(Image credit: Gregory H. Revera)
In the long, cold winter nights that the northern hemisphere get right now, the Moon is often very high in the sky. The full Moon especially is impressively bright and beautiful as it shines almost directly above your head and illuminates the dark winter night. If you looked at it last night, like I did, you probably even saw a very bright “star” close to the Moon (It wasn't actually a star, but Jupiter). It was a wonderful sight, and inspired me to write about the Moon for a bit. Unlike with the Sun, I don't really have a singular story, and will instead just tell several quite interesting things about the Moon.
(Image credit: NASA)
1: The Moon was created when Earth collided with another planet. The planets formed from a disc of gas and debris around the proto-Sun. Little particles would collide with each other and attach to each other, forming pebbles, which would keep colliding to form rocks, and as these series of collisions kept going, the rocks became bigger and their collisions more violent; especially once they became big enough that their gravity began to play a role. In the outer Solar System, this led to four heavy planets being formed, which had gravity strong enough to keep hydrogen and helium, the most common atoms, from escaping, and as a result collected gigantic atmospheres of those two gases. But closer to the Sun, the planets formed were smaller and not heavy enough to get atmospheres of hydrogen and helium, but only of heavier, far rarer gases. These five planets were Mercury, Venus, Earth, Theia, and Mars.
Hold on, Theia? Well, Theia was a planet about the size of Mars; two times smaller and ten times lighter than Earth. It happened to have formed in almost exactly the same orbit as Earth. Normally, Earth's gravity would've caused the two planets to collide and form a single one with them being so close together, but Theia was in exactly the same orbit, orbiting the Sun in one year, just like Earth. As a result, the two planets never came close to each other since they orbited at the same speed.
So why doesn't Theia exist any more then? Well, the Sun's gravity isn't the only thing that affects a planet's orbit. It's the most important one, but not the only one. The planets also affect each other's orbits a little. It's a very small effect, but the little pulls Venus, Mars, and Jupiter gave Theia and Earth were enough to slightly speed one planet up and slow down the other, causing them to approach each other over the course of 50 million years. Finally, they got close enough to be affected by each other's gravity, so they sped towards each other and collided.
The catastrophic energy of that impact was enough to instantly destroy the Earth's entire surface and melt and deform the planet, while Theia was completely pulverised. Within hours, the collision was clouded in a huge sphere of debris as the two molten planets became a single one that glowed almost as hot as the Sun, vaporising the top layer of rock so it added to the debris. Much of the debris rained back to this molten new Earth, and much of it escaped the gravity and began following its own orbit around the Sun, but the rest of the debris stayed in orbit of Earth. And as the Earth slowly cooled down from the collision with Theia, the debris coalesced into bigger and bigger clumps in a small-scale repeat of the origin of the planets. Eventually, these clumps became two large moons, which finally collided to become the single one that still orbits us. (The traces of this second moon can still be seen in the form of the highlands on the far side of the Moon)
(Image made using SpaceEngine)
2: The Moon used to be far closer. It's not quite sure how much closer, but it is thought it might be as much as eight times farther away now than when it was newly formed! Imagine how gigantic the Moon would've been in the sky at such a distance; it would create planet-wide solar eclipses every month! But if it was this close, how come it's now much farther away? The answer is the tides. The tides are caused by the Moon's gravity, and when the Moon was eight times closer, its gravitational influence on Earth would've been 64 times greater. That means the sea didn't rise up six or seven metres at best with flood, as it does now, but about 400 metres!
Imagine a gigantic tsunami like that rising up every single day and washing deep inland. And there's something else: days used to be shorter back then. The Earth rotated in only six hours, so there were days and nights of three hours each. Every day, you'd get this giant tsunami caused by the Moon. All that water in a 400 metre high tidal wave had a lot of mass together. Every time the flood would hit a continent, the Earth would get a pretty big hit in the direction opposite to its rotation by all that water. The Earth is incredibly heavy, of course, far heavier than the massive flood, but all that water crashing into the continents every day eventually slowed its rotation down over millions of years. By the time the dinosaurs walked the Earth, days lasted 22 hours. They're currently 24 hours and still increasing with a few microseconds each year due to the tides.
But there's a natural law saying energy can't simply disappear, and rotation is a form of energy. So all that energy the Earth lost as it rotated slower needed to go somewhere. In fact, it went into the Moon's orbit. Every day, the tides made the Earth rotate a bit slower and made the Moon orbit a little faster, so it moved into a higher orbit, and that's why it's now much farther away than it used to be.
(Image credit: NASA)
3: The Moon also rotates. Earth obviously rotates once a day, causing day and night. Yet the Moon always has the same side turned to the Earth. The reason one side of the Moon is always facing us is because it rotates once every 27 days; precisely the same time it takes to orbit the Earth. What an incredible coincidence, both its orbit and its rotation take 27 days! What are the odds?
