Light-year refers to the distance that light can propagate in a year in a vacuum environment. This distance is about 9.46 trillion kilometers. It is a unit of considerable distance. With the development of human space technology in recent years, light-year is becoming more and more popular, just like the smallest unit of length in the world. Many references have been made, and light-year is mistaken for a unit of time by many people who are not familiar with this astronomical unit because it contains the word “year”. Even in the bullet screen websites on the Internet, light-year, a unit of length, is spoofed as an IQ testing unit.

The largest distance unit is not light year, but second difference, 1 second difference = 3.08568 *10 M = 3.2616 light year.