Moon

[fchaman]

Is it possible to see the dark side of the moon?

[tracer]

Sure. Whenever the moon isn't full, at least part of the side facing the Earth is on the dark (non-sunlit) side of the moon.

Or are you asking if it's possible to see the far side of the moon?

[daver]

And of course we can sometimes see perhaps 18% of the farside. So the answer is either "yes" or "some", depending on what was meant by dark side.

[fchaman]

I think I meant the far side of the moon. What I want to know is there any possibility that any side of the moon is hidden from us?

[Grand Vizier]

I think I meant the far side of the moon. What I want to know is there any possibility that any side of the moon is hidden from us?

Yes - we had no knowledge of most of the features on the farside of the Moon until the Russian Luna 3 probe flew past it in 1959. But, as another poster pointed out, only about 40% is completely hidden from Earth, due to an effect called libration.

[Peter B]

I think I meant the far side of the moon. What I want to know is there any possibility that any side of the moon is hidden from us?

The Moon turns once on its axis in the time it takes to complete one orbit of the Earth. At the most basic level, this means that one hemisphere stays facing the Earth. This is the Near Side. All the Apollo missions landed on the Near Side, to remain in radio communication with the Earth.

We can't see the Far Side of the Moon from Earth. However, spacecraft orbiting the Moon have photographed the Far Side in considerable detail.

[Psionyx]

This is also why the side of the moon we can see is considerably smoother and more picturesque than the far side. The far side of the moon is pitted and scarred, and pockmarked with craters, and looks rather like Mercury. I would venture to guess as to why the near side of the moon DOESN'T look like that is that it's facing a stronger gravity well, and them asteroids are lazy and don't want to put out the effort required to swing completely around the moon to hit it from the front. In other words, the Earth acts as a big honking shield for the near side of the moon. Hits there happen, just a lot less often.

Other things might have had an effect on the smoother appearance of the near side of the moon, such as direct gravitational friction between the moon and the Earth, and other things, but I'm no astronomer, I'm just a layman putting in my guess. Am I right? :)

[freddo]

layman here also - I think you are right that there are other forces at work but they would have a much less than appreciable effect... Meteroid/ite impacts on the Moon are by far the most significant factor for the Moon's unique topography.

[tracer]

I would venture to guess as to why the near side of the moon DOESN'T look like that is that it's facing a stronger gravity well, and them asteroids are lazy and don't want to put out the effort required to swing completely around the moon to hit it from the front. In other words, the Earth acts as a big honking shield for the near side of the moon. Hits there happen, just a lot less often.
Nice try, but no cigar! 'Fraid you've got it backwards.

3-4 billion years ago, before the moon had locked into synchronous rotation with the Earth, lots of volcanoes on what is now the near side of the Moon erupted, covering over all the craters in their wake with dark lava. (The darker "seas," or maria, on the moon are the remnants of this volcanic activity.) As it turns out, the maria are denser than the normal not-so-dark moon rock. As tidal friction with the Earth slowed the moon's rotation, the side of the moon with the denser material on it gravitated (literally) toward the Earth. The moon finally settled down in the synchronous tidal "lock" we see today, with the densest part of its surface facing toward the Earth and the least-dense part of its surface facing away.

The areas of the moon that aren't part of the maria represent the oldest surfaces, which have been exposed to impacts basically since the moon's crust first cooled to a solid. The maria are younger, having been covered over with lava hundreds of millions (or even billions) of years after the moon's crust formed. In the non-maria areas, there are just as many craters on the near side of the moon as there are on the far side of the moon. The difference is, most of the near side is maria, and most of the far side isn't.

[Psionyx]

See, I didn't even know about them ancient lunar volcanoes. Whooee, Jethro, you learn sumpin' new ev'ry day! :D

[kilopi]

In other words, the Earth acts as a big honking shield for the near side of the moon. Hits there happen, just a lot less often.
I see that tracer has already weighed in on this, but I'd just like to point out that there is a common misconception of how big of a "shield" the Earth is for the moon, or vice versa. The moon sometimes seems big to us, standing on the ground, but this is where a model comes in very handy. The relationship of the moon to the Earth, size-wise, is the same as a marble to a tennis ball. And that marble would be about five feet away from the tennis ball. Hold a tennis ball, and have someone hold a marble five feet away, and try to imagine that marble protecting the tennis ball, or the tennis ball protecting the marble.

