The International Astronomical Union (IAU) is a group of scientists who study and understand things about space, including stars and planets. They meet together to talk about their discoveries and make decisions about how to name and organize all the things they find in space.
One thing that the IAU does is create something called a "circular." A circular is a document that they send out to scientists all over the world. It's like a letter that tells them about important things they need to know or do. It's kind of like when your teacher sends a note home with you from school to tell your parents something important.
In the case of the IAU, when they send out a circular, it usually has information about new discoveries in space or changes they want to make to how things are named or classified. It's important for scientists to read these circulars because they help them stay up to date on all the new things happening in space.
The circulars can contain information about things like new planets that have been discovered, new stars that have been named, or new scientific theories that have been developed. For example, scientists might discover a new planet and the IAU would send out a circular to let everyone know about it. They might also decide to change the name of a star, so they would send out a circular to announce the new name.
The circulars are sent out all over the world and scientists read them to learn and share information with each other. It's like a big network of people who are all interested in space and want to learn more about it. So the circulars help scientists stay connected and work together to understand the universe better.
In summary, the IAU circular is a letter that the International Astronomical Union sends out to scientists all over the world. It contains important information about new discoveries and changes in space that scientists need to know. It helps scientists stay connected and work together to understand the universe.