Currently, The Moon is approximately 239,000 miles from The Earth. However, because of gravitational effects and tidal acceleration, The Earth's rotation is slowing and The Moon is moving away by about 3.8 cm a year. At this rate, in about a billion years or so, the distance from the Earth to the Moon will have increased by almost 25,000 miles.
Narrator: At last count, over 150 moons populate our solar system. Neptune claims 13 of them, Saturn has 48 and Jupiter hosts an astounding 62. Earth, on the other hand, has just one, but it's a special one. Our moon, Luna, as the Romans named it, is remarkable in its size. It is by no means the largest moon in the solar system, several others are bigger. One of Saturn's moons, Titan, for instance, is twice the size; but our moon is the largest in relation to its host planet.
William Hartmann: It's a quarter of the size of The Earth, it's really big compared to The Earth. If you looked through a telescope at The Earth from a distance, you'd see The Earth and this other big thing. If you look at Jupiter, or any other planet, you got the big planet and then tiny little moons right next to it. So our moon is so much bigger … it's the only one of the now eight planets that has that situation.
Narrator: The relative sizes of the two bodies are close enough that some astronomers go so far as to refer to the Earth-Moon system as a double planet.
Narrator: But while a space suit could protect lunar tourists against the vacuum of space, a lack of oxygen, temperature extremes, and lethal solar radiation, a potential hazard a space suit would do little to protect against is high velocity micrometeorites. They pummel the lunar surface frequently.
William Hartmann: Tiny meteorites, the little ones that burn up in our atmosphere and make shooting stars, those - there's no atmosphere on The Moon, they're coming right down, hitting the surface.
Narrator: They pulverize the lunar surface, generating a dusty blanket of gravel-like material called "regolith".
Narrator: Really big meteorites have also hit The Moon in the past. In fact, these massive impacts are responsible for the dark circular regions on the lunar surface, the shapes that, to many observers, seem to be arranged like the eyes, nose and mouth of a human face - they make up the illusion of the man on The Moon. They blasted huge basins in the moon's surface, some of them are 700 miles across. Dark lava eventually burst through at the impact points and flooded the basins. Today, we call these dark regions "Maria", the Latin word for seas. This dates back to the 17th century when Renaissance-era observers looked up at The Moon. They speculated that the dark areas might be oceans, and astronomers of the time gave the many dark spots, or seas, whimsical names.
William Hartmann: When we see mountain ranges on The Earth, they're mostly caused, first, by the continents moving around, crashing into each other very slowly, buckling up and creating mountain ranges, which then erode into all the spectacular shapes that we see, the Matterhorn and so forth, from water erosion.
Narrator: On The Moon, there are no continental plate tectonics, the surface is static.
William Hartmann: Yet you still have mountains on The Moon, and the reason is that those big impact basins - and that's a big explosion, excavates a pit, throws material up on the outside - so you have these rings of mountains, arcs of mountains, that surround the impact sites. But they're impact features, and they're caused by the external forces, not the internal forces.
Narrator: For 15,000 years at least, man revered The Moon as a source of light, as a navigational guide, as a reference in agricultural pursuits, and most of all, as a convenient timekeeper. In the days before our modern systems, timekeeping was no simple task. Early timekeepers had two choices : they could monitor The Sun or The Moon.
Dana Mackenzie: If you think about trying to keep track of dates, if you use a solar calendar, like we do nowadays, there are 365 days in a year, and that's an awful lot of days to keep track of - and it's not something that the ordinary person can do very well. Compare that with a lunar calendar : everybody can tell when the full moon is, when the new moon is. You don't see The Moon at new moon, you see it as big and round at full moon. So, it's easy to tell, there are only 28 to 29 days in the lunar cycle, so it's easy to count them - and so, most societies actually start out with a lunar calendar.
Narrator: The Moon stabilizes Earth's climate. The gravitational effect of The Moon keeps the degree of tilt in The Earth's rotational axis constant. This tilt is what maintains the repeatable cycle of seasons as The earth orbits The Sun.
