Moving the Greenwich Meridian

Astronomically-related chat

Moderators: joe, Brian, Guy Fennimore

Post Reply
brian livesey
Posts: 5425
Joined: Mon Jan 09, 2006 11:05 am
Location: Lancashire
Contact:

Moving the Greenwich Meridian

Post by brian livesey »

The Prime Meridian at the Royal Observatory at Greenwich divides the east and western hemispheres. But, since the advent of GPS systems, it's been discovered that the line lies 102 metres to the east, across a Greenwich Park footpath near a litter bin.
Researchers at the US Naval Observatory and the National Geospatial-Intelligence Agency found that GPS technology, introduced in 1984, is better at calculating longitude than the traditional method used by astronomers.
The original meridian position was established by using the positions of stars in relation to Greenwich. The meridian had to be perfectly perpendicular and to do this astronomers used a basin of mercury as a spirit level to get the precise vertical line from the telescope to the "clock stars".
Unfortunately, the astronomers at the time didn't know that Earth isn't perfectly round and causes gravitational variations around the oblate globe, so the mercury wasn't perfectly flat.
Earth satellites are free from these gravitational variations and were able to calculate star positions more accurately than the 19th-century measurements.
The anomaly only occurs at Greenwich, so it doesn't mean that all lines of longitude have to be shifted 102 metres east.
brian
Brian
Posts: 3691
Joined: Fri Dec 03, 2004 9:58 pm
Location: Wellingborough
Contact:

Re: Moving the Greenwich Meridian

Post by Brian »

If it's 102metres at Greenwich what is it at the equator? :?
Brian
52.3N 0.6W
Wellingborough UK.

254mm LX90 on Superwedge, WO ZS66SD, Helios 102mm f5 on EQ1, Hunter 11x80, Pentax 10x50
ASI120MC Toucam Pros 740k/840k/900nc mono, Pentax K110D
Ro-Ro roof shed
M54
Posts: 205
Joined: Sun Apr 27, 2008 2:59 pm
Location: Cambs
Contact:

Re: Moving the Greenwich Meridian

Post by M54 »

So come September we will be using GFPT (Greenwich Foot Path Time) not GMT anymore.
JohnM
Posts: 389
Joined: Fri Dec 10, 2004 10:34 pm
Location: Surrey
Contact:

Re: Moving the Greenwich Meridian

Post by JohnM »

Is this news ? See http://www.thegreenwichmeridian.org/tgm ... ?article=7 for a history of the problem. In fact there are already 3 Greenwich meridians one passing through each of the transit telescopes.
The good news is that the south of England is drifting NE at about 2.5cm / year so the Greenwich Meridian will eventually line up with the WGS84 zero longitude line. See http://sgf.rgo.ac.uk/analysis/gnss.html for a graph
I believe there is also a problem with the zero longitude used by the ordinance survey - I think it goes through one of the other transit telescopes not the one that has the zero longitude marker. I think this was discovered when they had to carry out a precise survey to determine where Herstmonceux observatory is relative to Greenwich so they could continue transit observations from there.
The other interesting thing from the Satellite Laser Ranger at Herstmonceux is that the land goes up and down as well due to tides, the amount of water in the chalk below the observatory and atmospheric pressure.
I have not found the paper explaining the latest conclusions on the ‘error’ but the first post relates the local gravitational variation causing the mercury not to be horizontal. Of course local masses will have an impact on the local vertical. This was used to determine the value of the Universal Gravitational Constant G by measuring the deviation from the vertical caused by the mass of the Scottish mountain of Schiehallion see https://en.wikipedia.org/wiki/Schiehallion_experiment>.

I wonder if the Greenwich hill has any significant impact on the local vertical ?
Engineer @ Work - Astronomer @ Play
Cliff
Posts: 6594
Joined: Sat Dec 11, 2004 8:18 pm
Location: Manchester
Contact:

Re: Moving the Greenwich Meridian

Post by Cliff »

Dear al(L)
About 1990, I got a half cocked, perhaps not so bright idea, of organising a group to re-enact Neville Maskelyne's 1774 Scheihallion expedition to weigh the earth.
Although not keen to go himself, but did so at the King's request ! Maskelyne and his team spent 4 months making 337 astronomical observations of 43 different stars, from two locations about a mile apart, north & south of Scheihallion's summit. Meanwhile Maskelyne's land surveyors mapped the mountain. Later Maskellyne's mathematician, Hutton, produced a very accurate map with contours devised by Hutton. (Although the French claimed to have originated contours first.)
Whatever, it wouldn't have been easy to emulate Maskelyne's efforts in detail. Furthermore, Elf&Safety wasn't a factor bothering Maskelyne two centuries before. Then the final celebration party downed a keg of whisky, their bothy caught fire and the fiddler's violin got burned. Replicating that was OUT (Later Maskelyne sent the fiddler a new one from London.)
So the 1990 expedition got watered down a lot. Only three. Fionna Vincent, Roger and me, went to Scheiallion in central Scotland - quite handy for Fionna & Roger, living in Dundee.
As it happens Scheihallion is almost as high as Snowdon, and the way we ascended it about as difficult as Snowdon via the Llanberis Path. Even so Scheihallion with a sprinkling of snow provided a splendid day out with fabulous views (including Loch Tummel's "Queen's View" seen in the opposite direction and super rainbow as well). Though it might have been different if Fionna & Roger hadn't pointed out near the start that I was taking the wrong path. If we'd carried on we might have ended up in Glasgow !
Scheihallion 3547 feet (OS48 - 1956) 3 miles walk, 2500ft ascent from the Braes of Foss car park (where a bronze plaque explains how Maskelyne weighed the earth).
Snowdon 3560 ft, 3.5miles walk and 3000ft ascent from Lanberis.
Best wishes from Cliff
PS My 2014 birthday treat was a Steam train ride up Snowdon.
There were only 3 of us all day on Scheiallion, but Snowdon's summit was absolutely hammered with people.
Post Reply