Definition
The time of day at the Prime Meridian (zero degrees longitude, running through Greenwich, England), used worldwide as a single common time reference for aviation, navigation, and weather reporting. Also known as Coordinated Universal Time (UTC), Zulu time, or Greenwich Mean Time.
Plain English
It is the clock time at Greenwich, England, used as one shared time that everyone in aviation around the world agrees on, no matter what their local time is.
Context Anchor
Seen in flight planning, weather reports, clearances, log entries, and any aviation information given in Zulu or UTC time.
Derivation
The Prime Meridian is the line of zero longitude, chosen by international agreement in 1884 to pass through Greenwich, England. 'Meridian' comes from the Latin meridies, meaning 'midday,' because every meridian is the line where the sun is highest at local noon. Using one fixed meridian as the world's clock removes confusion between time zones.
Why Pilots Care
Gives pilots and controllers a single shared time reference when flights cross time zones or operate internationally.
Analogy
It is like everyone agreeing to use one master clock instead of each person using the clock on their own wall.
Intuition Check
Do not read this as a special kind of time that only exists on the line itself. It means using the prime meridian as the agreed reference for time worldwide.
Example Sentence 1
The METAR was issued at 1755Z, which is the time along the Prime Meridian, not local time.
Example Sentence 2
Converting local departure time to time along the prime meridian simplified the international flight plan.