Why do we only see one side of the moon?

Lunar libration.  Image credit : wikimedia user  Tomruen .

Lunar libration. Image credit: wikimedia user Tomruen.

The simple answer (and one that you’ve probably heard before) is that we only see one side of the moon because the moon rotates around the Earth at the exact same speed as it rotates around its own axis, so that the same side of the moon is constantly facing the surface of the earth.  This means that one full ‘day’ of the moon (meaning the length of time it takes for the moon to rotate around itself once) is about 4 weeks long.  If the moon didn’t rotate at all, we would see all of its sides; the only way for us to see such a constant face of the moon is if it’s also rotating. There’s a great visualization of this below.

Tidal locking results in the Moon rotating about its axis in about the same time it takes to orbit the Earth. Except for libration effects, this results in it keeping the same face turned towards the Earth, as seen in the figure on the left. (The Moon is shown in polar view, and is not drawn to scale.) If the Moon didn't spin at all, then it would alternately show its near and far sides to the Earth while moving around our planet in orbit, as shown in the figure on the right.  Image credit : Wikimedia user  Stigmatella aurantiaca

Tidal locking results in the Moon rotating about its axis in about the same time it takes to orbit the Earth. Except for libration effects, this results in it keeping the same face turned towards the Earth, as seen in the figure on the left. (The Moon is shown in polar view, and is not drawn to scale.) If the Moon didn't spin at all, then it would alternately show its near and far sides to the Earth while moving around our planet in orbit, as shown in the figure on the right. Image credit: Wikimedia user Stigmatella aurantiaca

If you watch the way the moon spins (or doesn’t), you can see that only the left side has a consistent side facing the surface of our planet (which, we must note, is not even a little bit to scale here).

However, the underlying reason why the moon rotates at this exact speed, forcing us to only see a single side of it, is because the moon has been tidally locked to the earth.  Tidal locking is a stable configuration, and relatively easy to get to, given enough time, so many of our solar system’s moons are found to be tidally locked, including the dwarf planet Pluto and its largest moon Charon, which are both tidally locked to each other.

The “lock” part of this name refers to the way that an object - like the Moon - is apparently fixed in position, with one side always facing the other object.  Any object which is found to be tidally locked will always have one side of itself facing the surface of the planet it’s orbiting.  The amount of time it takes to orbit around the planet will vary from object to object (Phobos, one of the moons of Mars, is tidally locked and orbits Mars every 8 hours - way faster than our Moon), but as long as the object is tidally locked, the rotation will match the length of time it takes to orbit.

However, it’s the “tidal” part of the tidal locking that gives us the real key to why tidal locking happens at all.

We’re most familiar with tides as the effect of our oceans rising and falling due to the position of the moon.  The Moon’s gravity pulls on the earth, and the water on the surface of the Earth closest to the moon responds to that pull by elongating towards the moon. The water on other parts of the earth feels the Moon’s gravitational pull as weaker, with the water on the opposite side of the earth feeling the weakest pull. However, these tidal forces also have another effect - they resist rotation.

The Moon was almost certainly not tidally locked when it first formed - at that time, it would have rotated at a faster speed, which meant that had any observer been on the early Earth, they could have seen all sides of the moon as it spun.  However, the gravitational pull from the Earth - which like the tides due to the Moon, pulls on the side of the Moon closest to the earth more than the far side, resisted this faster rotation. This resistance due to the gravitational pull of the Earth gradually slowed down the faster spin of the Moon until the Moon was no longer rotating faster than it was orbiting.  Once the Moon’s rotation had slowed so much that a single face was always facing the surface of the Earth, it had officially been tidally locked, and has stayed in this configuration ever since.

The Moon also has the same influence on the Earth, but since the Moon is so much less massive than the Earth, this resistance to rotation takes a much longer time to impact the Earth's spin.  However, it’s still a measurable effect! The Moon is slowing down the rotation of the Earth by about 15 microseconds every year, gradually lengthening our days.


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Why is it called an exoplanet?

Dear Astroquizzical: why is it called an EXOplanet? What’s the opposite of an exoplanet?

We haven’t had a naming question in a while!

An exoplanet is also called an “extrasolar planet” - both terms simply mean a planet which is in orbit around a star which is not ours.

The ‘exo’ part comes from the same root as an “exoskeleton”, “exothermic” or “exotic”. The first tells us that an animal’s skeleton is an ‘outer’ skeleton such as those of spiders and insects, not an interior skeleton like mammals have. If you’re a chemistry person, you’ll recognize “exothermic” as a sign that a chemical reaction produces more heat than it consumes. Put another way, it’s dumping heat “outside” of the reaction. “Exotic” simply means that it comes “from outside” where you’re from.

The opposite of “exo-“ is “endo-“, which means “internal” instead of “external”. While we don’t tend to use the word “endoskeleton” to mean an internal skeleton, we do use “endothermic” to mean something that must suck energy out of its environment.

