antiscion

from The Century Dictionary.

• noun In astrology, a sign of the zodiac equidistant with another sign on the opposite side. Signs having north declination are called commanding, those with south declination obeying.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun astrology The mirror image, or shadow, of a planet, based upon a line of reference which is the axis of 0 Cancer/0 Capricorn (the position of the sun at the solstices).