meridianus

Meridian derives from Latin meridianus (of or belonging to midday, southern), from meridies (midday, noon, south), a compound of medius (middle) and dies (day) — literally 'middle of the day.' The dies root gives English 'diary,' 'diurnal,' 'dismal' (from dies mali, evil days), and 'sojourn' (through Old French sojour, from Latin subdiurnare, to spend a day). In Latin, meridies meant both noon and south, because the Sun, when it reached its highest point in the sky — its transit of the meridian — was due south for observers in the Northern Hemisphere. The compound gave meridional (southern, or of the meridian) and prime meridian. A meridian is the circle on Earth's surface that passes through a location, the north and south poles, and the zenith directly overhead — the line the Sun appears to cross at local noon.

The practical importance of the meridian lies in its role as the definition of local time. Before the invention of clocks and the standardization of time zones in the nineteenth century, time was local: noon was the moment the Sun crossed the meridian, and local clocks were set accordingly. A city 15 degrees of longitude to the east experienced noon one hour earlier than a city 15 degrees to the west. This local solar time was perfectly adequate when travel and communication were slow, but the railways of the nineteenth century — which coordinated schedules across hundreds of miles — created an urgent need for standardized time. Britain adopted Greenwich Mean Time in 1847 for railway scheduling; the United States adopted four standard time zones in 1883; the International Meridian Conference in Washington in 1884 established the Greenwich meridian as the Prime Meridian.

The choice of Greenwich for the Prime Meridian — longitude zero — was a political and practical decision. Greenwich had been the reference for British Admiralty charts since 1676; by 1884, roughly 70 percent of the world's shipping used charts based on the Greenwich meridian. The conference voted to adopt it internationally, with France abstaining and declining for decades to use Greenwich in its own cartography. The zero meridian was inscribed in the paving of the Royal Observatory at Greenwich, and the Airy Transit Circle telescope that defined it still stands. Today, GPS calculations use a slightly different zero meridian — the IERS Reference Meridian, about 100 meters east of the Airy Transit Circle — because the satellite-based system is tied to Earth's center of mass rather than any surface monument.

The meridian's role in astronomy is equally foundational. A celestial meridian is the great circle on the celestial sphere that passes through the observer's zenith and the north and south celestial poles. When a star transits — crosses the meridian — it is at its highest point in the sky for that observer on that night. Meridian transit observations were the primary method for determining the positions of stars for the star catalogs that underlie all positional astronomy. The transit telescope, a telescope mounted to rotate only in the plane of the meridian, was the standard instrument of the great national observatories for two centuries. Every precision time signal and every geodetic survey of the nineteenth and twentieth centuries traced its accuracy back to meridian transits observed at one of these instruments. The midday line turned out to organize the measurement of everything.