The Moon Line in Astrocartography

Obama's birth certificate gives 7:24 PM, Honolulu time, on August 4, 1961: 05:24 UTC. His Moon was at 3°21′ Gemini, 69°11′ behind the Sun in the zodiac: a waning crescent that had risen in the small hours and was below the horizon by evening. The four lines are the site's, from the API call the astrocartography map makes; our generating script checked each point against the Swiss Ephemeris and measured every city in the site's list against each line.

The Moon's orbit is tilted to the ecliptic, so the Moon spends most of the month north or south of the Sun's path; between 1900 and 2050 it reached 5°18′ on either side. It crosses the ecliptic at two points, the lunar nodes, and is farthest from it midway between them. Obama's Moon still had 83°56′ to travel to reach the north node at 27°18′ Leo, so it was close to that midway point, and 5°14′ south of the ecliptic.

The site draws zodiacal lines: it finds where the Moon's zodiac degree, 3°21′ Gemini, sat on each angle, and ignores the latitude. Jim Lewis's original maps used the Moon itself, its right ascension and declination, a method called in mundo. For a body on the ecliptic the two agree. For Obama's Moon they do not, because a point 5°14′ south of 3°21′ Gemini has a declination 5°09′ lower than the degree itself: 15°40′ north instead of 20°49′.

The meridian lines move only a little: the Moon's own MC and IC lines run 1°02′ east of the zodiacal ones, about 116 km at the equator. The rising and setting lines move much more, and the gap grows with distance from the equator, because a change in declination shifts the time of rising most where the stars' daily paths meet the horizon at a shallow angle:

At 60° N the two methods put his Moon rising line 731 km apart, farther than the 300 to 400 miles either side that Jim Lewis first allowed a line. Any city near a Moon line on one kind of map can be well off it on the other: drawn in mundo, his Moon rising line would pass Toronto, New York and Philadelphia rather than Detroit, Columbus and Raleigh, and his setting line would cross southern Korea at Busan and Daegu.

The birthplace as a check

Honolulu gives a way to see the difference in time rather than distance. The zodiacal IC line lies 4°39′ east of the city, so by the site's map the Moon's zodiac degree reached Honolulu's lower meridian 18 minutes after the birth. The Swiss Ephemeris, timing the Moon itself, has it crossing at 7:47 PM, 23 minutes after the birth. About 4 minutes of the difference is the latitude (the 1°02′ shift in the meridian line); the rest is the Moon's own eastward motion while the Earth turned.

Which line is right depends on what you take a line to mean. If it is the place where you would have seen the Moon itself on the horizon or the meridian, the in mundo line is the honest one. If it is the place where your relocated chart has the Moon's zodiac degree exactly on an angle, which is how a relocated chart places it, the zodiacal line is. Both are in use; for the Moon the choice is worth hundreds of kilometers, so it is worth knowing which one a map shows.

When Obama was born the Moon was moving 12°40′ a day along the zodiac, against the Sun's 0°57′. That speed makes the date matter. Keep his birth time but make the date a day later, and the Moon MC line shifts 12°22′ east, about 1,376 km at the equator; his Sun MC line moves 0°01′. Two people born at the same clock time a day apart share their Sun lines almost exactly and not their Moon lines.

It does not make the birth time matter more. It is easy to assume the fastest body needs the most exact time, and some guides add that MC and IC lines are safer with a rough one. Neither holds up. Every line moves west by about a degree for every four minutes of birth time, because the Earth turns, and that holds for the curved lines as much as the straight ones. The Moon's own motion takes a little back: four minutes moves Obama's Moon lines 0°57′, a shade less than the Sun's 1°00′, because the Moon creeps east while the Earth turns it west.

How wide a Moon line runs is a separate question, and a matter of convention. Jim Lewis's first handbook allowed a quarter of an inch of his printed map on each side, roughly 300 to 400 miles, and his 1982 Cyclo*Carto*Graphy handbook stretched that to 700 miles. At 700 miles, 938 cities out of the site's 2,292 (41%) sit near at least one of Obama's Moon lines, more than for any of his other nine bodies, because his four Moon lines between them cross Europe, Africa, eastern Asia and both Americas.

Start with the Moon's distance from your lunar nodes, which you can read from a birth chart or the North Node calculator. Within about 10° of either node the Moon is less than a degree off the ecliptic, and zodiacal and in mundo maps barely differ. Near 90° from the nodes, as in Obama's chart, the Moon can be five degrees off it, and the rising and setting lines on the two kinds of map can sit hundreds of kilometers apart at mid-latitudes.

Then look at the lines themselves on the map, with the other planets switched off. The MC and IC lines are the safest to read, not because they are less sensitive to the birth time, but because latitude moves them least. For the curved lines, give more weight to the stretches near the equator, where the two methods agree, than to the high-latitude ends, where they part. You can compare your own map with Obama's by opening his map, pre-filled.