Starship Asterisk*

Lunar Nearside

Explanation: About 1,300 images from the Lunar Reconnaissance Orbiter spacecraft's wide angle camera were used to compose this spectacular view of a familiar face - the lunar nearside. But why is there a lunar nearside? The Moon rotates on its axis and orbits the Earth at the same rate, about once every 28 days. Tidally locked in this configuration, the synchronous rotation always keeps one side, the nearside, facing Earth. As a result, featured in remarkable detail in the full resolution mosaic, the smooth, dark, lunar mare (actually lava-flooded impact basins), and rugged highlands, are well-known to earthbound skygazers. To find your favorite mare or large crater, just slide your cursor over the picture. The LRO images used to construct the mosaic were recorded over a two week period last December.

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We have the best moon! I love the Zoomify image, and am glad for the labels, too.

I was curious why the impact craters are so symetrical. Since this is the near side, any craters in the center of the moon would have had to come directly from earth (in my simplistic view) to get a symetrical crater. Seems like there should be more side impact craters on the earth facing side since they would have to be passing by the earth to hit the moon. I guess it must be gravity is strong enough to arrest the side motion or the moon rotated at one time...

Thoughts,

M

Why no detailed, untouched, pictures of the far side of the moon. Are there structures there that we aren't supposed to see?

dobrosailor,
In fact, an impactor at almost any angle other than the vertical produces a circular crater.

There is one set of Moon craters that show the effect of a very,very low angle, Messier and Messier A.
You can see them in the Mare Fecundatis, which is at three o'clock on the LROC image, about a quarter of a radius in from the rim.
Zoom ij there and you can see two oval craters, with two rays pointing towards the centre of the image.
Even so the ovality is not great - they could eaily be taken for circular craters seen from an angle
It is considered that the impactor that caused Messier must have only just missed not rounding the Moon and going on by. It was on an extremely tangential course. The original body broke up in falling towards the Earth/Moon (see 'crater chains') and the two bits caused the two craters - it didn't bounce!

For more on the Messiers, see: http://seds.org/messier/xtra/m-crater.html

John

Johnny,
Yes, the giant Coke neon sign and the Chinese car factory
Nhoj

Johnny wrote:Why no detailed, untouched, pictures of the far side of the moon. Are there structures there that we aren't supposed to see?

Far side of the moon http://en.wikipedia.org/wiki/File:Cara-oculta-luna.jpg

This composite photo is just brilliant! But I agree with johnny and wish we could see a similar composite of the far side of the moon. (Hint. Hint. Please. Please.)

But there are two features of this photo I wonder about

• I notice parallel N-S striations on this photo, especially in the area around Copernicus and to the NE. These are not the rays that center on craters, but faint parallel lines. I assume these are artifacts of the imaging process. I guess it just surprises me that the contrast in LRO images isn't better balanced.

• The caption says these photos were captured over a two week period. If so, why do all the shadows seem to indicate that the sun is in about the same position, to the right of the moon?

dobrosailor wrote:I was curious why the impact craters are so symetrical. Since this is the near side, any craters in the center of the moon would have had to come directly from earth (in my simplistic view) to get a symetrical crater. Seems like there should be more side impact craters on the earth facing side since they would have to be passing by the earth to hit the moon. I guess it must be gravity is strong enough to arrest the side motion or the moon rotated at one time...

In addition to JohnD's comment that impacts by their nature produce round craters, keep in mind that the Earth, as seen from the Moon, isn't even 2° wide. You could hold your arm out and cover it with your thumb. So it isn't really blocking the Moon from nearside impacts. In fact, it has a slight gravitational focusing effect, which might increase lunar impacts. Finally, the Moon undergoes libration- rotating back and forth a bit- over a greater angle than the Earth subtends in the sky, so all of the nearside is regularly exposed to unoccluded sky.

Chris Peterson wrote:

dobrosailor wrote:
I was curious why the impact craters are so symetrical. Since this is the near side, any craters in the center of the moon would have had to come directly from earth (in my simplistic view) to get a symetrical crater. Seems like there should be more side impact craters on the earth facing side since they would have to be passing by the earth to hit the moon. I guess it must be gravity is strong enough to arrest the side motion or the moon rotated at one time...

