The ASC Line in Astrocartography

A degree of the zodiac culminates on one meridian and rises on the eastern horizon some hours earlier. How many hours depends on the latitude and on the degree's declination, its distance north or south of the sky's equator, which is the Earth's equator projected outward. The arc it travels from rising to culmination, the semi-diurnal arc, has a simple formula: its cosine is minus the tangent of the latitude times the tangent of the declination. At the equator the arc is 90 degrees for every degree of the zodiac, so every ASC line crosses the equator exactly 90 degrees of longitude west of the same planet's MC line. Away from the equator, a northern degree has a longer arc in the north and a shorter one in the south, and the line leans from southeast to northwest; a southern degree leans the other way.

The larger the declination, the more the line bends. On Steve Jobs's map the Moon, at 7°44′ Aries and only 3°04′ north of that equator, draws a nearly straight ASC line that spreads 13°51′ of longitude from 66°S to 66°N. Jupiter, at 20°30′ Cancer and 21°52′ north, spreads 128°50′: near 66°S its ASC line runs 25°34′ west of its MC line, and near 66°N 154°25′ west, closing in on its IC line. The most any degree can spread across this map, at 0° Cancer or 0° Capricorn, is 153°50′.

An ASC line is close to a meridian in the tropics and swings toward east–west as it nears the polar circle, because the semi-diurnal arc changes fastest there. Steve Jobs's Jupiter ASC line moves 0.40 degrees of longitude for each degree of latitude at the equator, 0.74 between 40° and 41°N, 3.39 on average from 60° to 66°N, and 5.26 in the last half degree. The Moon's line, with almost no declination to work with, never moves more than 0.32 degrees of longitude per degree of latitude.

The bend stops at the polar circles, 66°33′ from the equator. Past them, some degrees of the zodiac stay above the horizon all day and others never come up, the Ascendant jumps instead of turning smoothly, and the lines break. The map stops at 66°, inside that limit, where every line is still whole.

The Sun sits on the ecliptic, so its ASC line is the line of sunrise at the birth moment, and together with its DSC line it traces the boundary between day and night. When Steve Jobs was born, at 03:15 UT on February 25, 1955, that boundary on the dawn side ran 1 km from Tehran. A separate Swiss Ephemeris sunrise calculation for Tehran, for the center of the Sun on the true horizon, gives 03:14:59 UT: the line and the clock agree within two seconds.

The published sunrise for Tehran that morning was earlier, 03:10:49 UT, because an almanac counts the Sun's upper edge and the refraction that lifts it into view about half a degree early. Astrocartography lines use neither: they mark the geometric center of the planet's degree on the horizon. The same line also passes Karaj (40 km), Mogadishu (47 km) and Yekaterinburg (78 km).

The claim that ASC lines need an exact birth time more than MC lines do mixes up two things. In a birth chart, the degree on the Ascendant can move quickly: at Steve Jobs's birth in San Francisco his Ascendant, 22°17′ Virgo, was advancing 0°49′ of the zodiac every four minutes, and in the far north some signs rise in minutes. On the map, though, a line is a set of places, and every line moves with the Earth: 1°00′ of longitude per four minutes at any latitude, ASC lines and MC lines alike.

What does differ is how much that east–west slide changes a city's distance from the line. An MC line runs due north–south, so the full slide counts. An ASC line leans, and the slide moves it partly along itself. At 60°N, where his Jupiter ASC line leans 48 degrees from north–south, a 15-minute error moves it 209 km east or west but only 139 km closer to or farther from any place beside it. The Moon's nearly straight line loses almost nothing.

The Greek name of the Ascendant, hōroskopos, means "hour-marker", and it gave the horoscope its name. Lilly's first house covers the person: life, stature, complexion, form and shape. Astrocartographers carry that over to the ASC line, reading it as a place where the rising planet shows in manner, health and the impression made on strangers. The notes below follow that reading, with Ptolemy's benefic and malefic labels where they apply; they describe interpretations, not likely outcomes.

This site draws zodiacal ASC lines: where the planet's degree of the ecliptic is on the eastern horizon. The planet itself is usually north or south of the ecliptic, with a different declination, and rises somewhere else. Its in-mundo ASC line uses the body's own right ascension and declination, and the gap between the two lines grows with latitude because the declinations differ. Pluto shows it most on Steve Jobs's chart: 10°32′ north of the ecliptic, the body had a declination of 22°58′ against 13°05′ for its degree, and its in-mundo ASC line runs 5°46′ west of the zodiacal one at 40°N and 19°42′ west at 60°N.