How Is GMT Time Calculated?
GMT (Greenwich Mean Time) is based on the mean solar time at the Royal Observatory in Greenwich, London, England — located at 0° longitude (the Prime Meridian).
The Historical Method
Before atomic clocks, GMT was calculated by:
Why "Mean" Time?
The Sun doesn't cross the meridian at exactly the same clock time every day. The difference between apparent solar time and mean solar time is called the Equation of Time — it varies by up to ±16 minutes throughout the year.
GMT vs UTC Today
Modern timekeeping uses UTC (Coordinated Universal Time), maintained by over 400 atomic clocks worldwide. UTC is more precise than GMT but numerically identical for everyday use.
| Standard | Based On | Precision |
|---|---|---|
| GMT | Earth's rotation / solar observation | ~1 second/year |
| UTC | Atomic clocks | ~1 nanosecond/day |
The Prime Meridian
The Prime Meridian (0° longitude) was established at the International Meridian Conference in Washington DC in 1884. Greenwich was chosen because the majority of the world's shipping already used Greenwich-based charts.
Quick Answer
GMT is calculated from mean solar time at the Royal Observatory in Greenwich (0° longitude). Today, UTC — maintained by atomic clocks — has replaced GMT as the technical standard, though the two are numerically identical.
