It is 4.5 billion years ago. The Universe is in a chaotic state; the Earth has barely just coalesced from the debris flung out in a flattened disk spinning about our Sun.
Then, hurtling out of the depths of space flies Theia, a massive celestial body the size of Mars. It punches into our planet; the violence of the impact is barely fathomable. The crust and mantles of both planets are ripped apart and the cores of the two planets are fused together while massive amounts of debris are cast out into the ether. Much of it is collected by the gravity of the “new” earth while a disk of debris forms, much like that which spun around the sun (obviously on a much smaller scale). And it is that disk which over the next 10 – 100 million years coalesced into a boiling molten sphere that slowly cooled to eventually form our Moon.
Something very interesting happened over this period. As the moon cooled, the gravity of the much larger earth “dragged” the core of the moon off-centre by an estimated 480km, 20% of the moon’s radius. And so, naturally, as the core of the moon settled into its offset position, the spin of the moon was affected. Eventually, the moon stopped spinning and instead began to rock back and forth with the core dragging it back and forth. This rocking motion slowed over the next several millennia until finally, the moon settled into its current state with only one face turned toward our now blue planet. This process is known as “tidal locking” and is the reason why each and every single night we only bear witness to one side of our planet’s only natural satellite (and why Pink Floyd was able to gift us with their famous album, The Dark Side of the Moon.)
Now pock-marking the visible side of the moon are what are known as Lunar Maria, the Lunar Seas. The Moon’s Maria are named after their dark appearance, which reminded early astronomers of the dark, calm waters of the earth’s seas. There are a total of 31 maria on the moon, covering about 16% of its surface. These “seas” are concentrated on our side of the moon, the hemisphere that always faces the earth. This is because the crust on the near side of the moon is far thinner than on the far side because of the offset core, and over the millennia, as space debris battered the little planetoid, certain large enough meteorites punched through the crust into the shallow core and massive lava flows hundreds of kilometres wide surged over the surface. And because the moon is too small to retain an atmosphere there is no air, which means no wind, which means no weathering. That means that although these events may have occurred millennia ago we are still able to see the scars they have left behind. And these scars are what give those in the Northern hemisphere the Man in the Moon and those of us in the southern hemisphere, the Hare.
Each of these mares has a name, the most famous of which is likely the Sea of Tranquility, Mare Tranquillitatis, in which Apollo 11 touched down just 54 years ago. The largest sea, the Sea of Showers, Mare Imbrium, measures a spectacular 1100km. Just imagine a sea of molten lava 700 miles across. I greatly enjoy the names, especially those that make up the bulk of our Hare in the Moon, Mare Nectaris, the Sea of Nectar and Mare Fecunditatus, the Sea of Fertility make up the ears while the Mares Tranquillitatis (Tranquility) and Serinatus (Serenity) make up the head. The bulk of the body is made up of Mare Imbium (Showers) and one of my favourite names, Oceanus Procellerum, the Ocean of Storms, so named for its immensity is over 4 million square kilometres (1.5 million square miles), making it larger than the continent of Australia! It is the only ocean amidst the plethora of seas.
And so, the next time you do some moon gazing (I encourage you to try using your binoculars), I hope you’ll see something completely new. It’s just one small part of the incredible beauty of our night sky.