Greenwich Mean Time

Greenwich Mean Time (GMT), or Universal Time (UT), mean solar time on the prime meridian. Universal Time does not increase uniformly and is therefore unsatisfactory for theoretical astronomical and frequency control purposes. Ephemeris Time, based on atomic clocks rather than solar time and coordinated from Paris, has taken the place of UT for purely scientific purposes but Greenwich, at longitude zero, remains the prime meridian for all navigational purposes. By the middle of the 19th century some dozen prime meridians were in use for navigational charts and many more for the calculation of time.

In October 1884 an international intergovernmental conference met in Washington, DC, with the purpose of fixing a Prime Meridian and Universal Day. After many months of probably fairly leisurely deliberation the conference agreed to recommend to the governments represented that they should adopt the meridian passing through the transit instrument at the Observatory of Greenwich as the initial meridian for longitude; that the longitude should be counted in two directions from this meridian; and that the universal day should be a mean solar day to begin for all the world at mean midnight at the initial meridian. In due course the recommendations were universally adopted, as was the time zone convention based on Greenwich.

Astronomers by and large prefer the term Universal Time to Greenwich Mean Time but for most nautical and navigational work GMT still prevails. Howse, D. , Greenwich Time and the Discovery of Longitude (1980).
——‘The Centenary of the Adoption of Greenwich as the Zero Meridian’, Journal of Navigation, 38 (1985), 191. Mike Richey