How Come We Only See One Side Of The Moon?

February 22, 2021

Good day to you traveller and thank you for seeking refuge from your travels in another edition of Ask ARSE. The place where our die-hard followers probe us for answers about research and space exploration.

Rest your weary head and slide on in to a question from a fan that resides in our exclusive Australian Space Society.

"Hey y'all I was wondering while staring at the moon with my new Opportunity Rover stubby holder, why we only see the one side of the moon and it's spin is the same as ours? Is it gravity? Cheers guys and thanks for all the good stuff you do." - Alan

Thank you twofold Alan, once for the question and twice for the support in grabbing an Opportunity Rover cooler.

The short answer to the question, "... it's spin is the same as ours?" assuming we're talking Earth, is that it isn't. The moon rotates on its axis every 28 days. Earth rotates on its axis every one day - that's what a day is.

What you should be asking is, "Why does the moon rotate on its axis exactly the same rate as it revolves around the Earth?"

And the answer to that is tidal locking.
Sometimes called gravitational locking, captured rotation, or spin-orbit locking.

Tidal locking is why the moon seems to always face the same way towards Earth and why we can visible only see the same parts. It's a form of synchronisation.

See above.

If the moon didn't rotate, the arrow would always face in the same direction off into space and not at Earth. Say, to the left.

But as it rotates, locked into Earth, one side always seems to face inwards despite the fact that our planet rotates faster than the spin. Which is why we have moon cycles and nights where the moon disappears.

A tidally locked body - like our moon - takes just as long to rotate around its own axis as it does to rotate around its partner - like our planet.

Usually, only the satellite or moon is tidally locked to a larger mass or planet, but there are exceptions.

If the difference in mass and distance between the two is relatively small, each may be tidally locked to each other. Pluto and its satellite Charon are an example of this as seen below.

So how does this happen?

It comes down to (you probably guessed it) gravitational pull.

When the feeling is mutual and both bodies exact gravity on each other, they influence how the other moves.

Because the moon is not a perfect sphere, Earth gravity creates a bit of torque on it that slowed the moon's rotation. But it's revolution sped up thanks to the law of conservation of energy.

Eventually the moon hit a balance point where one rotation was equal to one revolution. Naturally, this happened to the moon first due to it being of smaller mass.

When one body reaches a state where there is no longer any change in rotation over a full orbit, it is considered to be tidally locked.

Why locked?
Because leaving this state would need energy and if you're a fan of Newton you'd know that energy has to come from somewhere. Like an object or a roaming invader planet disrupts the satellite with its own gravitational pull.

But there's still enough gravitational pull from our moon for it to influence Earth and surprisingly over a great deal of time it will happen to our planet too.

Then only one side of Earth will face the moon and the other side of Earth will NEVER see the moon!

How strange?

This will create a lot of different effects on Earth, namely the tides of our oceans.

But keep in mind we are talking over millions of years.

Image 1: Earth's tides

The moon's gravitational pull drags Earth's water away from the planet which is why we have different tides.

Namely:

High-high tide from the closest water to the moon,
Low tide at the "sides" of the moon's pull
Low-high tide is residual water on the furthest side from the moon. This is water that is directly pulled by the moon yet can't be dragged through the mass of the Earth.

We get some tides that are much higher than usual like Spring Tides which are high-high tides with the added gravitational pull of the sun as shown below.

Image 2: Spring Tides

Tides that should be high but are lower than usual are called Neap Tides.

Neap Tides occur twice a month during the first and last quarters of the moon when our sun and moon are at a right angle to one another as shown. Without the sun's influence this would be regular high tide. But with the added pull of the sun on a 90 degree angle, the sun and moon compete for a high tide that results in a stalemate and a lower high tide than usual.

Image 3: Neap Tides

So what will happen when only one side of Earth will face the moon in a few million years?

The side locked to the moon will have its water constantly pulled towards the moon and the new average will be a high tide. The other side will be locked into a low-high tide and the sides will much lower on average. Even its larger Spring Tides will resemble the regular high tides we have today.

See Image 1 above.
When the Earth and moon are locked much like Pluto and Charon, our world will be statically locked into those tides formations - save for the influences of the sun.

Interestingly, there are situations where a larger body and its satellite are not entirely synchronous, like Mercury and the sun.

Astronomers thought for years that Mercury was tidally locked with the sun, rotating once for each orbit so that the same side of the planet faced the sun in an eternal daytime/nighttime scenario.

Much like how the moon always faces us, except in this case Earth is the sun and the moon is Mercury.

But in 1965, radar observations proved that Mercury had a 3:2 spin to orbit resonance with the sun. Meaning Mercury spins 3 times for every 2 rotations around the sun.

Why?

Mercury's orbit is eccentric, meaning its orbital speed around the sun varies as its path gets closer and further from the sun, but reliably so. Since its spin rate is constant but the speed of orbit changes, it's impossible for one side of Mercury to always be facing the sun.

Check its whacky ways below.