Earth's Moon: Why One Side Always Faces Us

[Music] what if the moon merely rotated we here on earth would be able to look up and see all sides of the moon right of course Earth spins - but to simplify our discussions let's hold it still for now also let's add a yellow line so to make the rotation of the moon more apparent note that as the moon rotates the line goes from vertical to horizontal to vertical to horizontal and and so forth that means it's rotating what if the moon didn't rotate and it only revolved around us like this would still be able to see all sides of the moon right what if the moon both rotated and revolved how about like this look carefully and you'll see that yes we still get to have a 360-degree view of the moon no hidden sides but what if the moon rotated just as fast as it revolved like this ooh check this out the moon is rotating right look at the line if you don't believe me vertical horizontal mm-hmm right but it's rotating at a rate that matches the revolution just so so that only one side of the moon ever faces earth and that's exactly the situation we find ourselves in today the only side we ever see we call the near side the side we don't ever see we call the far side actually the first time we ever saw the far side was from photographs sent by a Russian lunar spacecraft in 1959 but no human ever saw the far side directly until the Apollo 8 mission in 1968 but why don't we here on earth ever see the far side why does the moon spin exactly as fast as it revolves good question and I mean very good question we could spend weeks studying this one question but I'll just give you the quick explanation hmm for starters let me tell you it's no coincidence Earth's gravity gets weaker with increasing distance right yeah but over the distance of a few meters or even kilometers this difference is not that much the Earth's pull Oh gravity on your head is for all intents and purposes the same as it is on your feet the moon however is thousands of kilometers wide so the near side of the moon experiences a significantly stronger gravitational field from Earth than does the far side what this means is that the moon's center of gravity relative to Earth is not the same as its center of mass the center of gravity huh the moon's center of gravity is the point upon which all of Earth's gravity appears to act you could scrunch the moon to the single point and gravitationally speaking there'd be no difference if Earth's gravity acted uniformly across the moon the center of gravity would be the same as the moon's the center of mass which is the average point of the mass of all the atoms within the moon yeah but because Earth's gravity is stronger on the near side of the Moon the moon's centre of gravity is shifted toward the earth like this now the moon's center of gravity can be considered to be the point upon which the earth tugs if the moon rotated a bit faster or slower the center of gravity would be lifted away from the earth and you know what happens to things lift it away from the earth that's right they're pulled back down the earth would apply what we call a torque which would align the moon right back to a position where the center of gravity is directly below the center of mass kind of like this this is what prevents the moon from rotating any faster or slower than it does the moon can only rotate so fast because earth won't let it do of wise as a consequence the moon rotates only once for every revolution and only one side of the moon ever faces us the difference is in Earth's gravity between the near and far side of the moon has other effects as well for example it has the effect of stretching the moon into a somewhat oblong shape this happens because the force of gravity isn't applied uniformly Earth's uneven gravity acting on the moon causes an oblong ish deformation remarkably the moon has the same effect on earth this is most noticeable with the Earth's oceans which are relatively easy to stretch as the Earth rotates beneath this oblong shape the sea level goes up and down as we pass under each bulge we experience high tide twice a day because there are two bulges the two low tides from when we pass under the narrow regions of this oblong distortion this oblong distortion tends to hold the oceans still relative to the moon so there's actually a bit of friction between the ocean floor in the water as the Earth rotates this friction combined with a small torque on us by the moon is slowing our rate of spin as I said earlier back in the time of the dinosaurs the day was about 19 hours long in a billion years or so a day will slow down to about 46 hours at that point only one face of the earth will face the moon much like only one face of the moon now faces the earth by the time this happens the moon will be much farther away because of the conservation of angular momentum in which we haven't talked about but like a fast spinning ice skater slowing down needs to throw her arms outward the moon slowing down earth can only do so by moving itself outward then the earth and the moon will be completely locked in there dance around each other right now the earth kind of shy and always turning away from the moon's constant gaze but when Earth finally returns that constant gaze when we're finally completely locked together the moon will appear at a single location in our sky day and night people on opposite side of Earth won't ever get to see the moon unless they go on vacation to the moon view side of our planet where I'm sure real-estate prices will be higher along with the sea level in a billion years from now we still be around oh I got a question for you does the far side of the Moon ever see the Sun think about that oh and can you ever see the Earth from the far side of the Moon lastly if you were right here and you looked straight up what would you see would it ever stray from being directly above you ooh good questions good science to you [Music]

Channel: Conceptual Academy

Views: 1,351,176

Rating: 4.5113416 out of 5

Keywords: Moon (Celestial Object With Coordinate System), Earth (Planet), Astronomy (Field Of Study), gravity, ocean tides, day length, moon, earth's moon

Id: j91XTV_p9pc

Length: 7min 58sec (478 seconds)

Published: Thu Apr 02 2015