Why dont we see the Once

Although the Moon is rotating, it always keeps one face toward us. This is called “synchronous rotation,” and it's why we only ever see the Moon’s nearside.

Published: Tuesday, August 28, 2012

Spacecraft have explored both the nearside (left) and farside of the Moon. Humans, however, have landed only on the nearside — the half that always remains visible from Earth — due to communication requirements. NASA/GSFC/Arizona State University

Why do we see only one side of the Moon from Earth? Have humans explored the other side, and, if so, is it different from the nearside?

—Sara O’Connor
Gresham, Oregon

Earth orbits the Sun once every 365 days (a year) and spins on its axis once every 24 hours (a day). The Moon orbits Earth once every 27.3 days and spins on its axis once every 27.3 days. This means that although the Moon is rotating, it always keeps one face toward us. Known as “synchronous rotation,” this is why we only ever see the Moon’s nearside from Earth.

All of the manned space missions to the Moon have landed on the nearside due to communication needs, so humans have physically explored this side much more.

We have remotely explored both sides of the Moon with orbiting satellites. In 1959, the Soviet Luna 3 first photographed the farside, and in 1968, as NASA’s Apollo 8 orbited the Moon, human eyes first viewed it. Since then, many satellites have sent back pictures and other data showing that the farside of the Moon differs from the nearside. (On Jan. 3, 2019, China's Chang'e 4 lander became the first lunar lander to touch down on the Moon's farside.)

The nearside is darker than the farside (which is ironic, as a nickname for the farside is “the dark side of the Moon” even though it’s actually the brighter side of our satellite). Mare deposits of dark basalt fill many large basins on the nearside, whereas the highlands of the farside are made from a light-colored mineral called feldspar. This may result from even another difference: The lunar crust is thinner on the nearside, which means that the mantle (the once molten, denser layer that underlies the crust) is closer to the surface there. Asteroid impacts could more easily fracture the crust down to the level of the mantle on the nearside, allowing magma to rise up to fill impact basins and form mare deposits.

Amount of cosmic debris reaching Earth has stayed surprisingly constant in last 500m years, say scientists

A composite of 50 photographs taken near Hawes, North Yorkshire, over a period of 25 minutes shows meteors and star trails during the Perseid meteor shower. Photograph: Danny Lawson/PA

Kate Ravilious

@katerav

Tue 3 Aug 2021 06.00 BST

This month it is worth turning your eyes to the night sky to watch the spectacular Perseid meteor shower. Peak viewing time will be around 12 August, when as many as 150 meteors an hour will whizz overhead. Generated by Earth passing through the debris left behind by Comet Swift-Tuttle, these meteors are a reliable event, but other meteors, such as the fireball that recently lit up southern Norway, are more random.

Most meteors burn themselves out in the atmosphere, but thousands of tons of cosmic dust do still make it to Earth’s surface every year. New research, published in Proceedings of the National Academy of Sciences, shows that the number of meteorite strikes has stayed surprisingly constant for millions of years.

Using powerful acids, the researchers dissolved pieces of limestone that had formed at different times, corroding everything except chromite – a hardy mineral found in some meteorites. They expected to see greater volumes of chromite following major collisions in the asteroid belt, because it was thought some of the additional debris would have headed Earth’s way.

But instead there was only one peak during the last 500m years. The researchers speculate that asteroid smashes have to occur in a specific place for their fallout to interact with Earth.

When we talk about the Moon we often describe it as having a ‘familiar face’, a nod to the distinctive pattern of bright highlands and dark lunar maria that has been turned towards us for millennia, visible to every human who has ever stood on Earth.

Advertisement

But why do we only see this one side of the Moon? We know that Earth spins about its axis, so why don’t we get to see the full lunar surface as our Moon does the same?

Full Moon, by Sarah and Simon Fisher, Bromsgrove, Worcestershire.

If you could have a bird’s-eye view of the Moon orbiting Earth, you would see that the Moon rotates once on its axis every 27.3 days, which also happens to be the same amount of time it takes to complete one orbit of our planet.

The result is that from our perspective on terra firma we see the same lunar hemisphere at all times.

Although this is not strictly true, either. We do get to see slightly more of the Moon than half of the Moon's surface at due to a wobbling effect known as lunar libration.

Thanks to lunar libration we can observe slightly more than half of the Moon’s surface. Credit: Pete Lawrence

If the Moon were to spin faster or slower than once per orbit we would see all of it.

In the proper astronomical parlance, we say that the Moon is ‘tidally locked’ to Earth

You may also come across the expression ‘synchronous rotation’, which means the same thing.

A diagram showing the formation of the Moon out of collision debris.

When the Moon formed some 4.5 billion years ago, it was spinning much more rapidly than it is today.

Earth’s gravity causes a rocky tidal bulge in our companion, which means it is lemon-shaped rather than a neat sphere, with a pinched end facing our planet.

Back in the Moon’s fast-spinning early history, the location of that bulge kept changing, shifting across the surface much in the same manner as our ocean tides.

This effectively acted as a brake, gradually slowing our companion’s spin speed until it fell into equilibrium with its orbital period. At this point the hemisphere facing us became locked in place.

The Moon may always keep the same face turned to us, but that face keeps changing. Image Credit: iStock

Though the Moon always keeps that same side towards us, even a cursory glance will show you that it is not consistently illuminated from one night to the next.

What you are seeing here is the changing phase of the Moon. By phase, we simply mean the proportion of sunlit Moon visible from Earth.

The essential point to remember is that although only a fraction of the Moon may be lit from our vantage point, a full 50% of the Moon is lit at any one time. We just can’t always see it.

The far side of the Moon, as seen by the Lunar Reconnaissance Orbiter. Credit: Credit: NASA/Goddard/Arizona State University

This is why astronomers don't like to refer to the side of the Moon facing away from us as the 'dark side', because this is not always true. Instead, we refer to the far side of the Moon.

The cycle of lunar phases (also known as a lunation) runs from new Moon to full Moon and back again, passing through crescent Moon, quarter Moon and gibbous Moon along the way, and takes 29.5 days to complete.

For more on this, read our guide to the phases of the Moon.

The phases of the Moon. The inner circle shows what the Moon looks like seen from above its north pole, while the outer circle shows the phase we see from Earth at that time. Credit: BBC Sky at Night Magazine

Although it takes the Moon 29.5 days to complete a lunar cycle (a period known as the synodic month), it only takes 27.3 days to complete one orbit of our planet (a sidereal month).

This discrepancy arises from one lunar cycle being defined as the time it takes for the Moon to return to the same phase as seen by an observer on Earth.

Because Earth itself is moving, hurtling through space on its own orbit around the Sun, it takes the Moon that little bit longer to catch up than complete an orbit of its own.

You may also wonder why, given the Moon sits in the middle of a line with Earth and the Sun at the point of new Moon, a solar eclipse is such a rare event.

And likewise, why we don’t experience guaranteed lunar eclipses at the time of full Moon. It’s because the Moon’s orbit around Earth is tilted by around 5° with respect to Earth’s orbit around the Sun.

Advertisement

What happens, in most instances, is a near miss.