GMT (Greenwich Mean Time)
gmt
The mean solar time at the Royal Observatory in Greenwich, London, historically used as the international time reference.
Meridian
meridian
Definition
A meridian is a half of a great circle on the Earth's surface, connecting the North Pole and the South Pole. Every point on Earth lies on exactly one meridian, and the angle between that meridian and the Prime Meridian (longitude 0°) defines the point's longitude. The term derives from the Latin 'meridies' (midday), since the sun crosses a given meridian at local noon.
The Prime Meridian
The Prime Meridian is the meridian designated as 0° longitude. It was established at the 1884 International Meridian Conference as the line passing through the Royal Observatory in Greenwich, England. This decision set the global reference point for the longitude and time zone systems. Today the international standard is the IERS Reference Meridian, which passes approximately 102 meters east of the historical Greenwich line.
Relationship to Time
Because the Earth rotates 360° in 24 hours, every 15° of longitude (15 meridians) corresponds to a one-hour time difference. All locations on the same meridian share the same solar time and observe the sun culminating at the same moment. Time zones are theoretically 15° wide, but in practice their boundaries follow national borders and administrative divisions rather than exact meridian lines.
Was this article helpful?
Related Terms
Time Zone
timezone
A region of the globe that observes a uniform standard time for legal, commercial, and social purposes.
UTC (Coordinated Universal Time)
utc
The primary time standard by which the world regulates clocks and time, serving as the basis for civil timekeeping globally.
Related Articles
How Time Zones Work - Understanding UTC Offsets and the Global Time System
Learn how time zones divide the world into regions with different local times, how UTC offsets work, and why some zones use half-hour increments.
The History of Standard Time - How Railways Forced a Global Clock
Before railways, every town kept its own solar time. Trains made that impossible, and within a few decades the world adopted a coordinated system anchored to the Greenwich meridian. This article traces the journey from local solar time through the 1884 International Meridian Conference to UTC.
What a Meridian Means - The Reference Lines Behind Time and Longitude
Meridians are the north-south lines that define longitude and underpin the world's time zones. This article covers what a meridian actually is, why Greenwich became the prime meridian, the relationship between meridians and time, and the 5-meter shift between the historical line and modern GPS.