Please expand your browser for the best viewing experience
Life is a tenuous thing. Earth is just within Sol's habitable zone, and constantly pelted with solar radiation and cosmic rays. Rocky scraps of cosmic afterbirth constantly cross Earth’s orbit, threatening to eradicate all terrestrial life. In point of fact, it is almost certain that countless Extinction-Level Events would have sterilized the surface of our plucky planet had it not been for our constant companion and benefactor; a body which unwittingly wards away many of the ills that could befall us:
Luna is unique among the observed celestial bodies; there is no other satellite closer in size and composition to its mother-planet (if one discounts the dwarf-planet Pluto), and the Earth/moon system is the only tidally locked pair. Furthermore, it also happens to be the only moon in the solar system which is circling an intelligent civilization-- a factor which may not be a mere coincidence.
It was 4.5 billion years ago last week that the young planetesimal Earth was forming from the sun's accretion disk of dust and boulders. In that era, the moon was much nearer Earth, and would have looked much larger--several times the size of the sun.
Observations of the solar system show us that the moon's birth was rather unusual. All of the other worlds either lack satellites or have captured them from other places. Of course the moon isn't Earth's only unusual resident; its surface crawls with all manner of strange and delicate carbon-based life forms. Adherents of the Rare Earth Theory postulate that a large moon such as ours is not merely a benefit for life, but essentially a requirement.
While Earth has had its share of ice-ages, the gravity of the moon has acted as a gyroscope, keeping the Earth's axis steady at 23.5 degrees and sparing us the wild environmental changes Mars faced. This long-term stability has given life a chance to arise amidst a cycle of regular seasonal changes.
A case can also be made that the tides have been invaluable to the evolution of life on our world. The sun alone would cause some tides to occur, though they would be far less than those the moon creates. The surfing would suck, and for many that wouldn't be a life worth living. The higher tides afforded us by Luna have made long swaths of coastline into areas of that are regularly shifted between dry and wet. These variable areas may have been a proving ground for early sea life to reach out of the oceans and test the land for its suitability as a habitat.
It's not only water being tugged by the moon's gravity. Perhaps the moon helps keep Earth's core and seas warmer than they would otherwise be. Since the moon circles the Earth once a month, and the Earth is spinning a full turn at a much quicker 24 hours, the moon's gravity is creating drag, hence friction, as it pulls at Earth's surface. This causes several things to happen: first is a perpetual morphing of the crust--like the amateurish kneading of bread--that contributes a clumpy, broken mess that we call plate tectonics.
Even Earth's rotation is slowed by virtue of the Moon's pull. Without the moon, the Earth might rotate much faster, causing a more turbulent atmosphere, and thus unending gales of life-hostile, skirt-blowing winds. As Luna's orbit slowly creeps away from the Earth at 1.5 inches per year, her gravimetric drag will eventually slow the Earth's rotation to match the pace of the moon's orbit. One day will be 9,600 hours long, and the moon will only be visible from one hemisphere, fixed in the sky.
Of course, by then the sun should be in an expanding red-giant phase, slowly engulfing its planets. The sun's coronal atmosphere could be creating drag against the moon, slowing it toward an eventual breakup as Earth's gravity tears it apart. The remnants of Luna will fall back to Mother Earth as meteorites, and while it may be a pretty show, it ought to prove bad for property values, and worse for the surf.
If the unlikely set of circumstances which brought forth our moon are as rare as they seem, perhaps ours is the only such planetary system in the entire, vast galaxy; or perhaps in our unfashionable limb of the universe. But every once in a great while, when the time is right, two protoplanets who love each other very much can touch each other in a special way, and make life together. Without that magic, astronomical ritual, we certainly would not be here.