Julius Schiller was a man who lived a long time ago and was interested in stars and the things you can see up in the night sky. He decided to make a big book with lots of pictures and information about all the stars and constellations that you can see when you look up at the sky. He made sure to draw all the pictures very clearly, so that people who didn't know much about stars could understand them too.
Schiller's book was called the "Coelum Stellatum Christianum," which is a really fancy way of saying "A Christian Starry Heaven." This meant that he wrote his book in a way that included Christian beliefs and symbols alongside his pictures of the stars, and he talked about how the stars and the night sky can teach us important things about God and our place in the world.
Because Schiller's book was so popular and helpful, lots of people used it as a guide for looking up at the stars and understanding what they were seeing. Even today, some people still use Schiller's book to learn about stars and constellations. So thanks to Julius Schiller, we can know more about the stars and the amazing things that are hiding up there in the sky!