Meaning and Notation

In astrology this lower angle is the direct counterpart of the culminating Midheaven, sharing the same meridian axis but marking its lowest point rather than its highest, and the astrological term differs in a precise way from the strictly astronomical nadir, the point on the celestial sphere exactly opposite the zenith along the vertical circle through the observer, which need not fall on the ecliptic at all. Astrologers borrowed the everyday word "nadir" as a convenient label for the properly named Imum Coeli, the angle Hellenistic writers already tracked as the point opposite the midheaven even before the Latin cardo terminology was fixed by writers such as Firmicus Maternus in the fourth century.

Medieval astrologers working from Arabic sources, among them the tenth century astrologer al-Qabisi, known in Latin as Alcabitius, built widely used house division systems on this same meridian and horizon framework, and Alcabitius's approach became a standard of medieval European astrology for centuries. In chart notation this angle is abbreviated IC, from Imum Coeli, paired with MC for the Midheaven just as DC pairs with AC for the Descendant and Ascendant. On the standard circular wheel chart the IC sits at the bottom, directly beneath the Ascendant-Descendant horizon line, completing the four angle framework that has organized horoscopic astrology since its Hellenistic beginnings.