The MC Line in Astrocartography

The Midheaven is the degree of the zodiac crossing the local meridian above the horizon. Which degree that is depends on one quantity, local sidereal time, and local sidereal time changes with longitude only: two places on the same meridian share it, whatever their latitude. So the set of places where a given degree culminates at one moment is a whole meridian, and every MC line on the map is a straight north–south line at a single longitude. The map draws it from 66°S to 66°N in half-degree steps.

The lines are spaced by right ascension, the coordinate measured along the sky's equator (the Earth's equator projected outward), not by zodiac degree. Because the zodiac is tilted 23°26′ to the equator, one degree of the zodiac near 0° Cancer or 0° Capricorn covers about 1.09 degrees of right ascension, and near 0° Aries or 0° Libra about 0.92. Steve Jobs's Jupiter (20°30′ Cancer) and Uranus (24°08′ Cancer) are 3°37′ apart in the zodiac, but their MC lines, at 90°47′W and 86°56′W, are 3°50′ apart. Chicago lies between them, 59 km from the Uranus line.

A planet's MC line also pins down its IC line, which is the same meridian moved 180 degrees around the globe. When MC lines cross the Americas, the IC lines of the same planets cross Asia or the Indian Ocean.

The Sun never strays from the ecliptic, so its MC line is exactly where the Sun stood on the meridian: where it was local apparent noon. Steve Jobs was born at 7:15 PM Pacific Standard Time on February 24, 1955, which was 03:15 UT on the 25th. At that instant the noon meridian was 134°34′E, through western Japan. Osaka, 84 km east of the line, had an apparent solar time of 12:03.

Every other MC line on the map has the same meaning for its own planet: the meridian where that planet's degree was at the top of the sky. Steve Jobs's Neptune line at 3°05′E runs through Algiers and passes 54 km from Paris; his Saturn line at 25°45′E runs from 27 km off Bucharest in the north to 14 km off Port Elizabeth in the south. One meridian serves both hemispheres.

The Midheaven was the mesouranēma of Hellenistic astrology, one of the four angles, and William Lilly's Christian Astrology (1647) gives the tenth house kings, princes and judges, honor, preferment, office, and a person's profession or trade. Astrocartography, which Jim Lewis made popular with the Astro*Carto*Graphy maps he began drawing for clients in 1976, carries that house reading to places: along an MC line, the planet is read as the one in the career seat.

Tradition then sorts the planets by nature. Ptolemy's Tetrabiblos (Book I, chapter 5) calls Jupiter, Venus and the Moon beneficent, Saturn and Mars maleficent, and the Sun and Mercury able to act either way. Uranus, Neptune and Pluto were found in 1781, 1846 and 1930, so what astrologers say about them is modern convention with no classical source. The notes below are readings, not forecasts.

Some guides say MC and IC lines can be trusted when the birth time is uncertain because they are less time-sensitive than the curved lines. They are not. A later birth time turns the Earth further under the same sky, so every line on the map slides west: 1°00′ of longitude for four minutes of clock time, 3°45′ for fifteen. At a given latitude an ASC or DSC line slides exactly as far.

The only difference between lines comes from the planets' own motion during those minutes, which trims the shift slightly. The Moon moves fastest, so its line moves least, 3°37′ in 15 minutes. Venus's moves 3°44′, and the other eight move 3°45′, the Earth's turn alone to the arc-minute. In distance, 15 minutes is 418 km at the equator, 330 km at San Francisco's latitude and 209 km at 60°N.

The map draws zodiacal lines: an MC line is where the planet's degree of the ecliptic culminates. The planet itself usually sits a little north or south of the ecliptic, and the meridian where the body culminates, the in-mundo MC line, is where local sidereal time equals the body's own right ascension. The gap grows with the planet's ecliptic latitude. On Steve Jobs's chart, Pluto was 10°32′ north of the ecliptic and its in-mundo MC line falls 3°49′ east of the zodiacal one; the Moon, 5°02′ north, lands 2°00′ west. The Sun has no ecliptic latitude, so its two lines coincide.