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A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: At the centre of time
From: Douglas Denny
Date: 2009 Oct 20, 16:59 -0700
From: Douglas Denny
Date: 2009 Oct 20, 16:59 -0700
Correction:- "....As part of this exercise, a check was made of the link between the two coordinate systems which depended on the survey made (I think in the late nineteenth century {someone will be able to correct me on this no doubt} ..." I'll correct myself: I meant of course late _eighteenth_ century as the Airey instrument was established in the late nineteenth. The power of the internet is amazing, here is the information about the Greenwich meridian(s), in all it's glory at:- http://gpsinformation.net/main/greenwich.htm And a relevant quote from the above link :- "Even today, it can be confusing as there are four Meridians all passing through the Old Royal Observatory. The earliest is Flamsteed's, named after the first Astronomer Royal, which was established in 1675. In 1725, Edmund Halley, the second Astronomer Royal established a second Meridian. The third was defined by another Astronomer Royal, James Bradley, in the mid 18th century, and is still used as the basis for map-making in Britain today. The fourth Meridian was established in 1851 by yet another Astronomer Royal, Sir George Airy, who set up new measuring equipment in a room alongside Bradley's original equipment. It is the positioning of this neighbouring equipment, just 5.79 metres (19ft) away, which eventually became the basis for international time. Due to the convergence of meridians of longitude towards the poles, Bradley's Meridian is 5.9m west of Airy's where they cross the South Coast of England, and 5.5m west where they cross the East Coast. As the pace of development and travel accelerated in the 19th century, it became clear there would have to be a common, world-wide standard for timekeeping. In 1884, 25 countries reached agreement at a conference in Washington, USA, that Airy's Greenwich Meridian would be adopted as the `Prime Meridian' - zero degrees - from which time could be set and from which other points of longitude could be calculated. Over a period of many years, countries which had not necessarily been party to this original agreement accepted and adopted the decision. So since 1884, the Airy line has been The Greenwich Meridian, although for practical mapping purposes in Britain (excepting hydrographic charts) the Bradley line continues in use as the zero meridian. The difference between the two is known and well defined - and is important scientifically - but for most day-to-day purposes has no real consequence." ----------------------- Douglas Denny. Chichester England. --~--~---------~--~----~------------~-------~--~----~ NavList message boards: www.fer3.com/arc Or post by email to: NavList@fer3.com To , email NavList+@fer3.com -~----------~----~----~----~------~----~------~--~---