On a cosmic scale, everything seems to be rotating. It’s not just our shared home planet Earth that enjoys “getting dizzy wit it”; there are tons of astronomical objects dancing around up there in space like it’s going out of style.
Every planet spins, the stars of galaxies seem to whir around a central point, and even asteroids spin… everyone is doing it!
But, my astronomical allies, this poses something of a conundrum when we consider moons, or more specifically, our moon, the moon.
Does our magical silvery button in the sky also rotate, or is it the one outlier in an unthinkably large cosmic system composed of spinning entities?
Why It’s An Interesting Question
Your first thought may be to say, well yes, of course the moon spins… everything does! But don’t be too hasty. Think a little deeper. Think perhaps of the Pink Floyd album, The Dark Side Of The Moon.
The reason this term, “the dark side of the moon” came about is simply that there is one half, or roughly 41% to be precise, that we can never see here on Earth.
The unseen portion of the moon isn’t necessarily dark, but as far as humanity is concerned, it is. Perhaps dark in our consciousness would be more of an astute way to put it.
Now, the reason we don’t see this side of the moon is, of course, because the same side is always facing Earth, which seems rather strange. If the moon was spinning along with everything else, and if you spent a decent amount of time observing it, you’d eventually expect to see the full picture, right?
So, does this mean the moon isn’t spinning at all? Not exactly! Read on to solve this moonly mystery!
Is The Moon Spinning On Its Axis?
Let’s be direct and answer the question first before elaborating. Place your bets now! The answer to our lunar conundrum is… the moon does indeed spin!
Yes, the moon rotates on its axis. In fact, it turns once every 27 days. This means that the entire surface of the moon will rotate by approximately 1 degree over a period of 27 days. That’s pretty slow compared to other celestial bodies, but it’s still quite fast for us Earthlings.
On a side note, the moon does not rotate around the Earth, but instead revolves around the center of mass of the Earth-Moon system (this is called the barycenter).
So while the moon spins, its rotation is perpendicular to the direction of motion, meaning that the top of the moon moves towards the Earth and the bottom of the moon moves away from the Earth.
This is why we have tides on Earth, and why the moon causes them.
But if the moon is rotating on its axis after all, that begs the question, why can we only ever see 59% of its surface from Earth?
Well, the truth of the matter is actually far more astounding than the idea that the moon is the only stellar object at the cosmic party that doesn’t feel like dancing.
Why Do We Only See One Side Of The Moon?
If the moon were rotating around the Earth, then we should be able to see both sides of the moon at any given moment.
However, due to the nature of the moon’s orbit, the moon appears to move across the face of the Earth, so that we only ever see one side of the moon. This is, as we picked up on earlier, what gives rise to the phrase, “the dark half of the moon”.
The reason we never see that fabled other half is that, although the moon is spinning, it’s always spinning in a constant with Earth.
What I mean by this is that the mechanics of Earth’s orbit, Earth’s rotation, the moon’s orbit, and the moon’s rotation are all such that, as everything wheels away, only that one side of the moon faces the Earth — nuts, right?
It rotates at roughly the same speed that it orbits the Earth-moon system, meaning that no matter where the Earth is in its rotation or orbit, the moon will always be showing us that one sunny side. The really bonkers thing is if things were any different, we would eventually see the moon in its entirety.
For instance, if the moon moved faster than it does, it would slowly wheel in the sky, gradually revealing different surfaces as days went by.
In fact, millions of years back, the moon did spin quite a bit faster than it does today, but the effect of Earth’s gravitational pull has slowed it to its current pace.
That’s not to say the moon will eventually stop turning, as it’s now in what’s known as a “tidally locked” state, which means its speed is essentially a constant.
On the flipside, if, like we considered at the start of this article, the moon wasn’t rotating on its axis at all, all its surface would be revealed to the Earth at a certain point of its orbit.
It’s just in this one bizarre scenario, the one actually occurring as we speak, that the moon only shows a single side in the sky.
Is The Dark Side Of The Moon Really Dark?
Being that we’ve established that the moon does indeed rotate on its axis, and it was never a question that it has a regular orbit, we can now officially declare the dark side of the moon spiel, well… spiel.
Being that the moon is turning, every part of its surface will revel in sunlight at a certain point in its cosmic meanderings.
However, momentarily, the sides that aren’t in direct sunlight would indeed be dark, like… really dark!
Do All Moons Rotate?
As far as we know, every moon rotates on its axis relative to the finer points of its orbital system, which isn’t too shocking.
But what really is quite amazing is that, like our moon, every other known moon in the solar system also exhibits the same uncanny rotation, showing just one side to the central object it’s orbiting.
That’s right, folks; from Lo to Titan, all moons constantly face inwards towards the object of their orbit, spinning at precisely the right rate to keep the rest of their surfaces a mysterious secret.
Why is this happening? Well, it’s not actually as mysterious as it seems. Remember earlier when I mentioned that the moon is tidally locked to the Earth? The same is true for every other known moon, and this is what causes their uniform behavior.
This is simply the way the gravitational force of a central object modifies the movement of its orbitals.
Why Can We See 59% Of The Moon?
The moon doesn’t follow a perfectly circular path around Earth, rather, it has an elliptical orbit, meaning its route follows an oval-shaped course.
This provides us with some pretty decent angles of observation as it makes its way around the Earth, allowing us to view slightly more than just 50% of the surface.
That extra 9% we’re seeing is actually a glimpse of very small areas beyond the moon’s average western and eastern horizons.
And there you have it — the moon is constantly rotating on its axis, but due to tidal lock, we only ever see one side, or, if you want to be exact, 59% of its surface.
The rest of it will always be facing away from the Earth, and all moons in our solar system exhibit the same tidally locked behavior.
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