Of course, it's not really a coincidence. This is another thing that's caused by the tides. Remember the tides being 64 times stronger when the Moon was newly formed? Well, just imagine how strong they were on the Moon itself. Earth is 80 times heavier than the Moon, so the tides on the Moon were about five thousand times stronger than they currently are on Earth! Of course, the Moon has no oceans for these tides to cause ebb and flood, but water isn't the only thing affected by the tides. Even the solid rock a planet consists of it affected by it. It's a far smaller effect, and as good as immeasurable on Earth, but when you make that effect 5000 times stronger, it becomes quite powerful. The Moon's surface would've risen up and down every day due to Earth's gravity, probably causing big earthquakes (well, moonquakes) and heavy volcanism (in fact, this might be what caused the dark “seas” of solidified lava on the Moon to form). While it's not quite as intuitive as the idea of a tidal wave smashing into the continents slowing the Earth down, this pulling on the Moon's surface also caused its rotation to slow down due to friction. And since the tidal effects on the Moon were 5000 times stronger, the Moon's days rapidly became longer and longer. All the way until it rotated exactly once for every orbit it made, always facing the Earth with one side.
(Clause Joseph Vernet – Seaport by Moonlight. Work in the public domain)
4: Moonlight is amazingly bright. If you've only ever been outside on a night with full Moon in a place with street lights, you may not have noticed this, but the Moon reflects a lot of light from the Sun. The full Moon gives off enough light to read by and easily find your way around with. It also means you see far fewer stars on night when the Moon is out: the dimmer stars just can't compete with the far brighter Moon. Moonlight casts clear shadows on the ground, and when it's partially clouded it often illuminates one side of the clouds but not the other, creating a very beautiful effect.
(Image credit: Claude Schneider)
5: You can see Earthlight on the Moon. Given how bright moonlight is, it stands to reason the Earth shines even brighter on the Moon. The Earth is four times the size of the Moon, so it has a sixteen times bigger surface to reflect light with. Added to that is that the Earth's surface simply reflects more light than the Moon: the Moon's surface consists of dark grey rock, while the Earth has lots of bright white clouds that reflect far more light. Oddly enough, the oceans are actually quite dark and don't reflect much light; the difference lies mostly in the clouds. All this means Earthlight on the Moon can get up to forty times brighter than Moonlight on Earth. This is actually bright enough to see the Moon illuminated by it from Earth with the naked eye (though binoculars or a telescope make it easier).
Full Earth gives off forty times more light than full Moon, so you'd think full Earth is the best time to observe Earthlight. Unfortunately, when it's full Earth, it's new Moon which means the Moon is very close to the Sun in the sky and invisible. The best time to look for Earthlight is when the Moon is a crescent: the Earth is still close to full seen from the Moon then, but the Moon is visible at night near sunrise or sunset. By the time it's half Moon, Earthlight is getting weaker (since it's also half Earth then) and the Sun's daylight on the Moon makes it harder to see the earthlight on the Moon's night side anyway. It's still visible with binoculars or a telescope at half Moon, but usually not with the naked eye.
(Image credit: NASA)
6: The Moon is the largest moon in the Solar System compared to its planet. It's not the actual largest (It's a big one, but three moons of Jupiter and one of Saturn are bigger), but all other moons are absolutely tiny in comparison to their planet. Mars has two moons the size of mountains. Jupiter, Saturn, Uranus, and Neptune also have big moons, just like Earth, but since these planets are all far larger than Earth, their moons are still tiny compared to them. Our Moon, on the other hand, is only four times smaller than Earth, and eighty times lighter. This is so close some people don't even consider it a moon, but instead consider Earth and Moon two planets that orbit each other (a double-planet). The Moon is certainly the size of a small planet: Mercury, the smallest planet, is only 1.5 times larger (and actually looks a bit like the Moon). The reason the Moon is so big compared to the Earth is probably the unique way it was formed: the other moons in our Solar System probably weren't formed by a planetary collision.
(Image credit: NASA and Buzz Aldrin)
7: Twelve humans have walked on the Moon. Their names are Neil Armstrong, Buzz Aldrin, Pete Conrad, Alan Bean, Alan Shepard, Edgar Mitchell, David Scott, James Irwin, John Young, Charles Duke, Eugene Cernan, and Harrison Schmitt. All of them did so between 1969 and 1972. However, India, China, and Russia all have plans for a manned Moon landing in the early 2020s.
During the six Apollo missions where a manned Moon landing happened, a bunch of stuff was left on the Moon. There's six American flags (which are by now probably just pieces of white cloth due to the Sun's radiation bleaching them), a lot of footprints (there's very little erosion on a world without air or water), six landing modules, six crashed ascent modules, and the three Moon buggies the Apollo 15, 16, and 17 crews used to ride around on the Moon and make bigger trips than they could walking. These are the locations where the Apollo missions landed; they're all visible with the naked eye on the Moon:
(Image credit: NASA)
By the way, if you're in the southern hemisphere you're usually seeing the Moon upside-down compared to this picture.
Well, I guess that's enough about the Moon for now. I hope you found it interesting.