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What is the Distance to the Moon?

Duration: 3:21 Uploaded: 29 June 2013

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What,is,the,Distance,to,the,Moon? In this short explainer, Universe Today publisher Fraser Cain demonstrates just how far away the Moon really is, and explains the methods scientists use to discover its distance. Did you know the Moon is slowly drifting away from us? --------- The Moon is such a familiar object in the sky, but did you ever wonder how far away it is? The answer is, on average, 384,000 kilometers from Earth. It's hard to put that into context, so let's do a demonstration. Here's the Earth... and here's the Moon to scale. Now how far away would I have to stand from the Earth for the distance to be accurate. Here? Here? Nope, I'd actually have to stand way back here. The Moon is traveling on an elliptical path around the Earth. It's getting closer and further, closer and further. Its closest point, known as perigee, is only 363,000 kilometers away. Its most distant point, the apogee, is 406,000 kilometers away. That's almost a difference of 44,000 kilometers. Meaning the Moon looks quite different when it's closer from when it's more distant. When it's a Full Moon, and it's a close Moon, that's a Supermoon; also known as a perigee-syzygy. The total illumination from the Moon can change more than thirty percent from Full Moon to Full Moon, depending on how far away the Moon is when things line up. So, how do we know how far away the Moon is? Astronomers calculate the distance to the Moon using the Lunar Laser Ranging experiment. When astronauts visited the Moon more than forty years ago, they left retroreflecting mirrors on the lunar surface. When they shoot a laser at the Moon, the light from the laser is reflected right back at you from one of these devices. For every 100 quadrillion photons shot at the Moon, only a handful come back, but that's enough. The light is moving at more than 300 kilometers per second, it takes a little more than a second to make the journey. And then it takes another second or so to return. Astronomers calculate the exact amount of time it takes for light to make the journey. They know exactly how far the Moon is at any time, down to millimeter accuracy. As a result, Astronomers have discovered that the Moon is slowly drifting away from us, at a glacial 3.8 centimeters each year. Billions of years in the future, the Moon will appear smaller in the sky than it does today. Don't worry, that'll happen long after the Sun has bloated up as a red giant, and blasted the surface of the Earth, possibly even consuming it.