The largest, shining object in Earth’s starlit night sky, our Moon has long been the source of mystery, myth, and poetry–a captivating inspiration for those who stare at the sky in wonder. But Earth’s Moon is a very real object–the only body beyond our own planet that we have actually set foot upon, leaving our lingering footprints in its distant, alien dust. Earth’s Moon has been with us almost from the very beginning, and even though it is our planet’s closest companion in space, it has nonetheless managed to keep some of its ancient secrets very well. In September 2015, astronomers released a new study that shed light on Earth’s bewitching companion’s ancient and secretive past. Although our Moon now appears to be unchanging, as if it has always been exactly the same as it appears now–going through its beautiful and, yet, familiar phases, and controlling our ocean tides–in the distant past, things were actually very different.
A billion years ago, our Moon was closer to Earth than it is now. As a result, it appeared to be a much larger object in the sky. During that ancient era, if human beings had been around to witness such a sight, it would have been possible to see the entire Moon–not merely the one near side face that we see now. A billion years ago, it took our Moon only twenty days to orbit our planet, and Earth’s own day was considerably shorter–only eighteen hours long. Stupendous, almost unimaginably enormous tides, that were more than a kilometer in height, would ebb and flow every few hours. However, things changed, as the lunar orbit around our primordial planet grew ever wider and wider. Annually, Earth’s Moon moves about 1.6 inches farther out into space. Currently, the lunar rate of rotation, as well as the time it takes to circle our planet, are the same.
There was a time when Earth had no Moon. About 4.5 billion years ago, when our ancient Solar System was still forming, the dark night sky above our primordial planet was moonless. At this time, the Earth was about 60 percent formed, although it did have a differentiated crust, mantle, and core. This was a very chaotic and violent era in our Solar System’s past, with planets first forming out of blobs of primordial dust, gas, and rock. During this era, frequently likened to a “cosmic shooting gallery”, collisions between the still-forming planets were commonplace. Orbits were not as orderly as they are now.
The most widely accepted scenario, explaining our Moon’s mysterious and ancient birth, is termed the Giant Impact Theory. According to this theory, Earth’s Moon was born as the result of a gigantic collision between our still-forming planet and a primordial Mars-sized protoplanet that has been named Theia. The tragedy that was the doomed Theia probably had an orbit that crossed Earth’s–making such a catastrophic collision difficult to avoid. It is thought that the impacting Theia hit our planet hard, but swiped it with a glancing blow at precisely the right angle. In fact, Theia came very close to bouncing off Earth, but was swallowed instead. The blast dispatched shock waves across our ancient planet, hurling debris and gas screaming into space. For a short time, Earth had a ring around it that was composed of this ejected material.