Astronomers and astrologists have used various symbols to depict all of the planets, and many of the minor objects in the Solar System. Perhaps two of the most commonly used are the Sun and Moon symbols.
The symbol for the Moon is… a picture of the Moon. Specifically, the symbol for the Moon looks like a crescent Moon in the last quarter. This symbol is very obvious, as it’s what ancient peoples saw in the sky for thousands of years, and it’s the same thing we see today.
Astronomers use both Sun and Moon symbols when they’re writing research journals. It’s much faster to just put in the symbol for the object.