Robin Canup: If we didn't have The Moon, or if we had a much smaller moon, for example, then you can mathematically show that the tilt of our North Pole would vary widely, with that angle going from say zero to 90 degrees - currently it's 23 ½ degrees - and it would actually vary chaotically. And so The Moon has played an important role in the stability of the axis of rotation of our planet, and therefore, in our climate.
Narrator: In 455 BCE, the Greek scholar, Anaxagoras, theorized that The Moon was simply a rock that was flung off by The Earth. Most of his contemporaries, on the other hand, were convinced that The Moon was a god, or maybe a huge ball of fire. So Anaxagoras' notion did not get much traction. A quiet speculation no doubt continued , but no hard information about The Moon came until 1609, when Italian astronomer, Galileo Galilei, pointed one of the first telescopes at The Moon, and recognized that he was looking at a landscape, the terrain of another world.
Narrator: In the last third of the 19th century, advanced theories about the origin of The Moon started to emerge. In 1873, French scientist, Edouard Roche, proposed that The Moon simply formed alongside The Earth out of essentially the same nebular cloud of particles and gases. But this idea had a fundamental weakness : The Moon has a much lower iron content than The Earth, it's much less dense.
William Hartmann: The big thing to remember about The Moon and its composition is that it doesn't have any iron core, like The Earth does. So you look at The Earth, and there's a very large central area, something like half the inside of The Earth, is iron, nickel iron - and that's metal that drained down to the center of The Earth when The Earth was hot, when it formed at the beginning. The Moon is more like just plain rock.
Narrator: In 1878, George Darwin announced his fission theory of lunar origin. This idea received some attention, in part because Darwin had a celebrated father : Charles Darwin, author of Origin of Species. In time though, George Darwin stepped out of his father's shadow and became known as England's leading expert on tides. And through extensive analysis of the tide-Moon relationship, George Darwin came to the realization that The Moon is gradually moving farther and farther away from The Earth.
Dana Mackenzie: It wasn't proved until 95 years later when astronauts landed on The Moon. They put little mirrors on The Moon, and you could shine a laser at The Moon and the laser will bounce off the mirror and come back and you can actually measure the exact distance between The Earth and The Moon. And the rate at which the distance is increasing is 3.8 centimeters per year.
Dana Mackenzie: He was one of the first Americans to actually get a doctorate in astronomy. He went to Germany and got a Ph. D. in astronomy which was, you know, almost unheard of back then. The U.S. was still, scientifically, a total backwater in the 1800's.
Narrator: See's duty station, Mare Island, had an observatory, and the Captain's job allowed him plenty of time to theorize. He had spent time analyzing both the co-accretion and the fission hypotheses regarding the origin of The Moon, and he wasn't convinced. Gradually Thomas See developed a completely different idea, it came to be called the "capture theory'. Essentially, See theorized that The Moon had actually formed in a different part of the solar system from The Earth, that it orbited The Sun, just like the other planets, but that at some point it had moved to close to Earth and was captured by Earth's gravity.
Narrator: The Moon rocks soon began to tell a fascinating tale. First, the rocks and soil samples contained particle indicating that The Moon must have been covered by a deep ocean of lave after it formed. This notion was reinforced by the discovery that the rocks were lacking significantly in what scientists call "volatile elements".
Robin Canup: Volatile elements are those that can evaporate easily, and therefore be lost when you heat up a rock - and some examples of volatile elements include water or potassium, for example. And if you compare the bulk Earth rocks to lunar rocks, you find that the lunar rocks are extremely parched. It's as if they've been heated and they've lost a lot of their volatile elements.
Narrator: But along with these stark contrasts, the lunar samples also revealed at least one astonishing similarity between the lunar surface and rocks and soil from Earth.
William Hartmann: The isotopes of individual elements, like oxygen in particular, you have different forms of oxygen. The Moon had exactly the same ratios of different forms as The Earth did. But all the other rocks we knew from elsewhere in the solar system, which are meteorites that fall out of space, all have different oxygen isotope ratios, which tells you that The Moon material and The Earth material are very, very similar.
