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 Sun looks like circle with a dot in the middle of it. Historians aren’t sure what it represents any more, but it’s the same symbol as the one used by the ancient Egyptians to represent Ra… the Sun god. It’s also possible that it looks like a shield.
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.
And Wikipedia has a great list of all the astronomical symbols.
We have recorded an episode of Astronomy Cast just about the Sun called The Sun, Spots and All.