What is Tidal Locking?

The Moon is tidally locked to the Earth, which means that it always shows one face to our planet. In fact, this is the case for most the large moons in the Solar System. What’s the process going on to make this happen?

Just look at the Moon, isn’t it beautiful? Take out a nice pair of binoculars, or a small telescope tonight and you’ll be able to see huge craters and ancient lava plains. Look again tomorrow, and you’ll be able to see… the exact same things. As you know, our modest Moon only shows us one face. Ever.

If you could look at the Moon orbiting the Earth from above, you’d see that it orbits once on its axis exactly as long as it takes to orbit once around our planet. It’s always turning, showing us exactly the same face. What’s it hiding?

The Moon isn’t the only place in the Solar System where this happens. All major moons of Jupiter and Saturn show the same face to their parent. Pluto and Charon are even stranger, the two worlds are locked, facing one another for all eternity. Astronomers call this tidal locking, and happens because of the gravitational interaction between worlds.

As you’re aware, the Moon is pulling at the Earth, causing the tides. In fact, the pull of the Moon is so strong that the ground itself rises up 30 cm, about a foot, as it passes by.

It’s even more powerful on the Moon. The gravity from the Earth distorts the Moon into an oblong shape. The sides pointed towards and away from the Earth bulge outward, while the others are pulled inward to compensate. It makes the Moon football shaped.

It’s no big deal now, but in the ancient past, shortly after its formation, the Moon was spinning rapidly. This meant that the part of the Moon bulged towards us was changing constantly, like water tides on Earth.

Vast amounts of rock need to shift and change shape to bulge towards the Earth and then settle down again, and this takes time. The position of the bulges on the Moon were always a little out of alignment with the pull of gravity of the Earth.

These bulges acted like handles that the Earth’s gravity could grab onto, and torque it back into place. Over time, the Earth’s gravity slowed down the rotation speed of the Moon until it stopped, forever.

Size comparison of all the Solar Systems moons. Credit: The Planetary Society
Size comparison of all the Solar Systems moons. Credit: The Planetary Society

This same process happened on all the large moons in the Solar System.
Because of its smaller mass, our Moon became tidally locked to the Earth billions of years ago. Now the process is continuing to make the Earth tidally locked to the Moon as well.

In the distant distant future, the Moon will stop moving in the sky, and hang motionless, visible from only half the Earth.

How distant? In about 50 billion years, long after the Sun has died, the Earth and the Moon will finally be tidally locked to each other, just like Romeo and Juliet, Fry and Leela, Pluto and Charon. The force of gravity is a powerful thing. Powerful enough to stop a moon in its tracks.

Did you have any other questions about the Moon? Post your suggestions in the comments and we’d be glad to make more videos and dig deeper!

You’ve Never Seen the Phases of the Moon from This Perspective: The Far Side

Sometimes, it seems to be a cosmic misfortune that we only get to view the universe from a singular vantage point.

Take the example of our single natural satellite. As the Moon waxes and wanes through its cycle of phases,  we see the familiar face of the lunar nearside. This holds true from the day we’re born until the day we die. The Romans and Paleolithic man saw that same face, and until less than a century ago, it was anyone’s guess as to just what was on the other side.

Enter the Space Age and the possibility to finally get a peek at the universe from different perspective via our robotic ambassadors. This week, the folks over at NASA’s Scientific Visualization Studio released a unique video simulation that utilized data from NASA’s Lunar Reconnaissance Orbiter to give us a view unseen from Earth. This perspective shows just what the phases of the Moon would look like from the vantage point of the lunar farside:

You can see the Moon going through the synodic 29.5 day period a familiar phases, albeit with an unfamiliar face. Note that the Sun zips by, as the lunar farside wanes towards New. And in the background, the Earth can be seen, presenting an identical phase and tracing out a lazy figure eight as it appears and disappears behind the lunar limb.

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The lunar nearside: A familiar view. Credit: Stephen Rahn.

What’s with the lunar-planetary game of peek-a-boo? Well, the point of view for the video assumes that your looking at down at the lunar farside from a stationary point above the Moon. Note that the disk of the Moon stays fixed in place. The Moon actually ‘rocks’ or nods back and forth and side-to-side in motions referred to as libration and nutation, and you’re seeing these expressed via the motion of the Earth in the video.  This assures that we actually get a peek over the lunar limb and see a foreshadowed extra bit of the lunar farside, with grand 59% of the lunar surface visible from the Earth. Such is the wacky motion of our Moon, which gave early astronomers an excellent crash course in celestial mechanics 101.

Now, to dispel some commonly overheard lunar myths:

Myth #1: The moon doesn’t rotate. Yes, it’s tidally locked from our perspective, meaning that it keeps one face turned Earthward. But it does turn on its axis in lockstep as it does so once every 27.3 days, known as a sidereal month.

