Tuesday, October 18, 2011

Why Does the Same Side of the Moon Always Face Us?

Many people have observed but are not totally aware of the fact that the Moon does not rotate on its axis while it orbits earth.  The same side always faces Earth.  Some call the other side the "dark side of the Moon"  which is an inaccurate term because all sides of the Moon get light and dark during the Moon's orbit around earth.  It should be called the "far side of the Moon".  At any given moment half of the Moon is in light and half is in darkness, just like Earth, eclipses not withstanding.  But throughout our recorded history, the familiar side of the Moon with the dark "seas" have always been facing Earth.  It doesn't change, it's locked that way.

But it's just too neat to be coincidental.  Knowing that there is continual variation in rotations and orbits of planets (see Defining Time), it occurred to me that there must be a force at work so I did some research.  (This is not the short answer.)  Earth and nearly every celestial body rotates on its own axis as it orbits something else.  Why doesn't the Moon do this?  Even with the slightest wobble or force, the Moon would shift a little bit and over time we'd be looking at another side.  But the constant struck me:  for all of recorded time, our view of the Moon has not changed.  What's going on?

It is believed that billions of years ago, the Moon did rotate on its axis.  So what changed that?  The answer is "Tidal Friction" slowed the Moon down. The earth's gravity pulled at the Moon and caused a bulge in the Moon  just like our ocean's bulge from the Moon's gravity creating tides (see Ocean Tides).  However, the Moon's bulge is solid, unlike the temporary bulges of ever changing oceans.  The bulge made the Moon go out of balance like your car's tire being out of balance.  This bulge generates heat through the friction of rock moving and shifting from the gravitational pull of Earth. Over time, this movement literally siphons energy away from the rotational momentum of the Moon, acting as a brake on the Moon's rotation.  Eventually, the Moon slowed down to a stop so that the same side always faced toward the earth.  Think of it as the Moon has a permanent high tide and the earth is pulling on that. Now, minor forces may try to nudge or affect the Moon's rotation, but the earth's gravity is stronger and has locked the "bulged" side of the Moon to face us.

The Moon gravity causes bulges in the earth as well, but we have mostly a liquid surface that rises and falls to absorb the gravitation force of the Moon so there is no permanent bulge.  The Moon's gravity is also far weaker but its "tidal effect" of constantly raising and lowering the tides helps in slowing the earth's rotation as well.  In 100 million years (a short period in geological time), the earth's day will be 1 hour longer.  Consider the energy of the Moon's gravity that it takes to lift the oceans of the world several feet twice a day.  Now consider the size of earth and you can imagine the force exerted on the Moon to stop it from rotating.

The Sun has this same effect on the planet Mercury the closest planet to the sun, has the greatest solar gravity forces than any other planet, has little gravity of its own and guess what, always faces the same side toward the sun.  The constant is gravity, it's the law.

1 comment:

  1. John,
    Interesting info, as ususal. As we discussed by email today, some slight confusion regarding rotational spin vs. orbital rotation, but all is good.

    Reminds me that on Pink Floyd's 'Dark Side of the Moon", one of the comments you hear in the background is, "There is no dark side of the moon, really. As a matter of fact, it's all dark".

    -- Steve B.