We see only one side of the Moon. There is no dark side of the moon.

Here’s Why We See Only One Side of the Moon [There’s No Dark Side of the Moon]

Why do we see only one side of the Moon? You have probably heard references to the “dark side” of the Moon – there’s even a Pink Floyd album with that name. However, there is no “dark side” of the Moon because our satellite is illuminated by the Sun, not by the Earth. All parts of the Moon get sunlight as it rotates.

The “dark side” misconception arises because we always see the same side of the Moon, known as the “near side.” The side we don’t see is more accurately called the “far side” rather than the “dark side.”

It may seem strange since the Moon also rotates on its axis, which would typically mean we should see all its sides over time. However, we don’t. We always see the same 50% of the Moon (actually a bit more, which I’ll explain later). Here’s why.

Tidal Locking is the reason why we see only one side of the moon

Tidal locking is a phenomenon where the Moon’s rotational period matches its orbital period around Earth. This synchronous rotation means that the same side of the Moon is always facing Earth while the other side remains hidden. Thus, while the Moon rotates and receives sunlight on all its surfaces, we only ever see one hemisphere from our vantage point on Earth.

In other words, the Moon rotates around the Earth at the same angular speed that it rotates on its axis, which is why we always see the same side facing us. This synchronous rotation results in a “moon day” being approximately 4 weeks long. If the Moon didn’t rotate, we would see all its sides as it orbited Earth. Its rotation is the reason we consistently see the same face.

In the video below, you can see and gain a better understanding of how the Moon’s same orbital speed and rotational speed match because of the tidal locking. This synchronized rotation is why we always see the same side of the Moon from Earth.

Tidal locking in Earth-Moon system. This is why we see only one side of the Moon. See how the Moon rotates while orbiting Earth, and how tidal locking means it always shows us the same face. Due to locking, the time taken for the Moon to rotate once on its axis and to orbit Earth are both 27.32 days. The video also explains the Moon’s phases.

The gravitational pull from Earth has influenced the Moon’s rotation rate. If the Moon had been spinning more than once per orbit, Earth’s gravity would have pulled against the Moon’s rotation, slowing it down. Conversely, if it had been spinning less, Earth’s gravity would have sped it up. Over a thousand years, Earth’s gravitational pull adjusted the Moon’s spin to match its orbital period, leaving one side perpetually facing away. This phenomenon, known as “tidal locking,” occurs when an object’s orbital period matches its rotational period.

An extreme example of tidal locking is the relationship between Pluto and its largest satellite, Charon. Due to their relatively small mass difference and distance, they are mutually tidally locked, meaning each only ever shows one face to the other. This wasn’t always the case for our Moon. When it first formed, after Earth was hit by a planet-sized body around 4.5 billion years ago, the Moon’s initial spin and orbit were not synchronized. It was much closer to Earth, and the gravitational interaction caused the Moon to develop a slight oval shape. Earth’s gravity kept realigning these bulges until tidal locking was achieved.

The Moon’s orbit is expanding at a rate of about 3.8 centimeters per year. Currently, the Moon orbits at a radius of about 384,400 km (238,855 miles), but it was much closer when it formed. This increase in distance is due to tidal interactions. The Moon raises tides on Earth, and because the side of Earth facing the Moon experiences a stronger gravitational pull, it creates tidal bulges. These bulges exert a gravitational force on the Moon, gradually increasing its orbit. Meanwhile, Earth’s rotation is slowed by the Moon’s gravitational pull on these bulges.

Tidal friction, caused by the movement of these bulges, transfers energy from Earth to the Moon’s orbit, causing the Moon to move farther away but at a slower speed. This process has been ongoing for billions of years. As a result, one hundred years from now, a day on Earth will be approximately 2 milliseconds longer than it is today. This same process also affected the Moon, but much quicker due to Earth’s larger mass. The Moon became tidally locked within a few tens of millions of years.

We see only one side of the Moon. There is no dark side of the moon.
Because of the tidal locking, we see only one side of the Moon. Photo by Griffin Wooldridge

We see more than 50% of the Moon’s surface, actually

Interestingly, we actually see more than 50% of the Moon’s surface. The Moon’s slightly elliptical orbit and its axial tilt allow us to peek beyond its average eastern and western horizons and see more of the lunar north and south poles.

Our changing view of the Moon makes it look like it’s wobbling. This wobble is called libration and lets us observe about 59% of the Moon’s surface, with 41% remaining hidden from view on Earth.

The term libration is derived from the Latin term libra, which means “scales” or “balance.” The term libra refers to the way the Moon appears to rock back and forth, much like a scale tipping side to side, allowing us to see slightly different parts of its surface over time.

Until 1959, no one had seen the far side of the Moon. It was first photographed by the Soviet spacecraft Luna 3 as part of the Luna program, which ran from 1959 to 1976.

Will Earth become tidally locked with the Sun?

It’s unlikely. Closer objects are more prone to tidal locking, but if Earth were close enough to the Sun for this to happen, it would be too hot for life. The habitable zone, where planets can sustain life, partially overlaps with zones where tidal locking is likely, making such planets less habitable.

On a tidally locked planet, one side would always face the star, resulting in extreme temperatures and potentially inhospitable conditions. However, Earth’s current distance from the Sun ensures it will not become tidally locked, preserving its habitability.

Sources

M. Özgür Nevres

Comments

Leave a Reply

Your email address will not be published. Required fields are marked *

This site uses Akismet to reduce spam. Learn how your comment data is processed.