NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: At the centre of time
From: Paul Hirose
Date: 2009 Oct 21, 15:01 -0700
From: Paul Hirose
Date: 2009 Oct 21, 15:01 -0700
Brad Morris wrote: > Aside from the political envy expressed by one reviewer as to why the prime meridian shifted, what were the reasons for that shift? The National Maritime Museum, which runs the old Greenwich Observatory site, has this explanation: "The Airy Transit Circle at Greenwich was originally the zero of longitude for the World. It is not at zero longitude in the WGS84 system, and the WGS84 system agrees with other modern systems (e.g. the International Terrestrial Reference Frame) to better than a metre. So at what stage was this origin lost from the International frames? "First it must be recognised that the only way that the position of any location can influence a terrestrial reference frame is if some positional measuring instrument is operated at the site, and the accuracy with which this position can be located in the frame is limited by the accuracy of the observations. "The latest astronomical observations made at Greenwich which contributed to the international terrestrial coordinate systems of that time were the observations made on the Small Transit instrument. These observations continued up to 1958. The observations made were the times and zenith angle of transit of stars, and were used to control Greenwich Time. Time signals from Greenwich were compared with time signals from other countries that were making similar observations, and from this a system of longitudes of the contributing observatories was obtained. The measured zenith angles give a system of latitudes. This coordinate system was compiled and maintained by the Bureau International de l'Heure (BIH). The zero point of this system was adjusted to give a constant value to the mean of all of them, rather than a precise value of zero for Greenwich, and so already at that time Greenwich had an offset of about 8 metres from the zero longitude of this system." So 50 years ago the prime meridian had already lost its physical connection to Greenwich and was starting to drift away. http://www.nmm.ac.uk/explore/astronomy-and-time/astronomy-facts/history/the-longitude-of-greenwich -- --~--~---------~--~----~------------~-------~--~----~ NavList message boards: www.fer3.com/arc Or post by email to: NavList@fer3.com To , email NavList+@fer3.com -~----------~----~----~----~------~----~------~--~---