William Hartmann: We realized during this mapping in the sixties that the big basins are actually impact features, very large asteroids hit The Moon and made these huge explosions, some of those are 600 miles across. How big an object does it take to do that, something like a hundred miles across. So we had large objects running around in the inner solar system as The Earth was forming and they were crashing into planets.
Narrator: For Hartman, the notion that 100-mile-wide asteroids once impacted planets begged a couple of questions : could planet-sized objects have ever collided, and could that have something to do with the formation of The Moon? By 1972, Hartman and fellow Tucson astronomer, Don Davis, had created a computer program to help them explore these ideas. The program made a rough attempt to simulate the accretion process in the early solar system. The astronomers wanted to see if any other planetary bodies formed near The Earth that could have crashed into it.
Narrator: In 1974, a new hypothesis explaining the origin of Earth's moon debuted on the world scientific stage. Its proponents dubbed it "the giant impact theory".
Robin Canup: The basic idea is that about 4 and a half billion years ago, Earth collided with an object roughly the size of the current planet Mars. It was a very large collision and it started The Earth spinning. It's what gave us our current 24-hour day, we believe. And this collision was so massive that it launched material into orbit around The Earth, and it's from that material that we believe The Moon later coalesced.
Robin Canup: Within a day after the impact, The Earth had reassumed a basically spherical shape, and any depression that the impact caused would have been smoothed over.
Narrator: Giant impact theory originator, Bill Hartman, presented his hypothesis at a scientific conference in 1974, but it received little attention for nearly a decade. Interest in The Moon dropped off sharply at the end of the Apollo missions. Finally, at a lunar conference in Hawaii, in 1984, 12 years after the last moon shot, the world's foremost astronomers reached a consensus. The giant impact theory was the most acceptable explanation to date for the origin of The Moon.
Dana Mackenzie: I think there are still some people who say, "Okay, it's not really proven yet." But, it's what I would call the default theory. If you look at any textbook of planetary science or geology, it's just the theory that's accepted, and there are certain small discrepancies which have yet to be explained - but, by and large, it works.
Narrator: Even with an agreed upon theory of how our moon came to be, scientists have not finished studying our closest cosmic neighbor by any means. In fact, talk of returning to the lunar surface with a permanent manned outpost has recently emanated from the White House and NASA headquarters. Such a base could provide a place to train astronauts to live in space long-term, as well as provide a more efficient launch point for eventual missions to Mars. But such lofty scientific goals are far from the minds of average residents of Earth, who occasionally peer up at the glowing, mesmerizing lunar disc in the night sky. For us, talk of lava oceans, regolith-forming micrometeorites and giant impacts will never diminish the fascination and romance of our mysterious neighbor-world, The Moon.
Narrator: It's so close. For thousands of years man has found comfort in its presence. It's been a beacon for nocturnal travelers, a timekeeper for farmers and a location finder for sailors at sea. For some cultures it's even been a god. It's the only cosmic body ever visited by human beings and today NASA is planning a permanent outpost there. But how did it get there in the first place? How did the Moon come to be? The answer is more astounding and spectacular than most residents of Earth have ever imagined.
The Moon is the only satellite in the solar system that perfectly eclipses The Sun; in fact, total, or spectacular, solar eclipses are extremely rare events anywhere in the universe. They happen here because of the apparent match in size of The Moon and The Sun. This illusion occurs because The Sun's distance from The Earth is about 400 times The Moon's distance, and The Sun's diameter is about 400 times the diameter of The Moon. Thus, because the ratios of the two objects are approximately the same, the sizes as seen from The Earth, appear to be approximately the same as well. This was not always, and will not always, be the case. Millions of years ago, The Moon was too close to The Earth for this illusion to occur, and millions of years from now it will be too far away, so enjoy it while you can.
Scientist believe that the moon was formed from the debris when a Mars sized object collided with the Earth.
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