An exoplanet is simply an “external” planet, in that it isn’t in our solar system. So the opposite of that would be something internal to our solar system, which also is not a planet! Any of the many moons, comets, and asteroids in our solar system (or Pluto) would qualify.

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What does Pluto have on it?

Pluto is very like an extremely large, dirty snowball. Or like a snowy dirtball, depending on your perspective - Pluto’s about 50-50 rock and ice.

One thing Pluto does have in abundance is rather a lot of moons! Its most famous companion is Charon, which is about 10 times less massive than Pluto, but considering that our moon is only about 1% of the mass of the Earth, this is a rather large moon, relatively speaking. Pluto also has four other moons, named Nix, Hydra, Kerberos, and Styx. Kerberos and Styx were officially named just a few weeks ago - there was an online poll to suggest names for them, and these were among the most popular suggestions. (Vulcan was also an option, but the underworld theme prevailed.)

The only images that we have of Pluto are from telescopes both on the ground on Earth and in orbit. We base most of our information on what Pluto is based on based on how bright it is - the brightness tells us something about how much of the Sun’s light is reflected back at us. The more reflective the object, the more icy it has to be. If you look at the brightness of an object in various different wavelengths of light, you can get a sense of what color the object is, and whether there are irregularities in the surface. It turns out that Pluto has variations both in brightness and in color, which makes it a very interesting snowy dirtball. One would normally expect it to be roughly the same all over. It’s tricky to make these measurements from so far away, and ideally we’d like to send a probe to Pluto and take a bunch of pictures and measurements from close up.

Fortunately, in 2006, we launched a satellite to do just that - New Horizons. New Horizons is scheduled to fly past Pluto in 2015, hopefully expertly navigating all of Pluto’s moons and any other objects that might be hanging around near Pluto in 2015. New Horizons is packed with cameras and other instruments, so it should - if all goes well - be able to provide much more detailed maps of Pluto than we’ve ever been able to obtain before, along with measurements of the atmosphere of Pluto, and any rapid changes that may occur on the surface.

So keep your fingers crossed for New Horizons, and soon we’ll have a much better answer to what’s on Pluto than we do now!

Have your own question? Something here not make sense? Feel free to ask!

Why couldn't Pluto have stayed a planet?

For an unassuming little ball of ice several billion miles away from the sun, a lot of people were strongly invested in Pluto’s status as a planet. It was the end of the mnemonic to remember the order of the planets - My Very Excellent Mother Just Served Us Nine Pizzas - and we had a lovable Disney animal named after it. It’s no real wonder that people were upset.

But the truth was, Pluto had never fit in very well with the other 8 planets. The orbit of Pluto, unlike the rest, which all lay in a very tightly defined plane, was wildly askew. It’s tilted by about 20 degrees relative to the rest of the planets. Also unlike the rest of the planets, which travel around the sun in almost perfect circles, Pluto’s orbit was extremely elongated. And it was a tiny planet. Pluto is big enough to have compressed itself into being a sphere (instead of the lighter and irregularly shaped asteroids), but it had a moon that was almost as big as itself. None of the inner 8 planets behave this way.

But we were more or less okay with ignoring these problems with Pluto and leaving it as a planet, since up until that point we hadn’t found any other objects of a similar size in the solar system; everything else was much smaller. It made sense to leave it as a planet if it was a unique object. But this is where Pluto started running into problems - we began to realize that Pluto wasn’t unique. Our ability to detect similarly sized objects got a lot better, and all of a sudden we had other objects that were about the same size as Pluto. Most notably, there was Eris, which was even further out from the sun than Pluto, and bigger than Pluto.

Now a decision had to be made.  Either we allowed Eris in to the solar system as the 10th planet (if Pluto was a planet, Eris surely was), or we would have to come up with a better definition of what a planet was.  The problem with including Eris and Pluto as planets was primarily that Eris’ discovery was proof that Pluto was not unique, it was merely the first in a class of objects we’d been unable to detect so far.  That lack of uniqueness meant that we’d be constantly adding new planets to the list as more of them were discovered, and we were guessing that there would be a lot of these objects out there.

So the International Astronomical Union decided to impose a set of 3 criteria that any object in the solar system must pass in order to qualify as a planet.  To gross public dismay, Pluto no longer qualified.  The criteria were the following: a planet must orbit the sun, and not another, smaller object. (In other words, it can’t be a moon.)  It must also be sufficiently massive to have compressed itself into a sphere, and finally, it should have cleared the area around its orbit of other objects.  Pluto failed the last criterion, which meant it went into the newly minted “dwarf planet” category. This dwarf planet classification distinguished them from the irregularly shaped objects that orbit the sun (like the asteroids), but swept them out of the main ‘planet’ category.

Ultimately, the decision to reclassify Pluto was a choice to make our definitions more consistent, which in the long term means there are fewer revisions of our textbooks. Our understanding of the solar system will gradually become more and more complex as time goes on, so it makes sense to let our classifications also reflect the complexity of nature. In the mean time, our very excellent mothers will just have to serve us nachos or naan instead of pizzas.

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