And, to prove the model, you can hold your hand out at arms length and measure the marble with your thumb and forefinger. If the moon is up, you can do the same to it--they'll be the same size!
The moon finally settled down in the synchronous tidal "lock" we see today, with the densest part of its surface facing toward the Earth and the least-dense part of its surface facing away.
Just a small nit--it doesn't face the Earth directly. I think it's about thirty degrees off. The largest deformation of the moon is due to the Earth tides, and that does line up with the Earth. The masscons being more on this side may be just a coincidence.

[tracer]

Hmmm! I hadn't thought about tidal deformations of the moon before. (I was repeating the story as I heard it in an undergrad Intro to Astronomy class.) I wonder if it was those tidal formations that triggered the massive volcanic eruptions on what-is-now-the-near-side in the first place.

[nebularain]

That's how I came to understand it. And it wasn't "volcanic eruptions" like we have on Earth, it was more like the magma seeping out of the ground through cracks.

Am I correct?

[kilopi]

That's how I came to understand it. And it wasn't "volcanic eruptions" like we have on Earth, it was more like the magma seeping out of the ground through cracks.

Am I correct?
Except for the part about it not being like we have on Earth. Check out the Deccan Traps (http://volcano.und.nodak.edu/vwdocs/volc_images/europe_west_asia/india/deccan.html). They are enormous lava "floods", and they are not the only example on Earth.

[nebularain]

D'oh! I really got to learn to clarify what I mean by what I said. When I think "Volcanic eruptions" I am thinking of volcanos, as in the mountain-types, blowing their tops or whatever. I don't believe the lava flows on the moon had that kind of pressure release, or was there?

[kilopi]

When I think "Volcanic eruptions" I am thinking of volcanos, as in the mountain-types, blowing their tops or whatever. I don't believe the lava flows on the moon had that kind of pressure release, or was there?
I think that there were such eruptions on the moon, but I couldn't find any links with a quick google. The mare are flood basalts, like the traps, I believe.

Some volcano mountains do blow their tops, like Mt. St. Helens twenty three years ago, but Kilauea (http://hvo.wr.usgs.gov/kilauea/update/main.html), a volcano mountain in Hawai'i has been erupting for many years without those spectacular explosions. It all depends upon the chemical makeup of the rock.

[sgartner]

The Moon turns once on its axis in the time it takes to complete one orbit of the Earth. At the most basic level, this means that one hemisphere stays facing the Earth. This is the Near Side. All the Apollo missions landed on the Near Side, to remain in radio communication with the Earth.

We can't see the Far Side of the Moon from Earth. However, spacecraft orbiting the Moon have photographed the Far Side in considerable detail.

This has always confused me (I've found it hard to visualize the motion).

The visualization that I always used was a tennis ball tethered to a pole (the gravitational center in the Earth-Moon system).

Let me try to explain what I "think" is happening and hopefully someone here will help me sort out what I have wrong (or right).

In order for the moon to be always facing earth it must be rotating in the reverse direction (clockwise to counter-clockwise) to its orbiting direction. However, I believe that I have also read that the moon precesses 10% back and forth so the "tether point" is moving on the surface of the moon. The tidal "lock" with the earth makes the moon tend to maintain this relationship (creating a feedback pattern between the moon rotational cycle and its orbital cycle).

I can visualize the moon "reverse" rotating once per month to match the monthly orbital cycle, but I don't understand how the rotation can be variable to allow for the 10% precession of the rotation so that we can actually see 60% of the moon's surface.

Help. How much of this do I have wrong? :-?

[Donnie B.]

Well, I think you have the sense of the rotation wrong.