In addition to JohnD's comment that impacts by their nature produce round craters, keep in mind that the Earth, as seen from the Moon, isn't even 2° wide. You could hold your arm out and cover it with your thumb. So it isn't really blocking the Moon from nearside impacts. In fact, it has a slight gravitational focusing effect, which might increase lunar impacts.

It should be kept in mind, however, that, for whatever reason,
there are far more crater impacts extant on the far side.

Probably the heavy cratering (and/or lack of maria) helped to make this the far side rather than it being the far side contributed to the cratering.

http://en.wikipedia.org/wiki/Far_side_of_the_Moon wrote:
<<The far side of the Moon is the lunar hemisphere that is permanently turned away from the Earth. The far hemisphere was first photographed by the Soviet Luna 3 probe in 1959, and was first directly observed by human eyes when the Apollo 8 mission orbited the Moon in 1968. The rugged terrain is distinguished by a multitude of crater impacts, as well as relatively few lunar maria. It includes the second largest known impact feature in the Solar System, the South Pole-Aitken basin. The far side has been suggested as a potential location for a large radio telescope, as it would be shielded from possible radio interference from Earth. To date, there has been no ground exploration of the far side of the Moon.>>

orin stepanek wrote:

Johnny wrote:Why no detailed, untouched, pictures of the far side of the moon. Are there structures there that we aren't supposed to see?

Far side of the moon http://en.wikipedia.org/wiki/File:Cara-oculta-luna.jpg

Still, it makes you wonder, why are we seeing hi-res image of the near side of the moon only?

Is there any doubt that the maria are flooded craters?
Giant flooded craters at that, where the lava has covered over any craters inside the big ones.
So it's this side that has taken the brunt.

But that was during the Late Great Bombardment, shortly after the formation of the Earth and Moon (however that was), when the Moon was still rotating faster than it went around the Earth. So the markings on the Moon are nothing to do with which way it faces.

John

Wes.D wrote:
Still, it makes you wonder, why are we seeing hi-res image of the near side of the moon only?

You are supposed to be fooled into thinking that this is just another earth photo of the moon.

http://www.hao.ucar.edu/education/bios/galileo2.php wrote:

Drawings of the moon as seen with Galileo's telescope. By his own account, Galileo first observed the Moon on November 30, 1609. Comparing patterns of light and shadow in the vicinity of the terminator (dividing line between light and shadow) at first and third quarter, Galileo could argue convincingly that there exists mountains and valleys on the lunar surface. Aristotelian doctrine stipulated that celestial bodies were perfectly smooth and spherical. Drawing reproduced from Galileo's 1610 Sidereus Nuncius.

---

[img3="The first telescopic observations of the Moon on record were carried out by the English man Thomas Harriot (ca. 1560-1621), on the evening of July 26, 1609. However, based on his extant correspondence, as well as entries in his notebooks, as in the case of sunspots, Harriot did not appear to have drawn any particular physical significance from what he saw. In 1609 and 1610, Thomas Harriot made a number of drawings of his telescopic observations of the Moon. In addition he also drew "Moon Maps" of what he believed the surface geography of the Moon to be."]http://galileo.rice.edu/sci/harriot_moon.gif[/img3]

That the moon is tidally locked with the nearside facing Earth is a given.

That it slightly rocks back and forth, called libration, is also a given, but less well known. As I recall, it rocks so that about an extra 10% of the Moon's surface becomes visible.

This raises two questions for me:
(1) if the Moon is core-less, then it should be largely frictionless (yes, I know that it does have small moon-quakes, so it isn't wholly frictionless) and so if a frictionless object is perturbed, as all bodies in orbit with at least two others are, then what is to stop the libration from changing? Could the rocking back and forth grow until it goes to a full loop, such as those Carnival rides that have egg shaped caged pods on a Ferris Wheel? The only thing that causes a mass concentration (MassCon) to tidally lock is friction to dampen out the rocking. Are the Moon-quakes adequate to the task?

(2) As it would seem the maria are all on the side facing Earth. Which is the chicken and which is the egg? Did the large impacts occur, causing MassCons and triggering the tidal lock? Or was it that the MassCons were already there and being already tidally locked exposed the nearside to greater risk of the gravitationally lens-focused impacts that caused the maria? Was the Moon tidally locked a very long time ago, such as when the Moon was far closer to the Earth? If so, the rate of rotation of the Moon would have to slow, so as to match the slowing of the rate of revolution around the Earth. (That might be a source of some Moon-quakes, yes?)

