Why do we See One Side of the Moon?

Phasa of the moon
Moon Phases during a Lunar Cycle

“Why do we see one side of the moon?  Does the moon turn round, because I always see the same patterns?”  Kam asked me these questions whilst we were looking at the included image showing different phases of the moon.

I don’t know if these are questions that small children ask.  I don’t remember asking these sorts of questions.

I’m not sure that it would have occurred to me to think that the moon has different patterns.

To me, the moon was the moon.  I do remember looking at the stars, but perhaps I accepted information without being as inquisitive.

 What happens to the moon?

We know that the moon orbits Earth and it takes about 28 days to complete a full orbit.  Throughout the orbit we see changing phases of the moon.  These are dependant upon the positions of the Earth, moon and sun.  If the moon spins or rotates on itself , why don’t we see different patterns on it, ie why do we always see the same face of the moon?

The moon does rotate around itself on its own axis.  It takes about 28 days to do this, the same length of time the moon takes to rotate around Earth.  This is known as Tidal Locking.  Tidal Locking is the reason why the same side of the moon always faces Earth.

Tidal locking happens when the time it takes for an object to complete an orbit matches the time it takes for that object to rotate once on its own axis.

Why does Tidal Locking occur?

It is because of gravity.  When we throw a ball into the air, gravity pulls it back down to Earth.  When we stand up, the Earth pulls down on us. We don’t float away, our feet stay on the ground.  

Our feet are closer to Earth’s centre than our head.  They are “our height” closer.  The Earth pulls with a slightly larger gravitational force on our feet than our head.  This small difference is called a tidal force.

When an object such as a Moon is close enough to a planet, these gravitational tidal forces cause the Moon to spin at the same rate that it revolves around the planet.  This is  tidal locking – why we always see the same side of the Moon

Other examples of tidal locking

There are plenty of examples of tidal locking in our solar system and universe.

– Mars’ moons, Phobos and Deimos, always have the same side facing Mars.

– Jupiter’s, Saturn’s, Uranus’, and Neptune’s moons are locked to their respective planets.

– Pluto and its largest moon, Charon, are locked to each other. Pluto sees one face of Charon and Charon sees one face of Pluto.

– It is thought an extra-solar planet, Gliese 581 c, is tidally locked to its Sun.

– Some asteroids that are bound to each other.

A note to you and me.

Thanks for reading, I hope you have enjoyed my thoughts about the moon’s one face!  I’ve written it so I have a general understanding to pass on to Kam when he next asks the question.

If I have misunderstood, plainly got it wrong, you have knowledge which will help us or ou just like it – please leave a message. We’d love to hear your thoughts. X

 

  • Animated graphic demo on wiki’s tidal locking section.
  • I’ve found a wonderful practical, crafty demonstration for us to make.  We’ll post the results when we do.  It will show us moon phases.  I’m trying to think of a way we can include tidal locking as well.  Kam, remind me.
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