Myth #2:  The Farside vs. the Darkside. (Cue Pink Floyd) We do in fact see the dark or nighttime side of the Moon just as much as the daytime side. Despite popular culture, the farside is only synonymous with the darkside of the Moon during Full phase.

Humanity got its first glimpse of the lunar farside in 1959, when the Soviet Union’s Luna 3 spacecraft looked back as it flew past the Moon and beamed us the first blurry image. The Russians got there first, which is why the lunar farside now possesses names for features such as the “Mare Moscoviense”.

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Our evolving view of the lunar farside over 60 years… Credit: NASA/LRO.

Think we’ve explored the Moon? Thus far, no mission – crewed or otherwise – has landed on the lunar farside. The Apollo missions were restricted to nearside landing sites at low latitudes with direct line of sight communication with the Earth. The same goes for the lunar poles: the Moon is still a place begging for further exploration.

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China’s Chang’e 5 T1 pathfinder mission looks back at the Earth and the lunar farside. Credit: Xinhua/SASTIND.

Why go to the lunar farside? Well, it would be a great place to do some radio astronomy, as you have the bulk of the Moon behind you to shield your sensitive searches from the now radio noisy Earth. Sure, the dilemmas of living on the lunar farside might forever outweigh the benefits, and abrasive lunar dust will definitely be a challenge to lunar living… perhaps an orbiting radio astronomy observatory in a Lissajous orbit at the L2 point would be a better bet?

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An artist’s conception of LRO in lunar orbit. Credit: NASA/LRO.

And exploration of the Moon continues. Earlier this week, the LRO team released a finding suggesting that surface hydrogen may be more abundant on the poleward facing slopes of craters that litter the lunar south pole region. Locating caches of lunar ice in permanently shadowed craters will be key to a ‘living off of the land’ approach for future lunar colonists… and then there’s the idea to harvest helium-3 for nuclear fusion (remember the movie Moon?) that’s still science fiction… for now.

Perhaps the Moonbase Alpha of Space: 1999 never came to pass… but there’s always 2029!

First Quarter Moon

Flying Across the Moon

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The first quarter moon is actually the third phase of the moon each cycle. In the Northern Hemisphere during this phase, the right hand 50% of the moon is visible during the afternoon and the early part of the night. In the Southern Hemisphere the left hand 50% of the moon can be seen. This lunar phase follows the new moon and the waxing crescent.

A lunar phase is the appearance of an illuminated portion of the moon as seen by an observer. For this article the observer is always on Earth. The lunar phases vary in a definite cycle as the moon orbits the Earth. The phases change based on the changing relative positions of the Earth, moon, and Sun. Half of the moon’s surface is always illuminated by the Sun, but the portion of the illuminated hemisphere that is visible to an observer can vary from 100%(full moon) to 0%(new moon). The only exception is during a lunar eclipse. The boundary between the light and dark portions of the moon is called the terminator.

There are 8 moon phases. These phases are: new moon, waxing crescent, first quarter moon, waxing gibbous, full moon, waning gibbous, last quarter moon, and waning crescent. The phases progress in the same manner each month. Earlier, it was mentioned that the lunar phase depends on the position of the Earth, moon, and Sun. During the new moon the Earth and Sun are on the opposite side of the moon. During the full moon the Earth and Sun are on the same sides of the Moon. The occasions when the Earth, Sun, and moon are in a straight line(new and full moon) are called syzygies.

When the moon passes between Earth and the Sun during a new moon, you might think that its shadow would cause a solar eclipse. On the other hand, you might think that during a full moon the Earth’s shadow would cause a lunar eclipse. The plane of the moon’s orbit around the Earth is tilted by about five degrees compared to the plane of Earth’s orbit around the Sun(called the ecliptic plane). This tilt prevents monthly eclipses. An eclipse can only occur when the moon is either new or full, but it also has to be positioned near the intersection of the Earth’s orbital plane about the Sun and the Moon’s orbit plane about the Earth, so there are between four and seven eclipses in a calendar year.

The first quarter moon is only one of eight lunar phases. You should research them all for a better understanding of the Earth/Moon system.

We have written many articles about the phases of the moon for Universe Today. Here’s an article about the 8 phases of the moon, and here’s an article about the moon phases for 2010.

If you’d like more info on the Moon, check out NASA’s Solar System Exploration Guide on the Moon, and here’s a link to NASA’s Lunar and Planetary Science page.

We’ve also recorded an entire episode of Astronomy Cast all about the Moon. Listen here, Episode 113: The Moon, Part 1.

References:
http://spaceplace.nasa.gov/en/kids/phonedrmarc/2004_march.shtml
http://starchild.gsfc.nasa.gov/docs/StarChild/questions/question3.html