The Mercury Line in Astrocartography

Mercury orbits inside the Earth's orbit, so from here it swings from one side of the Sun to the other and back about three times a year without ever getting far away. On a map that means each Mercury line sits near the matching Sun line: the Mercury MC line near the Sun MC line, the Mercury ASC line near the Sun ASC line, and so on. We measured the largest possible gap day by day from 1900 to 2050: the Mercury and Sun meridian lines never stood more than 27°26′ of longitude apart, a figure reached around July 3, 1900.

In Obama's chart Mercury stood at 2°19′ Leo, trailing the Sun (12°32′ Leo) by 10°12′. Mercury on that side of the Sun rises before it in the morning and sets before it in the evening, so it is a morning planet, and its lines run west of the Sun's: his Mercury MC line lies 10°24′ west of his Sun MC line. The birthplace shows the same thing in clock time. The Swiss Ephemeris has Mercury setting in Honolulu at 6:30 PM, 53 minutes before the birth, and the Sun's center following it down at 7:04 PM.

For reading a map this matters more than it sounds. A Mercury line should not be judged in isolation from the Sun line beside it, because a city near one is often within a few hundred kilometers of the other. On the usual readings, a Sun line with a Mercury line beside it describes a place where you are both seen and heard.

The chart is Barack Obama's, from his birth certificate: August 4, 1961, 7:24 PM in Honolulu, or 05:24 UTC on August 5, 1961. The site's API drew the lines, exactly as the map does for his chart, and our script tested every point of every line against the Swiss Ephemeris and every city in the site's list against every line.

In Obama's birth chart Mercury and Jupiter are in opposition: Mercury at 2°19′ Leo, Jupiter at 0°51′ Aquarius, 1°28′ from an exact 180°. On the map an opposition turns into two pairs of parallel meridians. Where one planet is on the MC the other is on the IC, so his Mercury MC line runs only 1°31′ east of his Jupiter IC line, and on the other side of the world his Mercury IC line runs the same distance from his Jupiter MC line.

Kolkata shows how close that is. It lies 19 km from the Jupiter IC line and 176 km from the Mercury MC line, so anyone reading his map for that city has to read both. New Orleans sits 14 km from the Mercury IC line and 133 km from the Jupiter MC line. Two lines this close behave like a crossing that runs the length of the globe: anywhere along them, both planets are near an angle at once. The crossings and parans page covers how astrocartographers read two planets angular together.

A retrograde Mercury draws its lines exactly like a direct one: the map uses the planet's position and nothing else. Obama was born 27 days after a retrograde period ended. Mercury had turned back on June 14, 1961 at 10°27′ Cancer and forward again on July 8, 1961 at 1°20′ Cancer, and by his birth it was moving 2°00′ a day, fast enough that his Mercury lines shift 1°05′ east for every day the birth date moves, against almost nothing for the Sun.

Retrograde periods matter for a different reason. Mercury's orbit is tilted to the ecliptic, and the tilt shows most when Mercury is closest to us, which happens in the middle of a retrograde, at inferior conjunction with the Sun. In our 1900 to 2050 scan its largest distance south of the ecliptic, 4°58′, came on July 26, 2045, with Mercury 0°42′ from the Sun and 0.587 AU from Earth. People born then have Mercury lines whose zodiacal and in mundo versions sit far apart; the site's map, like any zodiacal map, ignores the latitude.

Obama's Mercury, 1.25 AU away and 1°04′ north of the ecliptic, is a mild case. Drawn from Mercury itself, the meridian lines would sit 0°15′ farther east, 29 km at the equator, and the curved lines part by more toward the poles:

Find your Sun lines first, then look for Mercury's within about 27° of them on the map. If your Mercury was retrograde, check the Sun lines closely, since Mercury then sat near the Sun in the zodiac and its lines run tight beside the Sun's. If your Mercury was near its greatest distance from the Sun, the two sets of lines are as far apart as they get, and they are easier to read separately.

For how far a line reaches, Jim Lewis put the figure at a quarter-inch of his original printed map in 1976 and later at up to 700 miles each side. That outer figure brings 468 of the site's cities, 20%, within range of one of Obama's Mercury lines, far fewer than the 779 near his Sun lines even though the two sets run within about ten degrees of each other. Small shifts across coastlines and borders move these counts a long way. A birth time off by four minutes moves all four lines 0°59′ west, whichever line it is. The pre-filled map shows Obama's Mercury lines next to his Sun lines if you switch the other planets off.