Earth view from Apollo 11
Greenwich Time, most commonly called Greenwich Mean Time(GMT), originally referred to mean solar time at the Royal Observatory n Greenwich, London, England. It is virtually the same as Coordinated Universal Time(UTC). This generally the standard time referred to when figuring the difference in hours between two time zones. Prior to the introduction of UTC in 1972, GMT was the standard astronomical concept used in many technical fields.
As the United Kingdom grew, British mariners kept at least one chronometer on GMT in order to calculate their longitude from the Greenwich meridian, which was by convention considered to have longitude zero degrees. This practice did not affect shipboard time, but when combined with mariners from other nations drawing from Maskelyne’s method of basing lunar distances on observations at Greenwich, led to GMT being used as a reference time independent of location. Most time zones were based upon a number of hours ahead or behind GMT. Many international transactions still use this as a time frame.
The daily rotation of the Earth is somewhat irregular and is slowing down slightly. Atomic clocks offer a much more stable timebase. On January 1, 1972, Greenwich Time was replaced as the international time reference by Coordinated Universal Time, which is maintained by atomic clocks around the world. Universal Time initially represented mean time at Greenwich as determined in the traditional way to accord with the originally-defined universal day. On January 1, 1956 this ‘raw’ form of UT was re-labeled UT0 and superseded by refined form UT1, which was equalized for the effects of polar wandering, then UT2. UT2 was a version further equalized for annual seasonal variations in Earth rotation rate. Leap seconds are now added or subtracted from UTC to keep it within 0.9 seconds of UT1.
While Greenwich Time has been replaced for scientific purposes, many transactions are completed using GMT as the base reference for the times recorded and accorded to those transactions.
We have written many articles about Greenwich Time for Universe Today. Here’s an article about Greenwich Mean Time, and here’s an article about the Coordinated Universal Time.
If you’d like more info on Greenwich Time, check out the website entirely about Greenwich Mean Time. And here’s a link to the Wikipedia page about Greenwich Mean Time.
We’ve also recorded an episode of Astronomy Cast all about Time. Listen here, Episode 117: Time.
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