Lunar arithmetic is a way of counting and doing math using numbers from the phases of the moon. When you look at the moon, you will see it changing shape (or phase) over time. These changes create 8 different moon shapes. We can use these 8 shapes to make numbers. Lunar arithmetic uses these numbers to help us add, subtract and do other math calculations. It can be a little tricky at first, but once you learn how it works, it can be fun to do math this way!