![]() ![]() The Sun’s gravity attracts the Moon and Earth. The fluid core affects the directions in space of the Moon’s north and south poles, which lunar laser detects.Įinstein’s theory of gravity assumes that the gravitational attraction between two bodies does not depend on their composition. This was a surprise when discovered two decades ago because many scientists thought that the core would be cool and solid. Love).Īnalysis of lunar laser data shows that the Moon has a fluid core. Measuring how much the arrays move has enabled scientists to better understand the elastic properties of the Moon (a measurement of this, called the Love number, is named after scientist A. In fact, the positions of the reflecting arrays vary as much as six inches (15 centimeters) up and down each month as the Moon flexes. In a similar way, Earth’s gravity tugs on the Moon, causing two tidal bulges of the lunar rock. The forward force causes the Moon to spiral away from Earth by 0.1 inches (3 millimeters) each month. The gravitational force between the tidal bulges and the Moon pull against and slow Earth’s rotation while also pulling the Moon forward along the direction it moves in its orbit about Earth. There are two tidal bulges, the second one half a day later. Tides in Earth’s oceans are highest not when the Moon is overhead, but hours later. The Moon orbits Earth at an average distance of 239,000 miles (385,000 kilometers), but lunar laser ranging has accurately shown that the distance between the two increases by 1.5 inches (3.8 centimeters) a year. Laser ranging measurements have deepened our understanding of the dance between the Moon and Earth as well. For instance, cameras on spacecraft in lunar orbit can see the reflecting arrays, relying on them as locations accurate to less than a foot (a fraction of a meter). The lunar orbit and the orientation of the rotating Moon are needed by spacecraft that orbit and land on the Moon. The orbit, rotation and orientation of the Moon are accurately determined by lunar laser ranging. This allows the distance to be measured to within a fraction of an inch (a few millimeters), and scientists at the Jet Propulsion Laboratory analyze the results. Four telescopes at observatories in New Mexico, France, Italy and Germany fire lasers at them, measuring the time that it takes for a laser pulse to bounce off the reflectors and return to Earth. The longevity of the experiment can be attributed at least in part to its simplicity: The arrays themselves require no power. Along with the Apollo 11 astronauts, those of Apollo 14 and 15 left arrays behind as well: The Apollo 11 and 14 arrays have 100 quartz glass prisms (called corner cubes) each, while the array of Apollo 15 has 300. Neil Armstrong, Buzz Aldrin and Michael Collins departed from the Moon 50 years ago, but one of the experiments they left behind continues to return fresh data to this day: arrays of prisms that reflect light back toward its source, providing plentiful insights. ![]()
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