The average distance between the Earth and the Moon is 380,000 kilometers. How is this measured?

The distance between the ball and the celestial body has always been the dream of ancient people at home and abroad. There are also many people who try to calculate the distance in the sky, like Li Bai, who once calculated that he could go up to the sky and catch the stars a hundred feet above the ground, so he built a building, but unexpectedly it fell down due to poor quality. Maybe Li Bai is better at drinking, practicing swordsmanship, writing books, and doing astrophysics. I think our Dabaichi Village is to commemorate this event. But Li Bai and Hammer, who are naturally proud, will not review themselves. They can only say that they are making further progress.

This incident tells us that we must be honest and trustworthy and choose a good construction team for construction. More than 2,000 years ago, the ancient Greeks used geometric methods to measure the distance from the earth to the moon (we can check the details online). The measured distance at that time was 410,000 kilometers. The average distance measured now from the earth to the moon is 380,000 kilometers. See See how big the error is? It was not until Apollo 11 landed on the moon on July 21, 1969 and placed the first angle reflector on the lunar surface that humans could accurately measure the distance between the earth and the moon. After that, four more were installed. How accurately can they measure distances? All in all, according to measurements, the Moon moves 3.8 centimeters away from the Earth every year. It is said that before the astronauts returned to the moon, measuring stations on the ground measured the distance between the earth and the moon using lasers.

China did not receive the signal until June 8, 2019, when the laser was sent to the moon. In the end, it took five months to observe all the signals reflected by the lunar pentagonal reflector. Corner reflectors are similar to bicycle taillights. Everyone has experience. When driving at night, no matter which direction the car light shines on the bicycle taillight, the taillight can reflect the light in the opposite direction. This is because the light is composed of multiple sets of three mutually perpendicular plane mirrors. , it is not difficult to prove based on optical knowledge that when a plane mirror faces light from any direction, the outgoing light reflected three times by three mutually perpendicular plane mirrors must be parallel and opposite to the incident light. The same is true for angle mirrors. As long as we shine light on it and record the time when the light arrives, and know the speed of light, we can calculate the distance to the moon according to the formula S=vt.

This may seem like a task that can be accomplished with a flashlight, but the problem is much more complicated. The light of ordinary flashlights is blocked by the atmosphere during transmission and will disappear quickly. Some people cannot see it even hundreds of meters away. It can be said that lighting up the moon is a one-way journey. To complete such a task, only lasers can be used. Nowadays, ordinary people can have lasers, but there are only a handful of countries that can actually measure the distance between the earth and the moon. Therefore, measuring the distance between the earth and the moon also has high requirements for laser technology.