It's rather difficult to look up at the night sky and not see the Moon. It is by far the most salient object circling our Earth. But, have you ever wondered where it came from? What its purpose is? And what would happen if we were to lose it? Mankind has been wondering about these questions for thousands of years. We have only recently began to understand what our celestial partner means to us.
In order to fully understand the Moon, we would necessarily start by asking – where did it come from? There have been several theories spanning many decades. Earlier versions of lunar formation theory included capture, whereby the Moon would have been a captured planetoid. Another theory was fission, where the earth was spinning so fast that it would have, quite literally, belched it out of itself to cool and form away from us. It is in fact this latter theory that led us to the one we currently espouse to, that is to say, the giant impact theory. According to this, there occurred a collision during the late stages of planetary formation in our solar system. At that point, planets were still very new and busy forming. It was at this time when Earth was just an embryo – a baby planet, when a collision would have occurred with Theia, a Mars-sized planetoid. This collision ripped apart early Earth's crust and that crust then coalesced. It snowballed into a whole separate entity, which we now call the Moon.
Every 27.3 days, the moon makes its way around our planet while also completing one turn on its axis. Known as synchronous rotation, this celestial dance means that the same lunar face always peers down at us. Viewed from Earth, the amount of the moon illuminated by the sun appears to wax and wane, creating the familiar cycle from new moon to crescent to full. This sequence is a combined result of the moon’s changing position relative to both the Earth and sun, requiring 29.5 days to complete one full lunar cycle. Even though the same side of the moon always faces Earth, there’s no true “dark side,” as many mistakenly call our lunar orb’s far side. Even the far side of the moon receives sunlight — we just can’t see it — and the section of the moon illuminated on any given day shifts depending on the moon’s position.
The moon’s steady change between new and full provided a rhythm for generations of humans, who eventually crafted calendars marking the moon’s many phases and their effects on Earth’s surface. One of the most obvious lunar influences is seen in Earth’s tides. The moon’s gravitational tug causes one bulge of water to form on the nearest side of our planet and another on the side farthest away. As the Earth rotates, the part of Earth affected by the lunar pull shifts, creating a high tide about every 12 hours at any given spot.
The moon also dampens the amount that Earth teeters on its axis, helping to keep our climate more stable. The wobble in our planet’s tilt affects how the sun’s energy is distributed across the Earth and can influence the frosty advance or retreat of ice ages. Without the moon, scientists estimate that our planet’s tilt could have varied by up to 85 degrees, causing wild swings in climate.
But each year, the moon’s grip on our planet grows just a smidgen weaker as it drifts about one and a half inches farther into space. This slow expansion in the moon’s orbit is a result of its role in Earth’s tides. Our planet rotates a little bit faster than the moon’s orbit. So the tidal bulge that rises on the side of Earth nearest the moon spins just ahead of the orb. This drags the moon along, slightly speeding up its orbit and inching it away. So, what would happen if we lost our little friend completely?
If we all woke up one morning to find the moon missing, most of us probably wouldn't notice. Those of us who live near the oceans would find the tides reduced, but not completely absent. While the moon does most of the gravitational tugging that creates our tides, the sun also plays a role. Coast dwellers would notice tides less than half as big as they are now.
Moonless skies wouldn't affect most of our day-to-day lives, but it would upend the lifestyles of many nocturnal animals. Animals like moths have evolved over millions of years to navigate by the light of the moon and stars. Newly hatched turtles use the moon's light to find their way to the ocean – and they are struggling enough as it is, thanks to artificial streetlights that lure them in the wrong direction.
An errant moon will also affect Earth's motion about its axis. The pull of the moon currently increases the length of a day by about 2 ms every century. If that increase stopped tomorrow, it wouldn't be very noticeable on our human timelines. However, if the moon had disappeared billions of years ago, our rotation today would be very different. Early on, the Earth had a four hour rotational day, which the moon slowly and persistently slowed to the 24-hour days we know now. Without the moon's presence for all those years, we'd still be spinning much faster – and feeling even more like there weren't enough hours in a day.
Another thing we have the Moon to thank for is holding the Earth's 23.5° tilt fairly steady. Without that steadying hand, Earth would wobble much more, dramatically affecting the planet seasons and climate. According to Paul Sutter, an astrophysicist at the Flatiron Institute, …
Everything we know about the seasons would be completely out of whack without the moon. At a certain angle, some places on the planet will hardly ever see the sign at all, and others the sun would be overhead for months on end.
The change in Earth's tilt would happen gradually, though, so we wouldn't notice it instantly or likely even within one human lifetime. But for animals that have evolved over millions of years to move in sync with seasons, it might be hard for them to adapt, even over what would seem to us like a long timescale.
Losing the Moon would also mean losing a wealth of information about the young Earth. Thanks to tectonic activity, there are no truly old rocks on Earth. But the geologically inactive moon serves as a repository of information about what the Earth and the solar system were like billions of years ago. For instance, the number of craters on the moon tell scientists that there was a period of heavy bombardment by asteroids some 4.1 - 3.8 billion years ago. Chemical analysis of moon rocks has also helped us learn how much of our planet water was brought to earth by comets and asteroids.
The moon gives us a record for understanding what was going on way back at the dawn of time. Because we think the moon and earth formed in a giant impact together, learning about the Moon is therefore learning about the formation of the Earth.
Thanks to the moon, we have a Goldilocks existence here on the third rock we inhabit from the sun. Without the Moon, our days would be around six hours long. Our seasons would vacillate between being almost non-existent and going on for decades. Our ocean currents and tides would cause worldwide devastation and mayhem, leading to the extinction of countless of species of ocean dwelling creatures. It could have so easily all been so different. We could have been like Mercury, where a year outstrips the length of a day. Like Uranus, where a day and a night each last 42 of our Earth years. Who knows if life would have taken root at all on a planet with no moon such as ours? I, for one, will always be grateful for God's foresight in giving us what was clearly an anchor in an otherwise vast and empty space.