Life appeared on Earth through a series of lucky coincidences, and that luck started with our Moon. None of the other planets of the inner solar system have significant moons. Space is lonely around Mercury and Venus. Mars does have two small moons, Phobos and Deimos (Fear and Despair, befitting companions for the God of War), but those are simply captured asteroids, lassoed in the not-too-distant past and doomed to eventually come close enough to their unloving parent to be torn to shreds by gravitational forces.
In fact, no other planet in the solar system – or any exoplanet known orbiting other stars – has a moon quite like the Moon. With the exception of Pluto and its companion Charon, no other planet has a satellite with the relative mass of Luna. The giant worlds like Jupiter and Saturn have some moons large enough to be planets in their own right, but they are insignificant next to the massive bulk of their parents. The Moon is roughly 1% the mass of the Earth, a percentage unheard of in the galaxy.
And, as is the nature of nature, we found ourselves with our satellite through the chance encounter of a violent collision. Billions of years ago, when our solar system was but a churning mass of gas and dust swirling around a fitful young star, the planets began to coalesce. But before they could become planets, they were mere planetesimals, agglomerations of rock and ice, dozens of them swarming in the chaos of those early days.
In the orbit of what would one day become the Earth, we were not alone. At some point, due to some accident of trajectory and conceit of momentum, a planetesimal the size of Mars struck us. The details of the collision and its aftermath are muddied; with no time machine we can only rely on computer simulations of the impact. But this much is clear: the cosmic accident vaporized part – and possibly the whole – of the Earth and its impactor, creating a ring of superheated plasma that looked more like a rage-filled donut than a proto-world.
But with time the fury ceased; the plasma cooled. The ring coalesced back into the shape of a sphere, but now with an orbiting companion. The traces of the impactor are almost lost to us, the evidence of its existence only slim. The Earth contains more heavy metals in its core than it should for a planet its size – a contribution from the interloper. And the Moon itself, when sampled to measure the composition of its fundamental elements, reveals itself to be made of the exact same mixture as the Earth. A common origin then, not an object formed elsewhere and captured by our gravity.
And we’re lucky to have that faithful companion. Day to day, the Moon doesn’t largely affect the Earth. It raises and lowers the tides in its month-long orbit, sharing the duty with the gravitational pull of the Sun itself. Some creatures, like dung beetles, use the polarized light of the Moon to guide their way back home after a night of collecting. But otherwise our satellite does nothing more than give us something beautiful to look it – there’s nothing quite like the cool blue light cast over freshly fallen snow in quiet winter nights.
But over the long haul, when we zoom our perspective out over billions of years, the Earth wouldn’t be the same without our sole friend. Our planet spins about its axis, but that spin is tilted with respect to the movement of the Earth in orbit around the Sun by 23 and a half degrees. This tilt gives us our seasons, with half our year spent with the northern pole facing the Sun, and the other half trading places with the southern pole.
Our planet could have had any orientation it wished. The other planets have lesser and greater tilts, with Uranus tipped completely over on its side and Venus rotating backwards. And there’s nothing to keep that tilt fixed over cosmic time. Our planet was born spinning, but the internal arrangements of its core, mantle, and crust, along with the ever-present gravitational machinations of Jupiter, can cause the Earth to wobble, shifting its tilt ever so slightly.
With every shift in the tilt, the seasons would radically change. Instead of regular, predictable changes year after year, we would experience ages with endless summers, or ages with violent but short winters, or anything in between. The rhythm of the seasons provides a pulse for life, which has the freedom to grow and evolve without trying to overcome great climactic shifts caused by a changing axis.
Luna acts as a great gravitational counterweight, stabilizing the motion of the Earth. By providing a source of gravity external to our planet, the Earth’s interior is free to shift and reconfigure as it pleases – the Moon steadies our hand and keeps us upright.