The Midheaven line is the terrestrial meridian where a planet culminates at the chart moment. It is not necessarily overhead at the zenith. On the globe it is a pole-to-pole line of constant longitude: λ = α − GAST.
The IC is the opposite meridian, 180° away: the same planet at lower culmination. Right ascension and apparent sidereal time determine both lines. Tropical and sidereal zodiac settings change labels and interpretive context, not this geometry.
A map makes culmination visible, but it does not make a place a promise. Meridian treats the line as an invitation to observe how visibility, responsibility, and private foundations feel there.