What does Shang mean in Shangshu?

The word "Shangshu" has many meanings. One refers to a book, one refers to an official name, and the third refers to the names of a group of constellations. As the title of the book, Shangshu means "the book of ancient times". "Shang" at this time means "ancient". As an official name, "Shangshu" is equivalent to "the secretary of the emperor". At this time, "Shang" means "Emperor". As a group of constellations, it includes five stars: "Four stars in Draco, Little Bear 1 star". Viewed from the angle of view, they are the closest to Polaris, because ancient people correspond the stars in the sky to people on the ground, Polaris to the emperor, and those five stars correspond to people around the emperor (the emperor's secretary). Therefore, this constellation is also called "Shangshu".