Why the Greenwich meridian moved | Hacker News Reader

Why the Greenwich meridian moved

leapsecond.com

113 pointsdikshie30 comments

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.

Thank you for that. I think this is very complex to explain without a lot of diagrams.

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".

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.

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

Actually, Reddit's ELI5 subreddit describes it as:

LI5 means friendly, simplified and layman-accessible explanations.

Not responses aimed at literal five year olds (which can be patronizing).

So it was a comment which would be appropriate for ELI5.

Even before my eyes glazed over I understood exactly 0° 0' 0.000" of that last paragraph. So thanks for both simplifying and expanding it.

So, should we all move 100m to the east?

I'm pretty sure some of us need move east, some need to move west, some need to start digging and others need to start building towers. Me, I need to dig into the ground for about 90 meters at a 43 degree angle to the north. But it's a small price to pay if it keeps my watch running.

This story was on BBC yesterday and it surprised me: 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...

[1] https://unop.uk/dev/how-to-watch-bbc-news-videos-on-a-deskto...

$ youtube-dl 'http://www.bbc.co.uk/news/magazine-33919429'

Didn't realise you could do that.

https://github.com/get-iplayer/get_iplayer is quite good too.

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.

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.

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.... 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.

Yes, I would like to find the person responsible for this decision and give him a piece of my mind.

And in the United Status, 95% of USGS maps use NAD27.

I believe that USGS maps published after 2009 use NAD83, which for practical purposes is identical to WGS84 used by GPS. Also, beginning in 1984, older maps have both NAD83 and NAD27 tick marks. If in doubt, check the datum designation at the bottom of the map.

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.

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. (-:

I was this March with the family at the observatory park and thought that there was some kind of problem with the gps receiver of my mobile. Very interesting!

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...

Link to the actual paper: http://link.springer.com/article/10.1007/s00190-015-0844-y

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.

I had a stroll there a few months ago: https://igcdn-photos-a-a.akamaihd.net/hphotos-ak-xfa1/t51.28... left leg in the western and right leg in the eastern hemisphere.

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

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

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...

> 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.

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

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

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