The DSC Line in Astrocartography

A degree of the zodiac takes as long to go from culmination down to the western horizon as it took to climb from the eastern horizon to culmination. On the map, that symmetry makes each DSC line the reflection of the same planet's ASC line across its MC line: wherever the ASC curve sits some distance west of the meridian, the DSC curve sits the same distance east. At the equator both are 90 degrees of longitude out. A degree north of the sky's equator stays up longer in the north, so its DSC line swings further east as it goes north, running from southwest to northeast; a southern degree does the opposite.

There is a second way to see it. The Descendant is always the degree opposite the Ascendant, so the places where Venus's degree sets are exactly the places where the degree opposite Venus rises. A DSC line is the ASC line of the opposite point of the zodiac. That is why oppositions in a chart show up on the map as setting and rising lines that run together.

Steve Jobs's Venus at 21°10′ Capricorn faces Jupiter at 20°30′ Cancer, 0°39′ short of an exact opposition. Wherever Venus's degree was setting, a degree within a whisker of Jupiter's was rising, so his Venus DSC line and his Jupiter ASC line run as a pair across the whole map: 79 km apart at the equator, 69 km at 40°N and 55 km at 60°N. Uranus at 24°08′ Cancer, 2°57′ from opposing Venus, adds a third curve 349 km away at the equator. On the other side of the globe the roles reverse, and his Venus ASC line runs beside his Jupiter DSC line, 51 km apart at 40°N; both pass within about 100 km of Moscow.

The pair never crosses. On a zodiacal map, a setting line and a rising line can meet only where two degrees exactly opposite each other share a horizon, and then they are one line. Measured in mundo, with the bodies' real declinations (20°04′S for Venus, 22°20′N for Jupiter), the picture changes: Venus sets as Jupiter rises along 7°33′S, and Venus rises as Jupiter sets along 7°33′N. Those are parans the zodiacal map cannot draw. The crossings and parans page lists all of them for this chart.

The Sun's DSC line marks where its center was on the western horizon at the birth moment: the sunset line. In San Francisco, the Sun's center reached the true horizon at 5:53 PM Pacific Standard Time (01:53:25 UT), according to a Swiss Ephemeris sunset calculation for the city. Steve Jobs was born 81.6 minutes later. In that time the sunset line had slid west over the Pacific, and at San Francisco's latitude it lay at 142°47′W, 1,779 km from the city. Put the other way, a birth 81.6 minutes earlier would have put the Sun's DSC line through San Francisco.

The almanac sunset for that evening, 5:57 PM, came 4.3 minutes later, because almanacs time the Sun's upper edge with refraction allowed for. The map's lines are geometric and ignore both.

Seven of the ten curves pass within 150 km of a city of a million or more:

Greek astrologers called this angle dysis, the setting. Lilly's seventh house holds marriage and love questions, but also open enemies, the opposing party in a lawsuit, and the other side in a war: it is the house of whoever stands across from you. Astrocartographers read a DSC line the same way, as a place where the planet describes partners, clients and rivals more than the person. With Ptolemy's benefics and malefics as the traditional frame, the usual readings run as follows; they are interpretations, not predictions about anyone's relationships.

The map's DSC lines are zodiacal: they follow the planet's degree of the ecliptic down to the horizon. The body itself sets where its own right ascension and declination put it on the horizon, and the gap between the two lines widens toward the poles. Pluto, 10°32′ north of the ecliptic on Steve Jobs's chart, sets 18°06′ east of its zodiacal DSC line at 50°N, 1,294 km along the parallel; at 20°S the two lines almost agree.