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<img loading="lazy" src="https://cdn.kahimyang.com/resources-7/nearfarsidemoon.webp" alt="Why We Never See the Far Side of the Moon" />
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(The moon's near side (left) is covered with dark splotches of lunar maria that look like a man's face when seen from Earth. The moon's far side (right), with its 

many craters and elevated topography, looks quite different. (Photos: NASA))
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Scientists now think the moon was <a href="http://en.wikipedia.org/wiki/Giant_impact_hypothesis" target="_blank" class="news_link" title="Giant impact hypothesis">formed from a very large impact</a>, according to Dr. Oded Aharonson, professor of planetary science at the California Institute of 

Technology (Caltech).


Something hit our planet long ago, ripping a big chunk of Earth out. Some of that original impact material collected together under its own gravity and orbit to 

form the moon.
When that happened, according to Aharonson, the moon was closer to Earth and it spun a lot faster.


If anyone was on Earth at the time, they’d have looked up and seen different sides of the moon on different nights, rather than the same static view we see 

today.
So what caused the moon’s rotation to slow down?


Aharonson points to a process called tidal dissipation. The moon, as it orbits our planet, raises <a href="http://aom.giss.nasa.gov/tides.html" 

class="news_link" title="Tides" target="_blank">tides</a> on Earth.



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<img loading="lazy" src="https://cdn.kahimyang.com/resources-7/main_grail.webp" alt="A closer look at the South pole of the far side of the moon" />
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(A closer look at the South pole of the far side of the moon as seen from the GRAIL mission’s Ebb spacecraft. (Photo: NASA/JPL-Caltech))
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At the same time, Earth also raised tides on the moon, causing it to bulge and change its shape slightly. This also caused the moon to dissipate energy in the 

form of heat. This process took energy from the rotation of the moon which caused its spin to slow down.


Over the years, the moon has fallen into a synchronous orbit with Earth.


“Like a ballroom dancer, the moon spins around the Earth at exactly the same rate as it spins around itself,” Aharonson says, “so it always maintains the same 

face towards its companion, Earth.”


Although the moon’s rotation speed has stabilized and is no longer slowing down, its orbit is <a href="http://curious.astro.cornell.edu/question.php?number=124" 

target="_blank" class="news_link" title="Is the Moon moving away from the Earth? When was this discovered?">evolving outward from the Earth</a>, slowly expanding 

its circuit around our planet.


Right now, it takes the moon around <a href="http://en.wikipedia.org/wiki/Orbit_of_the_Moon" target="_blank" class="news_link" title="Moon's orbit">30 days to 

orbit the Earth</a>, but in the future, says Aharonson, both the moon and Earth will eventually synchronize to each other. The consequences of this 

synchronization, according to Aharonson, are that Earth would no longer experience changing tides and we would no longer observe the moon go through its monthly 

phases.



That’s when we would really see the exact same thing night after night when we look at the moon.


The process of tidal dissipation has also affected the shape of the moon. Looking at a big bright full moon, we see a perfectly round sphere. But it’s not, 

according to Aharonson, who says it is actually an ellipsoid, shaped a little like an American football. The moon has three axes and each one of them is a 

little different.


Why we only see the near side of the moon was not due to random chance, but depended on the two sides of the moon being asymmetrical.


The side facing us is filled with large, dark, <a href="http://en.wikipedia.org/wiki/Lunar_mare" target="_blank" class="news_link" title="Basaltic plains">basaltic plains</a> formed by <a href="http://volcano.oregonstate.edu/oldroot/volcanoes/planet_volcano/lunar/Overview.html" target="_blank" class="news_link" title="Volcanism on the Moon">ancient volcanic eruptions</a> which are dense as well as low and closer to the moon’s 

center of mass.


And, with all due respect to the Pink Floyd album “Dark Side of the Moon,” the other side of the moon isn’t really necessarily dark. When it’s illuminated by 

the sun, it can appear much brighter than the side we see.


Its moonscape is quite different, too. It is filled with mountains rather than the basaltic maria and basins of the near side.


If the moon were flipped around, giving us a view of the far side, Aharonson says we’d notice a little bit more mass and, because of its high mountains, it 

appear to be closer to Earth.


Scientists still don’t know why the moon is so asymmetrical; why one side of the moon is so different than the other.



Source


<ul class="article-sources">
<li>
<a target="_blank" class="news_link" title"Source" href="http://www.voanews.com">
Voice of America News
</a>
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</ul>