Origins
The word 'meridian' entered Middle English around 1384 from Old French 'meridien,' from Latin 'meridianus' (of midday, pertaining to noon, southern), derived from 'meridies' (midday, noon). 'Meridies' is an alteration of the older form 'medidies,' a compound of 'medius' (middle, from PIE *medhyo-) and 'dies' (day, from PIE *dyew-, sky, heaven, day, god). A meridian is, at its etymological core, the midday-line — the line that connects noon to the structure of the Earth.
The astronomical meaning is the original one. As the Sun traces its apparent daily arc across the sky (from east at sunrise, upward to its highest point, then downward to the west at sunset), it crosses an imaginary line running from due north to due south through the point directly overhead. This crossing — the 'transit of the meridian' — occurs at local noon, the moment when the Sun reaches its maximum elevation for that day. The meridian is thus defined by a temporal event: it is the line where noon happens.
From this astronomical definition, the geographical sense follows directly. A meridian on the Earth's surface is a line of longitude — a great circle passing through both poles. Every point on a given meridian experiences noon simultaneously (approximately). The prime meridian — the zero line of longitude — was fixed at the Royal Observatory in Greenwich, England, by international agreement in 1884. The choice of Greenwich was not inevitable; France, for centuries, measured longitude from the Paris meridian, and Spain from the Cadiz meridian. The standardization of the prime meridian was a political as much as a scientific decision.
Proto-Indo-European Roots
The PIE root *medhyo- (middle) is one of the great structural roots of Indo-European vocabulary. In Latin, 'medius' (middle) produced 'medium' (the middle thing), 'median' (the middle value), 'medieval' (the middle age), 'mediate' (to stand in the middle), 'Mediterranean' (the middle of the lands — the sea surrounded by continents), and 'meridian.' In Greek, the same root gave 'mesos' (middle), producing 'Mesopotamia' (between the rivers), 'mesosphere' (the middle atmospheric layer), and 'mezzanine' (via Italian). In English, 'middle' itself descends from Old English 'middel,' from Proto-Germanic *midja-, from PIE *medhyo-. The root's centrality in spatial vocabulary reflects a fundamental cognitive orientation: humans navigate by locating the middle.
The second element, PIE *dyew- (sky, heaven, day, the shining one), is one of the most ancient and sacred roots in the Indo-European family. It produced the name of the supreme deity in multiple traditions: Sanskrit 'Dyaus Pitar' (Sky Father), Latin 'Iuppiter' (from 'Diu-pater,' Sky Father), Greek 'Zeus' (from 'Dyeus'), and Norse 'Tyr.' In everyday vocabulary, *dyew- gave Latin 'dies' (day), 'diurnal' (daily), 'diary' (a daily record), 'dial' (a daily timekeeper), and 'divine' (pertaining to the sky-god). The meridian thus conjoins two PIE roots of exceptional depth: the middle of the day, the center of the sky.
The figurative sense of 'meridian' — the peak, the high point, the zenith of achievement — derives naturally from the astronomical meaning. The Sun at its meridian is at its highest; a person at their meridian is at the apex of their powers. Joseph Conrad's phrase 'the meridian of his life' and similar literary uses exploit this metaphor of solar culmination. The post-meridian decline — the afternoon of a career, a civilization, or a life — follows as inevitably as the Sun's descent toward the western horizon.
Legacy
The abbreviations 'a.m.' (ante meridiem, before noon) and 'p.m.' (post meridiem, after noon) that structure the English-speaking world's daily timekeeping are direct descendants of 'meridian.' Every time someone specifies '3 p.m.,' they are situating an event relative to the midday-line — the moment when the Sun crosses the meridian, dividing the day into its two halves. The word, coined to describe the Sun's position in the medieval sky, continues to organize the hours of modern life.