The sign that is positioned directly opposite a planet's sign of exaltation is considered to be its fall. As the exaltation is a place of awareness for the planet, the fall is a position of weakness concerning the function of the planet, including everything else the planet represents.
The signs of planetary fall are as follows:
Sun: in fall in Libra (exalted in Aries)
Moon: in fall in Scorpio (exalted in Taurus)
Mercury: in fall in Pisces (exalted in Virgo)
Venus: in fall in Virgo (exalted in Pisces)
Mars: in fall in Cancer (exalted in Capricorn)
Jupiter: in fall in Capricorn (exalted in Cancer)
Saturn: in fall in Aries (exalted in Libra)
Uranus: in fall in Taurus (exalted in Scorpio)
Neptune: in fall in Gemini (exalted in Sagittarius)
Pluto: in fall in Aquarius (exalted in Leo)
