Out on the pull: Why the moon always shows its face

Mar 27, 2013 by Jonti Horner, The Conversation
The moon has no choice but to show its good side. Credit: Nuranna

Technically, Pink Floyd had it wrong. The space-facing side of the moon isn't dark (except at full moon when the Earth is between the sun and the moon). Not that you'd know that, given we always see the same side of our nearest neighbour.

To understand why we only see that one side, we need to explore the relationship between the and Earth, and the forces that will slowly, but inexorably, sling the moon from our orbit into space.

As the moon orbits Earth, its raises "tidal bulges" on our planet. Both solid ground and oceans respond to this pull, causing the moon to raise land and .

At the same time, the sun also raises tides on Earth which, while noticeably weaker than those caused by the moon, adds a level of complexity to the tides we experience.

When the moon and sun are aligned correctly (either at new moon – when the moon is approximately between Earth and the sun – or at , when Earth is approximately between the moon and the sun), the tides induced by the moon and sun add together, and we get extra-high and extra-low tides. These are commonly known as "spring tides".

Equally, when the moon and sun are pulling at right angles to one another, their influence cancels out, to some extent, and we get "neap tides" – high tides are at their lowest, and low tides their highest.

Slow spin-down – the long-term influence of tides

Beyond just causing the daily ingress and egress of the ocean onto land, tides raised by the moon on Earth have another interesting effect – they are slowly causing Earth's rotation to slow.

This video is not supported by your browser at this time.
The moon’s effect on tides, explained.

As we all know, Earth spins on its axis once a day, but the moon takes almost a whole month to orbit our planet. As a result, the location of the tidal bulges from the moon move around our planet significantly more slowly than Earth's surface spins.

Friction causes the bulges to be pulled along with Earth's motion, to some degree, and they end up slightly ahead of the location directly beneath the moon.

While friction with Earth tries to pull the bulges ahead of the moon, the moon's gravity tries to keep the bulges aligned beneath it. The end result of this conflict is to cause Earth to slowly spin down, losing rotational energy to the drag from the tidal bulges.

That energy is transferred to the moon, causing it to speed up in its orbit, and therefore gradually swing away from Earth.

We're slowly growing apart – the moon's recession

The rate at which the moon is receding from Earth is relatively small, but easily measured (using the retroreflectors left on the Moon's surface by the Apollo astronauts, among other techniques).

Currently, the recession is only around 22 millimetres a year, causing one Earth day to lengthen by about 23 microseconds a year.

This video is not supported by your browser at this time.
1:1 spin-orbit resonance, or synchronous rotation, of the moon.

While that doesn't sound like much, it means the moon was once much closer to Earth, and Earth was spinning far faster than its current 24-hour rotation.

Again, these are both properties best explained by the "big splash" that created the Earth-moon system.

Synchronous rotation

Interestingly, that same tidal evolution is the reason the moon now keeps one face continually pointed towards Earth.

Earth exerts tides on the moon, just as the moon exerts tides on Earth. Since Earth is comparatively massive, the tides it raises on the moon are much greater than those raised by the moon on Earth. And those tides long ago slowed the moon's rotation so that it spins on its axis exactly once in the time it takes to orbit our planet once. This is called "1:1 spin-orbit resonance" or synchronous rotation.

As the moon recedes from Earth, its orbital period will increase, but the strength of 's tides will ensure its spin slows, so it will always continue to show the same face to our planet.

Explore further: Rosetta spacecraft sees sinkholes on comet

More information: I see the moon: Introducing our nearest neighbour: phys.org/news/2013-03-moon-nearestneighbour.html

Related Stories

Image: Super Perigee Moon

Mar 21, 2011

(PhysOrg.com) -- The full moon is seen as it rises near the Lincoln Memorial, Saturday, March 19, 2011, in Washington.

What if the earth had two moons?

Dec 28, 2011

The idea of an Earth with two moons has been a science fiction staple for decades. More recently, real possibilities of an Earth with two moon have popped up. The properties of the Moon’s far side has ...

Probing Question: Why does the Earth rotate?

Aug 06, 2007

We spend our lives on a spinning globe -- it takes only 24 hours to notice that, as night follows day and the cycle repeats. But what causes Earth to rotate on its axis?

I see the moon: Introducing our nearest neighbour

Mar 20, 2013

The moon. Our nearest neighbour. The main source of the ocean's tides, and a beacon that drives the lives of animals across the globe. And also, to date, the only object beyond Earth on which humans have ...

Space image: Earth and its Moon as seen by Voyager 1

Aug 22, 2011

This picture of a crescent-shaped Earth and Moon -- the first of its kind ever taken by a spacecraft -- was recorded Sept. 18, 1977, by NASA's Voyager 2 when it was 7.25 million miles (11.66 million kilometers) ...

Why is the harvest moon so big and orange?

Oct 01, 2010

Ever wonder why the moon sometimes looks so big and orange? Professor Emeritus John Percy of the Department of Astronomy and Astrophysics explains the mystery behind the harvest moon:

Recommended for you

Rosetta spacecraft sees sinkholes on comet

15 hours ago

The European Space Agency's Rosetta spacecraft first began orbiting comet 67P/Churyumov-Gerasimenko in August 2014. Almost immediately, scientists began to wonder about several surprisingly deep, almost perfectly ...

Me and my world: The human factor in space

18 hours ago

The world around us is defined by how we interact with it. But what if our world was out of this world? As part of NASA's One-Year Mission, researchers are studying how astronauts interact with the "world" ...

Radar guards against space debris

19 hours ago

Space debris poses a growing threat to satellites and other spacecraft, which could be damaged in the event of a collision. A new German space surveillance system, schedu- led to go into operation in 2018, will help to prevent ...

Why we need to keep adding leap seconds

21 hours ago

Today at precisely 10am Australian Eastern Standard time, something chronologically peculiar will take place: there'll be an extra second between 09:59:59 and 10:00:00.

User comments : 0

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.