Beijing time, September 3, according to foreign media reports, we have long been used to using the earth’s position relative to the sun to distinguish the four seasons of the year. It’s just that, while the earth’s journey around the sun is very important to humans on earth, it’s not much compared to the epic journey of the sun – and our entire solar system – around the center of the Milky way. According to Keith Hawkins, an assistant professor of astronomy at the University of Texas at Austin, it takes about 220 to 230 million Earth years for the sun to orbit around the Milky Way – the time it takes for the earth to make one revolution around the sun. In other words, if we use the Galactic “clock” to measure time, our earth is only about 16 years old (in terms of galactic or cosmic years), and the formation of the sun is only 20 years old, and even the whole universe is about 60 years old. The process of the solar system’s orbit around the Milky way is very similar to that of the earth around the sun. However, Hawkins explained that the solar system does not revolve around a star, but around a supermassive black hole in the center of the Milky way. And this supermassive black hole has a huge gravitational pull on all the matter in the galaxy. < / P > < p > “the sun’s revolution is very fast – about 230 kilometers per second, or 500000 miles per hour, and the sun is constantly circling around the center of the galaxy” so it won’t be pulled toward the black hole, Hawkins said. The Galactic year represents a larger unit of time than the earth year. But in the Milky way, this unit of time is not uniform. What we earth people call the Milky way is relative to the position of the earth in the galaxy. < / P > < p > “we can say that a galactic year is about 220 million or 230 million Earth years. But on other planets in the galaxy, their galactic year is different, “Hawkins said. The diameter of our galaxy is about 100000 light-years, and our earth is 28000 light-years away from the center of our galaxy. “If you think of the galaxy as a city, then our planet is probably in the suburbs,” Hawkins explained. If it is a star orbiting near the “center” black hole, then a galactic year is relatively short. But in the suburbs, where our solar system is located, “the Galactic year is a little bit longer,” he said. < / P > < p > similar rules apply to different planetary year lengths. Mercury, for example, is the innermost planet in the solar system. It takes only 88 Earth days for mercury to orbit the sun. Uranus, the seventh planet in the solar system, orbits the sun for 84 Earth years. Pluto, the distant dwarf planet, has a period of 248 Earth years. < / P > < p > although the physical mechanism of planetary orbits is similar to that of shaping the orbit of the solar system around the Milky way, there is still a question worth mentioning: how do astronomers calculate the length of the Galactic year? Hawkins said that in fact, this is a fairly basic science, which has been very clear since the early development of modern astronomy. < / P > < p > “it’s mainly about observing the movement of stars around the Milky way,” he said. “You can observe the movement of stars around the Milky way, and then deduce the length of the Galactic year from the velocity and direction of other stars.” Didi Qingju bicycle has entered 150 cities