The map behind the lines
Astrocartography is a birth chart projected onto the globe. Take the exact moment of birth, hold it fixed, and ask where on Earth each planet would have been rising, setting, overhead or underfoot. Every answer is a line, and a full map carries four for each body.
The method dates from the 1970s and was developed by Jim Lewis. It is modern, and pretending otherwise does it no favours. The older idea it rests on is that angularity strengthens a planet, which appears throughout traditional astrology in the treatment of the four angles of a chart.
Four Mercury lines, two shapes
The MC line follows the longitudes where Mercury was culminating on the upper meridian. The IC line follows those where it was at the lowest point, on the meridian below. Both draw as straight north to south lines, since meridians are fixed circles through the poles and do not care about latitude.
The AC and DC lines mark where Mercury was rising in the east and setting in the west. These curve, and the reason is worth stating because so much writing glosses over it. The horizon meets the ecliptic at an angle that changes as you travel north or south, so the point of the sky that is rising shifts with latitude. Above the Arctic and Antarctic circles the curves can swing violently or break off entirely. Any map that shows four straight parallel lines per planet is drawn wrong.
What Mercury's lines are read for
Mercury signifies speech, writing, trade, short journeys, paperwork and the handling of information. The MC line is read as work that involves those things being placed in front of people, teaching, publishing, negotiation, anything where the output is words. The stated difficulty is a tendency to be judged on how well you explain rather than on what you do.
The IC line is read as thinking done privately, study, research, the household full of books and half finished notes, with restlessness at home as the usual complaint attached to it.
The AC line is read as quickness of speech and a place where a person is met as clever or talkative, and the difficulty given is scattering, saying more than is meant. The DC line reads through others, associated with negotiation, contracts and business partners, and with the risk of being talked into things.
These are traditional associations. They describe a symbolic emphasis and nothing about what will happen to a person who goes there.
Orb, parans and honest limits
The customary orb is a few hundred kilometres. The figure most often printed is seven hundred to eight hundred, with the reading strongest on the line and weakening outward. It is a convention, repeated because it is useful for keeping maps readable, not a result anybody measured. Some practitioners work tighter, at four hundred, and the maps look different as a result.
Alongside the lines you may see parans, latitudes where two planets are angular at the same time. They run horizontally and are read as a mixture of the pair. And if you want to see how a place changes the chart rather than just which lines pass nearby, a relocated chart will show the houses redistributed for that city.
The framing that matters is this. The geometry is exact, which is why lines can be drawn precisely and why any two programs agree. What a Mercury line signifies where it crosses a city is convention layered on top. Take it as a way of thinking about a place, not as advice to move to one.
Questions people ask about this
What is a Mercury line read for?
Speech, writing, trade, study and the handling of information, with the emphasis depending on the angle. An MC line puts that work in public view, while an IC line points inward toward private study.
Are all four Mercury lines the same kind of line?
No. The MC and IC lines are meridians and draw straight from pole to pole. The AC and DC lines follow the horizon and therefore curve with latitude, sometimes very sharply.
How near does a Mercury line have to be?
The usual convention is within seven hundred to eight hundred kilometres, strongest at the line. Practitioners vary the figure, which tells you it is a working habit rather than a measurement.
Is Mercury retrograde relevant to its lines?
The line is drawn from position, so retrograde motion does not change where it falls. Some readers factor a retrograde Mercury into the interpretation, which is a matter of style rather than of geometry.
When was astrocartography invented?
In the 1970s, by Jim Lewis. It is a modern application of the older principle that a planet on an angle is emphasised, and it only became practical with computers.
Can a Mercury line make me a better writer?
No line does anything to a person. The tradition reads Mercury lines as a place where verbal and commercial matters are foregrounded, which is an interpretation, not a claim about outcomes.