

Why the Greenwich meridian moved - dikshie
http://leapsecond.com/pages/meridian/

======
dzdt
Explain it like I'm five version : a plumb bob at the Greenwich observatory
doesn't point at the center of mass of the earth. This is because of different
densities of rocks below the surface, and of mountains on the surface, and so
on. Prior to the satellite era, the meridian plane was defined to be parallel
to local surface gravity at Greenwich (so the plumb bob string lies in the
meridian plane) and perpendicular to the rotation of the earth as determined
by observations of stars. When satellites started to be used, the new meridian
plane was kept parallel to the original but moved over to go through the
center of mass of the earth. The center of mass of the earth is a nice
reference point for satellites orbiting the planet. The new system chose to
keep noon (the time when the sun is directly overhead, as measured by local
gravity direction) at Greenwich fixed instead of keeping the location of the
meridian line fixed. That was because lots of people depended on the Greenwich
Mean Time standard and not so many people depended on the exact longitudinal
coordinates of any location, which prior to GPS had been very hard to measure
anyway.

~~~
Htsthbjig
Thanks. You are not explaining it like we are five, but as if we were humans.

The explanation in the article is a non explanation, "look at me how smart I
am that could obfuscate knowledge, so I look smarter".

~~~
animefan
Re your second sentence, I think it's unfair to attack the author in this way.
By my reading, the author didn't attempt took describe the discrepancy and
just included the abstract of a paper that does. We should thank the author
for their best effort.

This touches me because I've often been in the situation where my explanation
was inadequate, and I was accused of trying to sound smart. We shouldn't be
too eager to take people down a notch.

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jsingleton
This story was on BBC yesterday and it surprised me:
[http://www.bbc.co.uk/news/magazine-33919429](http://www.bbc.co.uk/news/magazine-33919429)

    
    
      Researchers have revealed how the Prime Meridian line came to be placed about 
      100 metres away from the true divide between east and west.
    
      It was worked out in the 1980s using GPS technology that the famous landmark at
      Greenwich was in the wrong place.
    
      The original calculations in [the] 1880s did not take into account the earth's 
      contours, according to a new study by astronomers at the University of Virginia.
    

That page is mainly a video BTW. If you want to watch it in Firefox [0] or
Safari [1] without Flash Player then I've written up some guides.

[0] [https://unop.uk/dev/how-to-watch-bbc-news-videos-on-a-
deskto...](https://unop.uk/dev/how-to-watch-bbc-news-videos-on-a-desktop-
without-flash-in-firefox/)

[1] [https://unop.uk/dev/how-to-watch-bbc-news-videos-on-a-
deskto...](https://unop.uk/dev/how-to-watch-bbc-news-videos-on-a-desktop-
without-flash-in-safari/)

~~~
mzs
$ youtube-dl
'[http://www.bbc.co.uk/news/magazine-33919429'](http://www.bbc.co.uk/news/magazine-33919429')

~~~
jsingleton
Didn't realise you could do that.

[https://github.com/get-iplayer/get_iplayer](https://github.com/get-
iplayer/get_iplayer) is quite good too.

------
robinhouston
The Airy ellipsoid is still used by the Ordnance Survey in Great Britain.
(Northern Ireland has a different national grid based on a different geodetic
system.) WGS84 may be increasingly “the standard”, but it isn't the only
geodetic system in widespread use. ETRS89 is anchored to the European
continental plate, and is therefore slowly drifting out of sync with WGS84.

~~~
david-given
The more I learn about mapping, geodetic systems and elipsoids, the more I am
coming to the conclusion that deciding to live on a planet made out of squishy
goo was a bad idea.

~~~
jerf
It is commonly thought that life would be much harder on a volcanically-
inactive world due to the fact the volcanic activity cycles nutrients on a
geological time-frame that would otherwise tend to collect on the bottom of
the ocean. See
[http://earthobservatory.nasa.gov/Features/CarbonCycle/page2....](http://earthobservatory.nasa.gov/Features/CarbonCycle/page2.php)
for instance, though it doesn't quite make this exact point, you can see how
it might be a problem if all the carbon on a planet is locked up in rocks on
the ocean floor after a mere 250 million years.

------
bensummers
Living close to Greenwich, when I was testing a bluetooth GPS receiver (in the
days when phones didn't include them), I spent a few puzzled minutes wondering
if it was working. Zero longitude seemed suspicious.

~~~
andreastt
This is wonderful, not only because I can see my house on the screenshots, but
because I’ve wondered about the exact same thing.

However, it turns out that my house is on the fake, tourist meridian line now.
I can live with that. (-:

------
inconshreveable
If you're interested in the historical setting of Longitude (and why it's in
Greenwich to begin with), I highly recommend Dava Sobel's _Longitude_:
[http://www.amazon.com/Longitude-Genius-Greatest-
Scientific-P...](http://www.amazon.com/Longitude-Genius-Greatest-Scientific-
Problem/dp/080271529X/)

------
Rifu
Link to the actual paper:
[http://link.springer.com/article/10.1007/s00190-015-0844-y](http://link.springer.com/article/10.1007/s00190-015-0844-y)

------
zaf
Ha, just a month or two ago I was asking one of the local experts about the
exact same thing.

It got so technical that he advised me to continue my enquiries with the
astronomers at the Royal Observatory.

Unfortunately it was a Saturday morning and, as you know, they were all in bed
after yet another crazy all nighter.

------
jnsaff2
I had a stroll there a few months ago: [https://igcdn-photos-
a-a.akamaihd.net/hphotos-ak-xfa1/t51.28...](https://igcdn-photos-
a-a.akamaihd.net/hphotos-ak-
xfa1/t51.2885-15/10785082_1512005252407496_613113322_n.jpg) left leg in the
western and right leg in the eastern hemisphere.

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cpeterso
Null Island is a (fictional :) island in the Gulf of Guinea, located where the
equator crosses the prime meridian, at coordinates 0°N 0°E:
[https://en.wikipedia.org/wiki/Null_Island](https://en.wikipedia.org/wiki/Null_Island)

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rtkwe
Tom Scott did a good video on this exact thing. The video goes into more
detail than the webpage if you don't want to read the 10 page paper.

[https://www.youtube.com/watch?v=DmvHZ4omB2A](https://www.youtube.com/watch?v=DmvHZ4omB2A)

------
7952
My favourite explanation of projections is by the Ordnance Survey (PDF). It
looks at things from a practical rather than academic perspective.

[https://www.ordnancesurvey.co.uk/docs/support/guide-
coordina...](https://www.ordnancesurvey.co.uk/docs/support/guide-coordinate-
systems-great-britain.pdf)

------
vorg
> Here at latitude 51.5° and given an Earth circumference of roughly 40,000 km
> — one degree of longitude is 69 km

The math is wrong here. If the earth circumference is 40,000 km, then divide
by 360 to get one degree equals 110 km. I notice dividing by 1.6 gives 69
miles, so perhaps there's a units error here.

~~~
tvb
One degree is 40,000 km / 360 = 111 km at the equator. But Greenwich is at 51
degrees latitude so you apply a cos (51.5°) = 0.622 factor. That's why 1
degree of longitude at latitude 51 is 69 km. Does that help clear it up?

Note also if you're walking in a circle 57 meters from the monument at the
South pole, then each 1 m step you take crosses one degree of longitude.
Google for: south pole ceremonial sphere

~~~
vorg
Thanks, I didn't think about it too hard.