Just some thoughts.

Craig Willford

"Since this is the near side, any craters in the center of the moon would have had to come directly from earth ..."
Not to dispute any of the other good explanations but I think, in fact, that much of the material did come directly from earth when the moon was formed from the impact of the small planet. Not in a straight lines, of course but from orbital paths that hit the moon's near side and far side.

If you don't agree with my point about the age of the craters, the Late Great Bombardment and the lack of a tidal lock at that time, as it was shortly after the Moon (and Earth) was formed, say so.
If you do, all this talk of 'coming directly from the Earth' is completely irrelevant.

John

Guest wrote:"Since this is the near side, any craters in the center of the moon would have had to come directly from earth ..."
Not to dispute any of the other good explanations but I think, in fact, that much of the material did come directly from earth when the moon was formed from the impact of the small planet. Not in a straight lines, of course but from orbital paths that hit the moon's near side and far side.

Actually, all it would take is for a small body to be passing between the Earth and Moon, and be sufficiently close to the moon to be dragged in by the Lunar gravity. Nothing "Has" to come from Earth to impact the center of the Near Side.
Don't forget, you can fit 30 Earths between us and the Moon. There is a lot of space between us.

Craig Willford wrote:That the moon is tidally locked with the nearside facing Earth is a given.

That it slightly rocks back and forth, called libration, is also a given, but less well known. As I recall, it rocks so that about an extra 10% of the Moon's surface becomes visible.

This raises two questions for me:
(1) if the Moon is core-less, then it should be largely frictionless (yes, I know that it does have small moon-quakes, so it isn't wholly frictionless) and so if a frictionless object is perturbed, as all bodies in orbit with at least two others are, then what is to stop the libration from changing? Could the rocking back and forth grow until it goes to a full loop, such as those Carnival rides that have egg shaped caged pods on a Ferris Wheel? The only thing that causes a mass concentration (MassCon) to tidally lock is friction to dampen out the rocking. Are the Moon-quakes adequate to the task?

(2) As it would seem the maria are all on the side facing Earth. Which is the chicken and which is the egg? Did the large impacts occur, causing MassCons and triggering the tidal lock? Or was it that the MassCons were already there and being already tidally locked exposed the nearside to greater risk of the gravitationally lens-focused impacts that caused the maria? Was the Moon tidally locked a very long time ago, such as when the Moon was far closer to the Earth? If so, the rate of rotation of the Moon would have to slow, so as to match the slowing of the rate of revolution around the Earth. (That might be a source of some Moon-quakes, yes?)

You are thinking in terms of a dynamic libration in which the moon is wobbling from side to side like a pendulum about some stable minimum potential.
This sort of dynamic swinging certainly exists (and could even be felt on the moon with sensitive instruments)
but it is very weak and must have a period of many decades if not centuries.
[img3="The high mass of the Nearside MasCons: Mares Humorum, Imbrium, Serenitatis, & Nectaris plus that of the Farside highlands contrast with the low mass "bullseyes" Mare Orientale & Mare Humboldtianum to make a weak quadrapole moment stabilized by the Earth's gravity."]http://upload.wikimedia.org/wikipedia/c ... av_150.jpg[/img3]
What we observe here on earth is three types of kinetic libration
caused by the accelerations of our own inadequate earthly observation post:

http://en.wikipedia.org/wiki/Libration wrote:
There are three types of lunar [kinetic] libration:

• * Libration in longitude results from the eccentricity of the Moon's orbit around Earth; the Moon's rotation sometimes leads and sometimes lags its orbital position.
  * Libration in latitude results from a slight inclination between the Moon's axis of rotation and the normal to the plane of its orbit around Earth. Its origin is analogous to how the seasons arise from Earth's revolution about the Sun.
  * Diurnal libration is a small daily oscillation due to the Earth's rotation, which carries an observer first to one side and then to the other side of the straight line joining Earth's and the Moon's centers, allowing the observer to look first around one side of the Moon and then around the other—because the observer is on the surface of the Earth, not at its center.

k1ns wrote:

• The caption says these photos were captured over a two week period. If so, why do all the shadows seem to indicate that the sun is in about the same position, to the right of the moon?