If you look down on the system from above Earth's north pole, the Moon revolves counterclockwise around the Earth. To keep the same face toward the Earth, the moon also has to rotate counterclockwise.

Think how you would have to twist your wrist if you were modelling the system. Paint a dot on a marble and hold it a few feet from a tennis ball. Move the marble in a circle around the tennis ball. To keep the dot facing the tennis ball, you have to turn the marble in the same sense that you're moving it around the ball.

In fact, if it was the other way around, by the time you got the marble 1/4 of the way around the ball (counterclockwise, say) you'd have turned it clockwise 1/4 turn, and the dot would be facing away from the ball.

This would be easier to show with a diagram, or better yet with animation. Where's Walt Disney when you really need him?... #-o

A good way to visualize it is from the Moon's point of view. Use your marble-ball model and imagine you're on the marble, and see how you move with respect to the walls of the room (never mind the Earth). Maybe that will help...

[sgartner]

Well, I think you have the sense of the rotation wrong.

If you look down on the system from above Earth's north pole, the Moon revolves counterclockwise around the Earth. To keep the same face toward the Earth, the moon also has to rotate counterclockwise.


Thanks, your description helped and you are right, I had the relationship backwards. My real quandry is how to figure out the precession. If the moon were rotating a little too fast or too slow we would eventually see the entire moon. How is it that the moon "jiggles" back and forth? Is its rotation speed slowing down and speeding up over the month? That seems unlikely as it seems, to me, that there is no source for the energy to make that change.

However, it does happen, so I am still missing bits.

Maybe that will help...

Thanks again for the helpful visualization.

[Donnie B.]

IIRC, the lunar libration (almost wrote "libation"... ha! must be the wine...) is due primarily to the fact that the Moon's orbit is not a perfect circle.

The ellipticity of the orbit means that the Moon moves closer and further away (at different points on the ellipse), and also faster and slower -- faster when close, slower when farther away. This means that it's sometimes lagging a bit, so we see its leading edge, and sometimes zipping further ahead, so we see its trailing edge.

There may be a secondary effect as well, but I can't recall what it is due to libation... 8)

[Added] To clarify, what I'm referring to as slowing down and speeding up is the revolution speed, or orbital speed, not the rotation.

[sgartner]

IIRC, the lunar libration (almost wrote "libation"... ha! must be the wine...) is due primarily to the fact that the Moon's orbit is not a perfect circle.

The ellipticity of the orbit means that the Moon moves closer and further away (at different points on the ellipse), and also faster and slower -- faster when close, slower when farther away. This means that it's sometimes lagging a bit, so we see its leading edge, and sometimes zipping further ahead, so we see its trailing edge.

There may be a secondary effect as well, but I can't recall what it is due to libation... 8)

[Added] To clarify, what I'm referring to as slowing down and speeding up is the revolution speed, or orbital speed, not the rotation.

Donnie,

Thanks, that does make sense, though 10% seems like a lot of change for just an angular viewing differential due to an eliptical orbit. Maybe I need some libation myself. It might help to visualize these things (maybe I can get the pink elephants to orbit each other).

Scott

[Donnie B.]

Heheh... we have big careers ahead of us as Pink Elephant Trainers...

Think of it this way: it doesn't take a very large angular change to expose a pretty good-sized sliver of the surface to our view. A couple of degrees +/- may be enough to produce that extra 10% (or whatever it is... a bit less than 10% IIRC).

There's an excellent animation of the lunar libration that shows the effect much better than words can describe it. It's been linked before on the BABB... let me see if I can find it with a search...

[Added] Here you go: http://www.theman.themoon.co.uk/librations/
[Added more] This site explains two other libration mechanisms (my secondary effect is one of them): http://www.inconstantmoon.com/not_libr.htm

[Hamlet]

Heheh... we have big careers ahead of us as Pink Elephant Trainers...

Think of it this way: it doesn't take a very large angular change to expose a pretty good-sized sliver of the surface to our view. A couple of degrees +/- may be enough to produce that extra 10% (or whatever it is... a bit less than 10% IIRC).

