A Chart Ruler refers to the astrological planetary body that rules the zoidion (zodiac sign) of the ascendant in a given horoscope. For example, if a horoscope has Cancer rising on the ascendant, the Moon is the chart ruler. In Natal astrology, astrologers consider the house and sign placement of this planet to be of special importance in the shaping of a persons identity. In Mundane, Horary and Electional astrology the ruler of the ascendant might not be as important as the ruler of the house most involved with the question asked if it is different from things related to the ascendant.
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Chart Rulership Wikipedia(Text) CC BY-SA