not actually a force.

From the perspective of someone rotating with the Earth-Moon system, the moon and everything on it is in a balance between gravity pulling it inward and "centrifugal force throwing it outward". But this is true for the Earth too! The Earth and Moon each rotate around a common center of mass that lies between them, the Moon makes big circles while the Earth makes small ones.

On the side of the Earth nearest the Moon, gravity is bigger than centrifugal force, so the oceans there are pulled toward the moon. On the far side, the opposite is true, so centrifugal force "pulls the ocean away".

From the perspective of someone rotating with the Earth-Moon system, the moon and everything on it is in a balance between gravity pulling it inward and "centrifugal force throwing it outward". But this is true for the Earth too! The Earth and Moon each rotate around a common center of mass that lies between them, the Moon makes big circles while the Earth makes small ones.

Small correction - the common center of mass is actually located inside the Earth, despite the Moon being relatively big.

You might like this. It think it a good explanation of the problems with using the centrifugal force.

Because, not only is the water being pulled away from the earth slightly on the near side, the earth is being slightly pulled more than the water on the far side. In a sense you can view the whole system being slightly stretched along the axis of the moon's gravity.

I understand the principle I think. On one side, the water is pulled towards the moon creating a bulge. The other side is farther away from the moon so it's not pulled nearly as much. But why does it create a seemingly equal bulge on the non-moon side? And wouldn't the water far side still flow towards the low tide instead of bulging creating a bit less of a bulge at least? No/less moon pull = as much bulge as a full Moon pull. I just can't make it add up.

Objects in space are in freefall. Imagine being inside, say, a small room that was airtight, falling through space. You will get pulled by gravity toward different bodies around you, but the room around you is being pulled as well. It's moving, accelerating, in lock step with you, so there is no relative motion, no perceived relative force.

Now imagine that your little room is attached to, say, a huge weight, a big chunk of Iron that weighs thousands of tonnes, along a huge pole that is a thousand kilometers long. This whole contraption is still in freefall, but things are a little different. Let's say that you are in orbit around the Earth and the pole is sideways relative to the Earth, meaning that both you and the weight on the end of the pole are precisely the same distance from the Earth's center. Both you and the contraption will fall around the Earth in orbit, and your motions will again be substantially identical, so you'll still have the experience of weightlessness.

But what if the contraption was vertical with respect to the Earth instead? What if the huge Iron weight was a thousand kilometers closer to the Earth than your little room? Well, the motion of the contraption will be dictated primarily by the gravitational forces acting on the weight, since it's so much heavier than you or your room. This means that your room will experience an acceleration that is equivalent to being 1000km closer to the Earth, since it is attached to the weight. And now the room moves in a way that's different from the way you are pulled around by gravity, and the two don't move in lockstep any more. The weight is pulled with a slightly greater acceleration toward the Earth than you are, so your room moves with a slightly greater acceleration than you do, leading to a net perception of a sort of "pseudo gravity" toward the ceiling, an almost negative gravitational acceleration relative to the position of the floor.

Now imagine flipping the contraption around, so that you are 1000km closer to the Earth than the weight. Now you are accelerated slightly more than the weight and thus the room. Again you experience a slightly differential acceleration, and again this is towards the roof of the room, because everything has been flipped around.

## Small tides

But Takaho Miura of Hirosaki University in Japan and three colleagues think they have the answer. In an article submitted to the European journal Astronomy & Astrophysics, they argue that the sun and Earth are literally pushing each other away due to their tidal interaction.

It’s the same process that’s gradually driving the moon’s orbit outward&colon Tides raised by the moon in our oceans are gradually transferring Earth’s rotational energy to lunar motion. As a consequence, each year the moon’s orbit expands by about 4 cm and Earth’s rotation slows by 0.000017 second.

Likewise, Miura’s team assumes that our planet’s mass is raising a tiny but sustained tidal bulge in the sun. They calculate that, thanks to Earth, the sun’s rotation rate is slowing by 3 milliseconds per century (0.00003 second per year). According to their explanation, the distance between the Earth and sun is growing because the sun is losing its angular momentum.