The Imum Coeli (most often called the IC for short) is the zodiac sign that was the lowest point in the sky at the moment the chart is calculated for (like a Natal chart). If someone is born around midnight, the Sun will be found close to the IC in their chart. It is one of the four angles in the chart, and in most house systems it is the cusp of the 4th House in the chart.
In natal astrology, the IC sign and 4th house reflect your home, family, childhood, ancestry, the past, and one of your parents (usually father).
The IC (short for Imum Coeli, latin for "bottom of the sky") is always opposite the Midheaven in the chart, and represents the North direction in the chart. Some modern astrologers associate the IC and 4th house with the zodiac sign Cancer.
