I hope everyone remembered to set their clocks back one hour last night after British Summer Time ended. We are, of course, now on Greenwich Mean Time until 27 March 2011.Coincidentally, I found myself in Greenwich today and, as pictured, stood on the Prime Meridian. These days, as well as a brass strip showing the line of 0 degrees longitude, there is also a green laser beam that shines into the night sky towards the O2 dome, which is visible for about 15 miles.
The Royal Observatory at Greenwich is well worth a visit and of particular interest are the time pieces on display of John Harrison, who solved the longitude problem (the basis of the Channel 4 drama "Longitude" in 2000).
"Oxford time" is 5 minutes later than GMT. I gather that the Great Tom Bell at Christ Church College is rung at 9:05 pm (9pm on Oxford time) every evening for this reason. However, I have to admit that I have never witnessed and confirmed this myself.
Sorry, the Google Earther in me makes me post this comment:-
ReplyDelete"It is true that a GPS receiver held over the Meridian Line at Greenwich will NOT read longitude 0˚ 00΄ 00˝ The WGS84 prime meridian is in fact 102.5 meters to the east of the 1884 Prime Meridian at Greenwich. WGS84 stands for “World Geodetic System 1984” and is used by today’s GPS satellite Global Positioning System.
With the advent of the GPS satellite system in the 1980s, accurate navigation could be done by any child using a hand-held computer with more compute power than existed in the entire world in 1954, and with access to 25 satellites in Earth orbit, each carrying two caesium atomic clocks. The system calculates position by receiving and comparing time signals from any three GPS satellites. It needs an internal “map” of the world in the form of a computer program. It wasn’t easy to create the computer map because the Earth isn’t a simple sphere. It has a complicated shape which required a technique called ‘best fit’ to develop the map based on an Earth geodetic model -- WGS84. This doesn’t fit the Earth’s surface exactly everywhere but juggles the map shape to find the position where it fits best at the most places it can. Try as they might, the best fit they could get at Greenwich put the WGS84 meridian to the east of Airy by a tad under 102.5 meters."
http://www.flamsteed.info/faswgs84.htm
Interesting. In fact, the Greenwich Meridian has itself shifted because it used to be just defined by the position of the telescope used by the Astronomer-Royal of the day(when it was difficult to move telescopes). I think the meridian has "creeped" several times in this way. For example, James Bradley (the third Astronomer Royal) has a plate fixed to the wall to show his meridian which is 5.9m away from the Airy meridian which has been the defining line since 1884. I believe it is the Bradley Meridian rather than the Prime Meridian, which is used in map making (eg. Ordnance Survey). I guess GPS could be thought of as a new "telescope" and carried this tradition forward of redefining the meridian.
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