Almanacs Converge

Almanacs Converge

Coordination of the Astronomical and Nautical Almanacs

Another interesting thing about time was the introduction of a distinction between various measures of time: first the time as observed, then the time corrected for polar variation (in the order of 100ms), furthermore, a time corrected and averaged in rate over a year by the application of a seasonal correction, and finally the time of the clocks and time signals.

"GMT" designation which simply continued to exist because of the reluctance of the British and American authorities to confuse the navigators (that is the most frequently heard explanation) was not adapted to these new needs. Instead, the iau established the system of universal time which comprised the ut0, ut1, ut2 and utc (universal time coordinated) in the meanings as listed above.

Finally when other professions entered the timing field and demanded even closer international coordination and consultation, the international telecommunications union (itu with its consultative body the ccir), and the International Bureau for Weights and Measures as part of the metric convention, in addition to the scientific unions, iau, ursi, and iugg, established a system of definitions and conventions which deal exclusively with the system of Universal Time, GMT being considered as obsolete.

For practical purposes gmt may be considered as the equivalent of The general expression ut. Ut without further index is in use wherever a precision of 1 second is sufficient, or where no greater precision is to be implied. For the use in almanacs where the hour angle of celestial objects is needed, ut1 is to be used. For all general timing of clocks or electronic systems, utc is the appropriate reference.

Any of these documents may be obtained from the usno time service upon request. Literature: Derek Howse (1980) Greenwich time and the discovery of the longitude.
Oxford university press, isbn 0-19-215948-8 - Qb213 79-40052