There's an excellent animation of the lunar libration that shows the effect much better than words can describe it. It's been linked before on the BABB... let me see if I can find it with a search...

[Added] Here you go: http://www.theman.themoon.co.uk/librations/

APOD (http://antwrp.gsfc.nasa.gov/apod/ap030810.html) also has a nice animation and write-up.

[Kaptain K]

Actually, all three links are to the same animation. :o

[Peter B]

Donnie,

Thanks, that does make sense, though 10% seems like a lot of change for just an angular viewing differential due to an eliptical orbit. Maybe I need some libation myself. It might help to visualize these things (maybe I can get the pink elephants to orbit each other).

Scott

Scott

Here are some comments about libration that I wrote for a thread in the Planet X section of the BABB:

As for libration, I like to use the analogy of a car driving circles around you in a car park.

Imagine you're standing in a car park, and a car is driving around you anti-clockwise (from right to left across your field of vision). Now imagine you keep turning on the spot to watch the car as it drives around. All you'll see is the left hand side of the car. You won't see the right hand side of the car (the Far Side of the car, if you like), and you'll get only a rough idea of what the front and back of the car look like.

Now imagine the car drives around you using a highly elliptical path. Some of the time it comes very close to you, and then it heads out into the distance, swings around, and heads back in. What will you see of the car now? Well, you still won't see the right hand side of the car, but you'll get a good view of its rear as it drives away, and you'll get a good view of its front as it drives towards you.

What does the Moon do? A lot closer to the first example than the second, but its path is elliptical enough that we get to see a bit of its leading and trailing hemispheres.

[sgartner]

Thanks to everyone. Both the animation and the car analogy were very helpful. I'm glad I started reading the book and subsequently found this site!

Scott

[kilopi]

Thanks, that does make sense, though 10% seems like a lot of change for just an angular viewing differential due to an eliptical orbit.
The moon's elliptical orbit has an eccentricity e of 0.0549 (http://www.planetscapes.com/solar/eng/moon.htm). That doesn't make it much different from a circle--if one were to draw such an ellipse on a piece of paper, and then draw a circle around it with a radius the same as the ellipse's semimajor axis, the semiminor axis of the ellipse would be off of the circle radius by only a half of one percent ( e^2/2). But the focus of the ellipse, where the Earth would be (or more precisely, the barycenter), would be offset from the center of the circle by 5 and 1/2 percent of the radius. That alone means that the Earth is shifted over three degrees (arcsin .0549) from center, on both sides of the orbit, but it also means that when the moon is at perigee it is going over eleven percent faster (1.0549/0.9451, Kepler's second law, which says the orbits sweep out equal areas in equal times) than at apogee. The area of the ellipse to the "right" of the Earth is about 14 percent (approx. 8e/pi) larger than the area to the "left," and so must take a proportional longer time to traverse that side. Its rotation rate stays essentially constant, and it "over rotates" as it revolves through its orbit.

[mike alexander]

kilopi wrote:I see that tracer has already weighed in on this, but I'd just like to point out that there is a common misconception of how big of a "shield" the Earth is for the moon, or vice versa.

Don't know answer, so must ask question: Aside from the physical blocking, how much does the earth's (or moon's) gravitational field affect impact frequency? I am thinking of the protective effect of Jupiter on the inner system. Are there local effects (realizing the much smaller gravitational potential available?)

[kilopi]

Don't know answer, so must ask question: Aside from the physical blocking, how much does the earth's (or moon's) gravitational field affect impact frequency? I am thinking of the protective effect of Jupiter on the inner system. Are there local effects (realizing the much smaller gravitational potential available?)
The thought experiment I suggested last May (http://www.badastronomy.com/phpBB/viewtopic.php?p=88512#88512) shows you how much "physical blocking" would be expected--not much. Gravitationally, I don't see that there is any added. Unless the object is headed for Earth, it will swing by Earth, and be deflected somewhat by Earth's gravity. But whether it would be deflected towards the moon or away from the moon might be pure chance, and so wouldn't make that much of a difference.