An astronomical constant is something that scientists use to help them understand the way things work in space. It's like a special rule or number that helps them do their job better.
One example of an astronomical constant is the speed of light. Scientists know that light always travels at the same speed, no matter where in the universe it's coming from or where it's going. This is a really important piece of information when studying things like stars and galaxies, because it helps scientists know how far away they are and how they're moving.
Another example of an astronomical constant is the gravitational constant. This is a number that tells us how strong the force of gravity is between two objects, like planets or stars. Scientists use this constant to help them understand how objects move around in space and how they interact with each other.
Astronomical constants are really important for scientists who study the universe, because they help them make predictions and understand things that might otherwise be really difficult to figure out. Without these constants, scientists would have a much harder time figuring out how things work in space!