The moon has always fascinated me. It’s always made me feel so connected to those who are close in heart but not in distance. It seems so familiar every time I see it. This may lead to us wondering if we always see the same side of the Moon.
Each night, we can walk outside and see the same moon hanging high above us in the sky and realize that we’re closer than we might think.
But during one of my nights gazing up at that giant rock in the sky, I wondered if we always see the same side of the moon. And the answer is yes, we do.
I’m sure you know phrases such as the dark side of the moon or heard it be referred to as a familiar face. These all stem from the fact that the moon always faces us in the same way.
In fact, as long as humans have been on Earth, they’ve always been staring up at the same side of the moon, smiling back down at us.
This then leaves you with a whole host of new questions. It’s common knowledge that Earth spins on its axis as does the moon.
So, how do we end up seeing the same side of the moon regardless? Continue reading to find out!
Why We See The Same Side Of The Moon
Okay, so let’s imagine that we can all float around in space. How cool would that be to get a great view of the moon and Earth (Also check out How Many Light Years Away Is The Moon?).
What you would see would look a little like this: The Earth is spinning on its axis, and the moon orbiting around it.
The moon itself will also spin around too, but here’s the catch.
The moon rotates only once every 27.3 days, which is exactly how long it takes for the moon to complete a single orbit around the Earth.
What this means for us humans is that from our viewpoint we always see the same lunar hemisphere consistently. This is at least mainly true.
Sometimes we will see ever so slightly more than just half of the moon’s surface. This is because of something known as lunar libration. Libration is a wobbling effect that shows us just a few extra bits of the surface here and there.
Interestingly, if the moon were just a little faster or a tad slower, we’d see the entire thing.
We see only one side of the Moon because it is tidally locked to the Earth.
How The Moon Became Tidally Locked To Our Planet
Did you know that a long time ago, around 4.5 billion years ago, the moon orbited quicker than it does today? Well, it’s true.
Earth’s gravity causes a rocky tidal bump in the moon.
Also, the moon isn’t the perfect sphere that we believe it to be. It’s closer to a lemon in appearance since it has this pinched end that faces toward Earth.
When the moon was rotated, the placement of that bump would shift across the surface of the rock, similar to the way our ocean tides shift and move around.
This essentially acted like the brakes of a car. It slowly but surely decreased the speed of its rotation until it fell into equilibrium with its orbital period.
It was at this point that the hemisphere of the moon facing us became locked. As the Moon orbits the Earth the same side always points towards Earth. This is shown in the animation above.
A consequence of the gravitational interaction between the Earth and the Moon is that the speed of the Moon’s motion around Earth is increasing. Due to this increase in speed, the Moon is moving away from Earth at a rate of 3.78cm (about 1.5″) per year.
The gravitational interaction between Earth and the Moon also affects Earth. The Moon’s increase in speed results in the length of Earth’s day lengthening. One billion years ago Earth’s day was about 19 hours. Even today the day is becoming longer and longer.
Why Does the Moon Look Different Sometimes?
It’s an easy misconception to make that the change in the moon’s appearance has to do with its rotation. But this isn’t the case.
The same side of the moon is always there, facing us; we can’t always see it. This is because the moon doesn’t have any illuminating properties. It simply reflects light from the sun.
This means that the different phases of the moon, such as a full moon, crescent moon, quarter moon, and gibbous moon, are simply the proportions of the moon illuminated by sunlight.
50% of the moon is always lit at any given time. However, because of the Earth’s position relative to the Moon and Sun, we don’t always see it.
Lunar Cycle/Orbit Discrepancy
Sometimes this information can all seem a little confusing, especially when you consider the number of days in the cycles.
So, it takes the moon 29.5 days to complete a lunar cycle but only 27.3 days to complete one orbit of our planet. This begs the question of the 2.2 days discrepancy.
Well, it comes from the way a lunar cycle is defined. A lunar cycle is the time it takes for the moon to return to the same phase as seen by an observer from Earth.
But because the earth moves around its own orbit of the sun, the moon lags slightly.
Essentially the moon takes a tad longer to catch up and complete its own orbit, and this is where that two extra days discrepancy can be found.
What About The Dark Side Of The Moon?
Unless you are talking about Pink Floyd’s album, there is no dark side of the Moon. Many use it to indicate the side of the Moon that we don’t see. However, that side of the Moon receives just as much light as the side we can see. ‘The dark side of the Moon’ is better called the far side of the Moon.
Final Thoughts
Considering that the moon spins on an axis much like Earth, it’s quite hard to imagine that we only ever see one side of the moon, but this is the case.
So, when you look up into the night sky and see that bright beacon in the sky, you are always looking at the same side.
This is due to the moon’s shape, which results in it becoming tidally locked to Earth.
Despite seeing different moon phases, it is always the same half staring at us from above.
The difference in appearance is actually due to the amount of visible sunlight reflecting off the moon.
If you are up for an observing challenge you may like to discover Lunar Rilles.
Hi, my name is Jason Anderson, and I am a Physics Professor. Ever since I was a kid, I’ve been fascinated with space, the universe, the moon, you name it. I spent hours and hours at the planetarium close to my hometown, wondering what else could be out in the universe.
Since then, I’ve been an avid stargazer and astronomer, and love nothing more than spending my time charting stars, observing planets, and finding constellations.
This is why I decided to start Telescope Guru. I only wish to share this fun pastime with the world. With this site, I hope to answer all of your questions relating to astronomy, telescopes, and stargazing.
