Prime Meridian
babylonian: ziqpu (Sumerian ZIQ.PI, 'that which stands upright') — the meridional reference stars
Definition
The great circle on the celestial sphere that passes directly overhead through both celestial poles and runs due north–south, cutting the sky in two. Where this line meets the ecliptic above the horizon is the Midheaven (MC); where it meets the ecliptic below is the Imum Coeli (IC). A star is said to be culminating when it crosses this line.
In Tradition
Across the tradition the meridian is the upper-and-lower-culmination reference: Crane records the modern framing — 'where the ecliptic crosses the meridian is the Midheaven (always due south in the northern hemisphere, due north in the southern)' — and notes that the meridian does not change position during the day, though it moves with the observer. The line is the structural axis of the upper/lower halves of the chart, paired with the horizon as the second of the two great-circles framing the angles.
In Practice
Practitioners work with the meridian in three principal ways: as the source of the MC and IC (the upper and lower angles of the chart); as the reference line for paran computation (a star culminating at the same moment a planet rises is in paran with that planet on the meridian); and as the framework for primary directions, in which the chart's degrees are 'directed' along the diurnal arc by measuring how long it takes a given point to reach the meridian. The line is also the basis for sidereal-time computation — the right ascension of the meridian advances by approximately 360° in a sidereal day.
Historical Origin
The meridional reference is foundational in Mesopotamian astronomy: MUL.APIN Tablet I iv 1-9 catalogues fourteen ziqpu stars — 'that which stands upright' — which transit the observer's meridian successively through the night and serve as the canonical clock for timing the risings and settings of all other stars. The doctrine carries forward into Hellenistic chart construction, where the meridian's intersection with the ecliptic gives the Medium Caelum, and into the modern observational and computational framework that Crane and the Brady-lineage fixed-star tradition use.
Etymology
Origin: Latin. Meaning: meridies = midday — the line over which the Sun stands at noon.
Further Reading
- Joseph Crane, Astrological Roots: The Hellenistic Legacy
- Hermann Hunger & John Steele, MUL.APIN
- Bernadette Brady, Brady's Book of Fixed Stars