The Facts Behind Planetary Symbols (As best as mythology will allow.)
Classical Planets - Those Planets Known to the Ancient Astronomers:
1) The Sun: Depicted as a shield with a circle inside, this symbol can be seen in the hieroglyphics of ancient Egypt, which were used to represent the sun god, Ra. Most symbols used for the Sun throughout history have contained the simple element of a circle. The element, gold, is associated with the Sun.
2) Mercury: The symbol represents the Roman, fleet-footed messenger god, Mercury, with the horns at the top of the circle representing the wings on Mercury’s cap. The planet was associated with Greek God, Hermes, also a messenger, but one who did not start out that way from birth. The symbol is also used to represent the element, mercury, in medieval times.
3) Venus: A stylized hand mirror for the Roman Goddess, Venus. The Greeks associated the planet with Aphrodite and Athena, all beautiful women in mythology. It is also the symbol most would recognize denoting the female gender. The symbol was also used in medieval alchemy for the element, copper.
4) Earth: The symbol depicts our globe bisected by latitude and longitude lines. It was also used by the ancient Greeks to symbolize a sphere. Earth was considered one of the four basic elements (Fire, Water, Earth and Air) by the ancients.
5) Mars: The shield and spear of the Roman god of war. The Greeks associated the planet with Ares, also a god of war. (Could the color of blood and the color of Mars have helped make this connection?) The symbol also represents the element, iron and is used to designate the male gender.
6) Jupiter: Some report that the symbol represents the arm of Jupiter (or Zeus, for the Greeks) holding a lightning bolt. The symbol is also the alchemist's symbol for the element, tin.
7) Saturn: The symbol may represent a stylized sickle that the god of the harvest may have used. Alchemists associated Saturn’s symbol with the element, lead.
Planetary Symbols of Modern Planets - Considered as Such Since They Were Unknown to the Ancients.
8) Uranus: To the ancient Greeks, Uranus (or Ouranos) was literally the sky, including all the stars and the heavens. The symbol appears to be a combination of the Sun and Mars’ symbols and is sometimes associated with platinum.
An alternate astrological symbol that is sometimes used for the planet shows a stylized “H” for the astronomer, William Herschel, who discovered the planet in 1781.
9) Neptune: The trident - the weapon typically shown with the Roman god of the sea. Alchemists sometimes used the symbol to represent the element mercury or the compound, lime.
10) Pluto: The monogram constructed from the first two letters of the planet’s name (It was considered a planet when it was discovered in 1930 by Clyde Tombaugh.) and the first initials of the astronomer who predicted its existence, Percival Lowell.
Pluto has an alternate astrological symbol that is sometimes used:
11) Earth’s Moon: Symbolized by the crescent and represented the element, silver.
Alchemy symbols: http://www.nationmaster.com/encyclopedia/Alchemical-symbol
Planetary symbols: http://solarsystem.nasa.gov/multimedia/display.cfm?IM_ID=167