Why We See Only One Side of the Moon’s Surface

Posted on Categories Discover Magazine

Take a moment to imagine the moon, conjuring its speckled surface inside your mind. Think of its imperfections, its spots and splotches, carved out and cratered from the impacts of asteroids, meteorites, and comets. Chances are that your imagination conjured something surprisingly similar to mine, and to the imaginations of countless others.

The reason for this similarity is simple: We almost always see the same surface when we stare at the moon, the same patterns of craters and cavities, thanks to the way that the moon moves, rotating and revolving around Earth. So, what about the other surface of our only natural satellite? Why is there a far side of the moon, and why is it so unfamiliar?

Read More: Our Moon on Earth

Why We Always See the Same Side of the Moon

The moon is always spinning. It rotates around its axis, and it revolves around Earth. But despite that, the signs of this spinning are surprisingly subtle. When we watch the moon on Earth, we almost always see the same swath of its surface, since the moon rotates around its axis and revolves around Earth in around the same time, completing one rotation and one revolution every 27 days.

Of course, the moon changes constantly in other ways, appearing in the sky at different times and along different paths depending on the day, and in different degrees of illumination, from full illumination to none at all.

These differences in the path and phase of our natural satellite result from the relative positions of the moon, Earth, and Sun in their orbits, constantly circling, moon around Earth and Earth around Sun. But because the moon is tidally locked, taking the same amount of time to rotate around its axis as to revolve around Earth, it tends to show a single side to its observers on Earth.

Read More: Here Are 4 Reasons Why We Are Still Going to the Moon

Debunking the Dark Side of the Moon

Almost completely concealed, the side of the moon that faces away from Earth is a constant source of curiosity. And while it inspires its fair share of conspiracies, it’s also credibly cited as a spot for looking for alien life, since it faces away from our steady stream of radio chatter. In fact, scientists say it’s a perfect place for finding faint radio signals from the Sun and from other stars that are difficult to detect on Earth, due to its relative “radio darkness.”

Spared from these signals and separated, almost entirely, from the sight of observers on Earth, the far side of the moon is something strange and set apart. Tellingly, it’s called “the dark side of the moon” almost as much as it’s called “the far side of the moon,” thanks to its reputation of mystery.

But to describe it in that way can cause confusion, and is sometimes frustrating for astronomers. While the far side of the moon is dark in the sense that it’s unfamiliar, it isn’t dark in the sense that it’s deprived of sunlight, as is sometimes surmised. Instead, the far side of the moon transitions through the same sequence of lunar days and lunar nights as the near side, each experiencing two weeks of lunar day and two weeks of lunar night in turn.

Read More: 5 Ways Life on Earth Would Be Different If We Had Two Moons

Has Anyone Seen the Far Side of the Moon?

Naturally, to say that the far side of the moon is almost always unseen is not the same as saying it is never seen. Around 18 percent is occasionally observable on Earth thanks to inconsistencies in the moon’s movement, slight twists and turns that show off slivers of the far side surface. And the other 82 percent is occasionally observable, too, thanks to the fleets of spacecraft that map the moon’s surface from space.

In the 1950s, for instance, the Soviet Luna 3 spacecraft snapped photos of the far side of the moon for the first time. From these photos, astronomers found that the far side features fewer of the mare plains, or maria, that span the near side — as smooth and vast as oceans or seas when viewed from above and formed from flows of ancient lava. In subsequent years, assisted by subsequent spacecraft, astronomers also found that the far side features more craters and more crust, raising all sorts of questions about the origins of the moon’s asymmetries.

Since then, many more missions have observed the far side of the moon, shooting new images and shining new light on its surface. And a series of new spacecraft have taken things a step further: In January 2019, China’s Chang’e 4 spacecraft became the first to successfully land on the far side of the moon, while China’s Chang’e 6 became the first to retrieve far-side samples in June 2024.

Read More: Sealed for 50 Years, Rare Apollo Lunar Sample Will Have Its Opening Day

Unearthing Soil from the Far Side

Landing in the South Pole-Aitken Basin, one of the oldest and largest impact basins on the far side of the moon, both Chang’e 4 and Chang’e 6 were launched to study the stuff of the far side surface. While Chang’e 4 accomplished the task with a rover — still operational — armed with a radar, a spectrometer, and other scientific instruments that scan the moon’s surface and subsurface and identify its minerals, Chang’e 6 collected and returned samples of rock and regolith with a robotic arm and scoop.

Both achievements are part of an ongoing mission to untangle the mysteries of the moon and its origins, far side and near. When scientists test the South Pole-Aitken samples from Chang’e 6, for instance, they could find fragments of the moon’s interior, disrupted and brought to the surface by the impact that formed the basin. Not only that, they could discover more about the differences between the two lunar surfaces, comparing the composition of far side soil with previously collected near side samples.

Tough to see and tough to study, astronomers say that the far side of the moon is more inaccessible than the near side, making the achievements of Chang’e 4 and Chang’e 6 all the more impressive. Turning to satellites to organize their far side operations (it’s impossible to communicate with the spacecraft that land there otherwise), the missions brought back incredible insights and an intriguing idea: While still strange and unfamiliar, the moon’s mysterious side may not stay mysterious forever.

Read More: Samples From the Moon’s Far Side Have Just Arrived to Earth

Article Sources:

Our writers at Discovermagazine.com use peer-reviewed studies and high-quality sources for our articles, and our editors review them for accuracy and trustworthiness. Review the sources used below for this article:

Sam Walters is a journalist covering archaeology, paleontology, ecology and evolution for Discover, along with an assortment of other topics. Before joining the Discover team as an assistant editor in 2022, Sam studied journalism at Northwestern University in Evanston, Illinois.

Leave a Reply