The IC Line in Astrocartography

The Imum Coeli, "bottom of the sky", is the degree of the zodiac on the meridian below the horizon, directly opposite the Midheaven. Because the two points are always 180 degrees apart in the zodiac, each IC line on the map is its planet's MC line carried halfway around the globe: the same straight north–south meridian, 180 degrees of longitude away.

Along an IC line the planet's degree is out of sight, at the lowest point of its daily circle. That holds everywhere the map reaches. Between the polar circles every degree of the zodiac rises and sets once a sidereal day, so none of them can stand on the lower meridian above the horizon.

With the Sun on the lower meridian, it is local apparent midnight. Steve Jobs's birth, at 7:15 in the evening in San Francisco, fell at 03:15 UT, and the midnight meridian then ran along 45°25′W through southeastern Brazil. São Paulo, 124 km west of it, showed an apparent solar time of 23:55, a few minutes short of its own midnight. In San Francisco, 77 degrees of longitude further west, the Sun had set an hour and 17 minutes before.

Venus at 21°10′ Capricorn and Jupiter at 20°30′ Cancer are 0°39′ short of an exact opposition. When two planets are opposite, each one's IC degree is close to the other's MC degree, so their meridians fold together. Venus's IC line (90°04′W) and Jupiter's MC line (90°47′W) run 0°42′ apart, 60 km at 40°N, through the Gulf of Mexico and Yucatán; Mérida is 47 km from the first and 121 km from the second. On the far side of the world the pair swaps roles: Jupiter's IC line (89°12′E) and Venus's MC line (89°55′E) run the same distance apart through Bangladesh, with Rangpur 4 km from Jupiter's line and 67 km from Venus's.

Two parallel meridians never meet, so the map lists no crossing between them, though a reader standing in Mérida is near both. That is the general rule for MC and IC lines: they meet only curved lines.

IC lines are sometimes offered as the safe lines for a birth time known only roughly. They are not safer. The whole map turns with the Earth, 0°15′ of longitude per minute of clock time, and every line at a given latitude moves by that amount. A more useful way to judge a line's position is to ask how many minutes of birth time separate it from a place. For a city a few kilometers from a line, the answer is a matter of seconds, finer than any birth record.

In Hellenistic astrology the IC was the hypogeion, the angle "under the earth". Lilly gives the fourth house fathers, lands, houses, tenements, inheritance and the end of any matter, and astrocartographers read an IC line as a place where the planet colors home, family, property and private life rather than public standing. The readings below follow that house; the benefic and malefic labels are Ptolemy's (Tetrabiblos I.5), and the outer planets carry modern readings only.

IC lines on this site track the ecliptic degree rather than the body, like every other line on its map. The body itself reaches the lower meridian when local sidereal time equals its right ascension plus 180 degrees, and for a planet off the ecliptic that is a slightly different meridian. The gap is the same as for the MC line, mirrored to the other side of the globe. For Steve Jobs it is 3°49′ for Pluto, which was 10°32′ north of the ecliptic, 2°00′ for the Moon, and under a degree for the rest. His Pluto IC line at 124°37′E becomes 128°27′E in mundo, 293 km east at Daqing's latitude.