/kloc-at the beginning of the 0/7th century, many scientists thought that there was no? Light speed? Things like that, they think that light can travel any distance at a time. Galileo disagreed, and he came up with an experiment to measure the speed of light: he and his assistant each picked up a lantern with shutters, and they stood on the top of the mountain a mile apart. Galileo lit the lanterns. As soon as the assistant saw Galileo's light, he should open the shutters. Speed is obtained by calculating time and distance, but the problem is that the speed of light is too fast to be measured in this way.

Therefore, the required experimental condition is that light travels a long distance. How to establish such an experiment? In A.D. 1670, Danish astronomer Ole Romer made a very careful observation of Jupiter's moons. The black spot on Jupiter observed on the telescope is the shadow of the satellite. The satellite orbits Jupiter every 1.76 days, so the time required for each orbit is always the same. To his surprise, he found that satellites don't always appear where they should. At certain times of the year, it seems a little behind schedule; At other times, slightly ahead. Romer noticed that when the earth is closer to Jupiter, the satellite seems to have exceeded its predicted orbit, but it is farther away from Jupiter and the satellite seems to be far away. ? If light cannot travel infinitely fast, it will take some time to reach the earth from Jupiter. Suppose it takes an hour. Therefore, when you look at Jupiter through a telescope, you actually see the light left an hour ago, so you will see what Jupiter and its satellites looked like in the past hour. When Jupiter goes further, it takes longer for light to get here from there, so Romer saw the satellite earlier than usual. When Jupiter is particularly close to the earth, the opposite will happen. So the satellite didn't change its orbit at all, just saw the time of arrival here, as if it were in different positions. Romer calculates the value of the speed of light by changing the position of the satellite, and the distance from the earth to Jupiter will also change.

In the next few years, with the development of better equipment and technology, many people can measure the speed of light more accurately. With today's technical resources, we can measure it with incredible accuracy. For example, astronauts have installed mirrors on the rocks of the moon. Scientists on earth can aim the laser at this mirror and measure the propagation time of the laser pulse-it takes about two and a half seconds to go back and forth.