- 2791 - MOON - measuring the distance? - The distance to the Moon is 240,000 miles. I learned that in High School. Today the average distance is measured to be 238,856 miles. Actually that distance can be measured to within less than an inch.
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-------------------------- 2791 - MOON - measuring the distance?
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- Lasers can measure the distance between surfaces of Earth and the Moon using laser ranging. Lasers at observatories on Earth are aimed at retroreflectors planted on the Moon during the Apollo program .
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- Laser light pulses are transmitted and reflected back to Earth, and the round-trip duration is measured. The lunar distance is calculated from this value.
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- The first successful tests were carried out in 1962 by observing laser pulses reflected from the Moon's surface using a laser with a millisecond pulse length.
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- Greater accuracy was achieved following the installation of a retroreflector array on July 21, 1969, by the crew of Apollo 11, and two more retroreflector arrays left by the Apollo 14 and Apollo 15 missions have also contributed to the experiment. Successful lunar laser range measurements to the retroreflectors were first reported by the 3.1 m telescope at Lick Observatory
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- The Apollo 15 array is three times the size of the arrays left by the two earlier Apollo missions. Its size made it the target of three-quarters of the sample measurements taken in the first 25 years of the experiment.
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- The distance to the Moon is calculated approximately using the equation:
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-------------------- distance = (speed of light × duration of delay due to reflection) / 2
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-------------------- 233,000 miles = 186,000 miles / second * 1.25 seconds
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------------------- Actually 238,856 miles using more precise data.
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- To compute the lunar distance more precisely, many factors must be considered in addition to the round-trip time of about 2.5 seconds. These factors include the location of the Moon in the sky, the relative motion of Earth and the Moon, Earth's rotation, lunar libration, polar motion, weather, speed of light in various parts of air, propagation delay through Earth's atmosphere, the location of the observing station and its motion due to crustal motion and tides, and relativistic effects.
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- The distance continually changes for a number of reasons, but averages 239,228.3 miles between the center of the Earth and the center of the Moon during its monthly orbits.
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- At the Moon's surface, the laser beam is about 4.0 miles wide and scientists liken the task of aiming the beam to using a rifle to hit a moving dime 1.9 miles away.
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- The reflected light is too weak to see with the human eye. Out of 1,017 photons aimed at the reflector, only one is received back on Earth, even under good conditions. They can be identified as originating from the laser because the laser is highly monochromatic.
- This is one of the most precise distance measurements ever made, and is equivalent in accuracy to determining the distance between Los Angeles and New York to within 0.01 inches.
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- The upcoming MoonLIGHT reflector, that may be placed during an attempt by the private MX-1E lander, is designed to increase measurement accuracy 100 times over existing systems. MX-1E was set for launch in July 2020, however, as of August 2020, the launch of the MX-1E has been canceled.
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- The Moon is spiraling away from Earth at a rate of 3.8 cm/year.
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- The Moon probably has a liquid core of about 20% of the Moon's radius. The radius of the lunar core-mantle boundary is determined as 381±12 km.
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- Einstein's theory of gravity and the general theory of relativity predict the Moon's orbit to within the accuracy of these laser ranging measurements.
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- During their historic moonwalk on July 20, 1969, Armstrong and Aldrin deployed the first laser retroreflector on the surface of Tranquility Base.
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- The 46-centimeter (18-inch) square array contained 100 corner cube prism reflectors, special mirrors that send any light striking them straight back to its source. Less than two weeks later, on Aug. 1, 1969, a laser pulse aimed through the lens of the 3-meter telescope at California’s Lick Observatory successfully hit the array for the first time.
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- With today’s more sophisticated laser ranging systems, the precision of the Earth-Moon distance measurement is now approximately 1 millimeter (0.04 inch), 25 times better than the first calculations in 1969.
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- By comparison, that’s approximately equivalent to determining the distance between New York and Los Angeles to 0.01 millimeters (0.0004 inch).
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- Along with the Earth-Moon distance measurement, the devices have revealed that the moon is moving away from the Earth at a rate of 3.8 centimeters (1.5 inches) per year, our satellite probably has a liquid core, and the universal force of gravity is very stable with measurements showing that Newton’s gravitational constant has changed less than 1 part in 100 billion since the laser experiments began.
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- Hitting one of the retroreflectors with a laser beam from Earth has been compared to using a rifle to hit a moving dime 3 kilometers (1.9 miles) away. Quite a good shot wouldn’t you say!
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- August 17, 2020 2791
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--------------------- Tuesday, August 18, 2020 -------------------------
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