Indeed, if you look carefully at the length of the shadows, the angle of the sun, relative to the local horizontal plane, is about the same across the face of the moon. To achieve that, you need to take pictures of different longitudes at different times, as the lunar day progresses, for about half a lunar day; i.e., for about two weeks. So, pictures of the right (eastern) limb of the moon must have been taken first, at a time when the western limb was dark, while pictures of the western limb were taken two weeks later, when the eastern limb was dark.

One consequence of the constant solar angle is that the picture looks remarkably flat. You don't get the feeling of the Moon being a sphere because the angle of illumination is the same across the entire disk.

Hey neufer, in your mar 03, 2011 2:53pm moon post, when i click on >Thomas Harriot<, i get this-->Forbidden
You don't have permission to access /images/photos/ozzy_and_harriet_std.jpg on this server.

How come?? Crooked link

Beyond wrote:
Hey neufer, in your mar 03, 2011 2:53pm moon post, when i click on >Thomas Harriot<, i get this-->Forbidden
You don't have permission to access /images/photos/ozzy_and_harriet_std.jpg on this server.

How come?? Crooked link

It works for me:

Dear Art Neuendorffer:

Your analysis about libration was thorough and illuminating. Thanks.

I was aware of, but hadn't fully thought out, what you now make clear as Longitudinal Libration, which is simply the viewing slightly around the corner because of the elliptical orbit of the Moon, causing it to be slightly ahead or behind in the relation of the revolution and rotation.

What I was thinking though is the dynamic libration, which you talked about. You are clearly right that the periodicity would have to be very lengthy, as the mass is very substantial. I should have known this, if only I had thought about it.

The perturbation of the other kinds of libration, though, could be the very largest source for causing some dynamic libration. The MassCons slightly leading or slightly trailing the center of gravity of the Earth-Moon system would effect work on the Moon, would it not?

Do you know if scientists have detected any dynamic libration?

Sincerely,

Craig Willford

neufer wrote:

Beyond wrote:
Hey neufer, in your mar 03, 2011 2:53pm moon post, when i click on >Thomas Harriot<, i get this-->Forbidden
You don't have permission to access /images/photos/ozzy_and_harriet_std.jpg on this server.

How come?? Crooked link

It works for me:

Hmm...I still get Forbidden clicking on Thomas and when i click on your white square with the red x, in your post I'm quoteing, i get nothing.
OH-WELL.

Johnny wrote:Why no detailed, untouched, pictures of the far side of the moon...?

Clearly it must have been dark in the back since it was light in the front! But that's just what they want us to think. You can see the obvious fake crater shadows even though the whole surface is lit.

And of course they also want us to think the orbiter would be out of Earth contact while on the other side, but we know better - it doesn't actually go all the way around!

But the REAL reason is that the set Hollywood built with government money in 1968 just doesn't have anything painted on the back side, as can be seen in this rare spy photo.
You can even see the old paintbrush strokes starting to show on the front in today's high resolution APOD.

-Noel

Craig Willford wrote:
The perturbation of the other kinds of libration, though, could be the very largest source for causing some dynamic libration. The MassCons slightly leading or slightly trailing the center of gravity of the Earth-Moon system would effect work on the Moon, would it not?

Do you know if scientists have detected any dynamic libration?

Apparently so, Craig, and it appears that I have overestimated
the period of the Moon’s free/dynamic librations
(of which there are again three in number {e.g., yaw, pitch & roll ? }):

http://www.csr.utexas.edu/mlrs/dda.html wrote:
Lunar Laser Ranging at McDonald Observatory:
P. J. Shelus, R. L. Ricklefs, J. G. Ries, A. L. Whipple, and J. R. Wiant
McDonald Observatory

<<[Lunar laser ranging (LLR)] analysis has also provided measurements of the Earth’s orbit precession, the 18.6 year nutation, the Moon’s tidal acceleration, lunar rotational energy dissipation and the Moon’s free librations, Earth orientation, and the determination of the obliquity and the equinox.

The lunar libration in longitude, with a period of only 2.9 yr, has a 69 yr beat with another term of similar period; a third mode of that libration has a 24 yr period with respect to the node.>>

Looks upside down to me.