I've recently spent a lot of time studying and observing the Moon. I even found some web sites that showed pictures from the side of the moon we don't see. At first I thought the Moon didn't rotate because we always see the same face. However after doing some homework, I found out that the Moon does rotate. So if there is Lunar rotation, even a very slow one, we at some point we should see the other side. I also found out that except for Hyperion, one of Saturn's moons, this is the same case for all the other moons of all the planets in the entire Solar System. We would always see the same face if we were standing on the other planets. The explanations I read as to why this happens were way over my head. I'm not a hard science guy so if possible, can somebody explain why this happens but in non scientific terms. Thanks! CG

The Moon rotates almost exactly once in the same time it takes to orbit the Earth, so we only ever see one side.

Slight irregularities result in a phenomenon called libration, which allows us to see about 9% more than one half of it. The Moon's center of mass is not in the same place as its geometric center, and its orbit is not circular. That, coupled with the fact that the Moon's gravity acts on our oceans as a 'brake' and creates tidal drag, serves to make the Moon "nod" and "wobble" back and forth as it rotates on its axes and revolves around the Earth.

Antonio Cidadao produced this animation by photographing the Moon nightly for one complete lunation:

 

It shows the libration quite well.

Our Moon is not tidally locked to the Earth. If it were, it would appear to hover over the same spot on Earth rather than to rise and set a bit earlier or later each night. However, given enough time and no catastrophic influences from outside the Earth/Moon system, the Moon could eventually become tidally locked.

 

Chipdatajeffb, Thanks for taking the time and answering my question. This clears it up for me. Also liked the libration animation. I had no idea the Moon wobbled as such. There truly is a lot more things going on with the Moon then I thought. CG

It really doesn't rotate nearly the same as moons do for any of the planets in our solar system.

http://www.moonconnection.com/moon-same-side.phtml

There are theories why we only see one side of the Moon. Some say we once had two Moons, which rotated much faster.

 

Indeed, the Moon’s average orbital period around the Earth is precisely the same as its rotation period on its axis.  This is due to tidal locking initiated by a slight imbalance in the distribution of the Moon’s mass.  This same effect is seen on almost all of the natural planetary satellites in the solar system.  This causes an observer on the central planet (such as Earth) to essentially see only one side of a natural satellite.

 

The reason we can over time see more than half of the Moon’s surface is due to what are called librations.  The Moon isn’t really wobbling while it rotates other than by a slight physical libration of less than 0.05°.  The noticeable libration effect is almost entirely due to the perspective of the observer relative to the Moon.

 

The libration in latitude causes us to see slightly over the Moon’s north or south poles over the course of a month. It is due to the fact that the Moon’s equator intersects its orbital plane at an angle of nearly 7°.  This effect is similar to the Earth appearing to nod relative to the Sun and produce seasons due a 23.4° inclination between its equator and orbital plane.

 

The libration in longitude causes us to see between 2° and 5° around the Moon’s eastern and western limbs over the course of a month.  This is due to the fact that it rotates on its axis at a virtually constant rate, while it circles the Earth at varying speeds because of its eccentric orbit.

 

There is also a topocentric libration due to the observer’s position on Earth relative to the Moon.  When the Moon appears on the horizon one can see about 1° further around its upper limb than can an observer who is watching the Moon in his zenith.

 

 

 Why do we see only one side of the moon? Because the mass of the moon is more on that side facing earth..thus, the positive and negative property of the moon is more on this side facing earth that is attracted by the negative and positive property, respectively,  of mass of earth.

 

jsaldea.

 

7.6.09

A picture truly is worth a thousand words!! That explains it perfectly!! So, this is what the earth would appear like were we standing on the sun and photographing it in the same manner? (Realizing of course that our feet would be REALLY hot...)

No, if you were at the Sun looking back at Earth, the "face" of the Earth would be changing continually because it rotates once every 24 hours and revolves around the Sun once every 365+ days. So, if you could suspend yourself above the Sun's north pole and stare at the Earth, with enough resolution to make out the continents and oceans, you'd see the Earth rotate completely once every 24 hours.

Not only that, but the Sun itself rotates, and its rate varies depending on the latitude. So the "picture" would be a good deal more complicated if you could somehow "stand" on the Sun's "surface" and look at the Earth.

In the case of viewing the Moon from the Earth, the Moon not only appears to wobble left/right and up/down, but it also gets nearer and goes farther during the course of one revolution around the Earth (and noticeably so ... that is why the image of the Moon in the animation appears to be changing size) because its orbit is elliptical.

chipdatajeffB:

No, if you were at the Sun looking back at Earth, the "face" of the Earth would be changing continually because it rotates once every 24 hours and revolves around the Sun once every 365+ days. So, if you could suspend yourself above the Sun's north pole and stare at the Earth, with enough resolution to make out the continents and oceans, you'd see the Earth rotate completely once every 24 hours.

Not only that, but the Sun itself rotates, and its rate varies depending on the latitude. So the "picture" would be a good deal more complicated if you could somehow "stand" on the Sun's "surface" and look at the Earth.

In the case of viewing the Moon from the Earth, the Moon not only appears to wobble left/right and up/down, but it also gets nearer and goes farther during the course of one revolution around the Earth (and noticeably so ... that is why the image of the Moon in the animation appears to be changing size) because its orbit is elliptical.

That’s a truly fine description, Jeff.  My only quarrel is with the final word “elliptical”.  The word “eccentric” would convey better meaning within the context.  Too often I read of various effects due to the elliptical (non-circular) orbits of planets or the Moon, when it’s the eccentricity (off-centeredness) that is pertinent.  The orbits of the Moon and planets are essentially circular.  A human eye looking at an accurate diagram could not detect the ellipticity.  But the Sun in the cases of the planets (Earth in the case of the Moon) is noticeably away from the center.

 

When Kepler began his analysis of the planetary orbits, he assumed (as did many others) that they were circular but eccentric (Sun off-center).  Only due to the precision of Tycho’s naked-eye data was Kepler able to determine a slight ellipticity in Mars’ orbit.  In the case of celestial mechanics, there is a correlation between eccentricity and ellipticity, but the ellipticity only becomes obvious when the eccentricity is rather large.  Mars’ orbit has fairly large eccentricity for a planet; just enough for Kepler to detect ellipticity.  With less precise data, he would have missed it.  With more precise telescopic data, perturbations may have confused him.   

chipdatajeffB:

No, if you were at the Sun looking back at Earth, the "face" of the Earth would be changing continually because it rotates once every 24 hours and revolves around the Sun once every 365+ days. So, if you could suspend yourself above the Sun's north pole and stare at the Earth, with enough resolution to make out the continents and oceans, you'd see the Earth rotate completely once every 24 hours

 

I missed that part, I can see it now...thank you!! :)

Awesome explanation with great video makes your teaching point indelible and unforgettable.  Thanks!

chipdatajeffB -

But could you please clarify your definition of tidal locking?  I have always been under the impression that tidal locking was when "the gravitational gradient makes one side of an astronomical body always face another," as per Wikipedia.  Would a true statement be that the moon IS tidally locked with the Earth, but the Earth IS NOT tidally locked with the moon?  Thanks!

bogenj:

Would a true statement be that the moon IS tidally locked with the Earth, but the Earth IS NOT tidally locked with the moon?

Yes.

Yes, Curt is correct! (good point)