Mercury lines
The four lines on your map where Mercury was on an angle when you were born: overhead, underfoot, rising and setting.
Mercury in astrocartography
Mercury is talk and traffic: conversation, writing, buying and selling, learning, the short trips that fill a diary, and the people you see without choosing them, which in astrology means siblings and neighbours. It's also nerves, the restless part of you that checks the phone and wants one more thing to do. On the map that splits four ways. The MC line is what you're known for saying, the IC the life of the house, the Ascendant your own speed of mind and the Descendant the people you trade words with.
Mercury never strays far from the Sun in the sky, so its lines always keep company with your Sun lines. The two MC lines are never more than about 28° of longitude apart, and often much nearer. If Mercury was within a few degrees of the Sun when you were born, the textbook calls it combust and reads its lines through the Sun's, so that on them what you say and who you are get hard to tell apart.
Mercury MC line: known for what you say
Mercury overhead makes communication your public calling card. Writing, teaching, trading, consulting and any work that lives on ideas and contacts tends to move fast here, and your name travels through what you publish and say.
Mercury IC line: a thinking home
With Mercury beneath you, the private life fills with books, conversation, study and movement. It is a good place to learn, to write at home, to run a business from the kitchen table, and to stay in touch with family across distance.
Mercury ASC line: quick and curious
Mercury rising sharpens you. You think and talk faster, pick up languages and local codes with ease, and people meet you as witty, adaptable and young in spirit.
Mercury DSC line: partners in conversation
Mercury setting brings people who engage your mind: talkers, traders, teachers, younger collaborators. Relationships here run on exchange and are often formed through work or study.
Where the lines run
Mercury's declination, how far north or south of the celestial equator it sits, decides where its rising and setting lines stop: at 90° of latitude minus the declination. For births between 1930 and 2025 it ran from 25.9° S to 25.7° N, so in some charts those lines end as close to the equator as about 64° of latitude, north or south. Beyond that point Mercury didn't rise or set at all on the day you were born, while the MC and IC lines carry on to the poles.
Mercury lines are drawn with Pro; the free map shows the Sun, Moon, Venus and Jupiter.