The Greenwich Meridian is in the wrong place

I am at the grenwich observatory in London hence this fantastic uh rainy British Spring weather and I'm here because I wanted to see uh line zero this is it the prime meridian this is what defines zero when it comes to longitude so your GPS will show you exactly zero if you're on this line except it doesn't zero is not here anymore it's over there in the park [Music] it's true I'm right on well I'm not right on the line I'm currently in the Western Hemisphere Eastern Hemisphere on the line and if I get my GPS

right now it is uh negative 0.1 14333 now obviously a phone is not super accurate but it is accurate enough to show me this is not zero and if I walk around trying to find zero it's about 100 met that way obviously this annoys tourists who visit the line look at their phone and realize it's not actually on zero and to give you a sense of what that might look like we have a dramatic reenactment by my mate Steve mold if I could leave the Greenwich Meridian zero Stars I would it was actually Steve who

originally told me about the zero line moving and he kindly joined me to make a video about it now because this is a standard Mass video we're going to do things very thoroughly so we're here on the incorrect zero a completeness I know later on I've uh sent future mat to go out into the park in fact future mat we have future M with us right now are you there yes we're here in the future on the line in the park how are things in the courtyard you know it's it's all going pretty well here

um in the courtyard apart from the disappointment at the zero lines in the wrong spot everything everything's great in the Park yeah I mean is your Steve complaining about the promise as well yes yeah oh absolutely all right so we're going to hand over to both of you in the Park thank you very much so we spent a while walking around earlier we had out your phone looking at the GPS location on future Steve's phone and uh we found it this is it we it it it comes straight down there across the path and off

that way this is your proper zero line should we label the line okay yeah of course if we we need to run a line north south okay uh this is the actual zero line for all GPS uh we're going to send back to Matt and Steve in the past thank you very much future Matt and future Steve now current St Just Go With It mate okay so when you first mentioned this to me and I asked I was like why how like how can this not be zero anymore was it ever zero what was going

on I'm going to paraphrase what I remember you said they were using the wrong down yeah yeah well so the wrong down the wrong up basically when they were figuring out where the line should be they needed a way of looking directly upwards okay and it turns out that there is more than one way to define directly upwards cuz looking at the stars which is why we're at an observatory yes for a line that's the way to do it and we now use a different up yes I know so you you explain this to me

and I don't know if I brought this up talking to you yeah or on the channel yeah wrote a book about trigonometry yes and the US edition is out this August 20th detail about the pre-order sweep stakes and free digital Bookmark at the end of the video but in it I wanted to mention this yeah and so I thought you know what I'm going to obviously your explanation crisp but I thought I'm going to dig into it a bit more and what I found out is did I get wrong no you were 100% correct you

were totally on the money uh but what I realized almost immediately is that this this is not the line oh what is the line then this is the Line This is a telescope actually but the telescope is what defines the line it's not like people worked out where the line should be in theory and they had to calculate where to put this brass strip the T the brow strip is just pointing this is where the telescope is well it lines up exactly with the eyepiece of the telescope in fact the eyepiece is the line nothing

is the line the only line is the eyepiece on this telescope everything else is just pointing to where that is that's zero which is quite pleasing because the eyepiece is shaped like a zero so that that is that is zero and the reason it's on a telescope and it's not actually a line is because it's not really about measuring anything it's all about timings and for that we're going to a different location before we get to the timing problem we need to Define some of our terms which is why I brought this model of the

Earth and we got some tape okay so this is the Earth the issue with the Earth is is a sphere yeah which makes working out where you are difficult in terms of like having an origin like where do you decide where Z 0 is like on a sheet of paper you could say it's this bottom corner is z Zer but on a on a sphere there's no obvious place to choose famously no corners yeah we get one line for free well we get one line and two dots right cuz the Earth is is spinning yes

so we get our axis of rotation so North is the top of the axis that's spinning around South is the bottom and we get the Equator so so we so we say Okay zero in this direction in the up and down direction will just Define as the Equator the middle bit the middle bit so we already have a zero it's it's the it's the other one that is a prop okay okay great we're going to mark that one so that's our zero latitude yes so you can have positive latitudes like we're at positive latitude netive

yep on negative yeah what we would love to have yeah is an equally unambiguous zero zero for this dire yes yes and uh we don't if I said um where is the Paris observatory in the past you would say the Paris Observatory is X degrees uh east of the London Observatory but wouldn't it be nice if you could have a zero so the everyone just um reference the same thing and you can say London is here Paris is here instead of it you know always talking in terms of relative to this relative to that it

was a big vote in 1884 right where all the countries got together not all the countries a lot of the countries got together had a big argument took a vote and where we're at right now one okay yeah because Paris I mean the French have been using their own line we've been using our line everyone else had different lines yeah and then they decided to be easy if we all use the same one there you go you want to Mark us on there somewhere this is me taking a guess at what our latitude right okay

yes yes entally I'll put good enough great great great doc thank you I did it across it's across from across this really oh excellent smudge so you're here yeah how do you work out your latitude and longitude yeah so so you look at the stars you look at the stars right okay yeah okay so turns out your latitude how far up you are is easy because if this is the Earth and where in this fantastic room at the Royal Observatory grage there were like there are books and things everywhere around this room around us yeah

we can consider them to be the Stars imagine they're the Stars they're the Stars they're the Galaxy around us okay oh relative to this relative to this earth fine and yes the Earth is going around the Sun and all that jazz but that's happening on such a small scale it doesn't make a difference oh I see so the the the fact that the Earth is kind of moving that way and that way and this way and this way because of its orbit around the sun doesn't matter because everything's so far away that they essentially don't

move they don't move we'll assume they're infinitely far away infinitely far away but but still visible yes and so if you like if there's like a star over there where the camera is yeah and you're on the equator yeah and you want to look at that star it's directly above you yeah cuz you're like you got to look straight up cuz it's over there but if you were up here on this Dot and you look straight up you're like I can't see that star I'm looking at a star over there yeah and then what you

can do is measure the difference between looking straight up over there and you're like I got to turn my telescope all the way down some number of degrees to be able to look at that the same star that was overhead here yeah yeah and the number of degrees you got to tilt it down latitude that's great that's so good so you could use um to any any star that is on that equatorial line you could use that as now I'm sure in practice this is much more complicated than I've made it out to be like

this is I'm sure there's a bunch of stars and we know the angles they should be and you measure the angle you're looking at them with I don't know with like a a telescope and a seon or something I don't know right but but you can work out that nice and easy yeah the question now is why doesn't that work for working out how far because you think well should work for this line right CU if you're on this line there's a star over there we're producing a CO yeah right directly above and over here

you're measuring the same angle so you just tilt until you're looking at that star so the issue is you have to know what time it is well that yeah exactly the the issue is this is happening all the time and the equator is fine that's not going anywhere yeah cuz that's defined by being the middle of the axis of rotation yeah so that's staying fixed but this lines all over the shop so so that that doesn't help you what you need to do is measure if you're here and you've got the producer the cstar yeah

you need to measure as you sweep above it what time is it overhead yeah that's what this is all about now we're at the front of the telescope this is it this is the telescope that did the timing and this is the point in the video where Steve gets very excited by explaining what up is so okay the way you use this for timing is you're looking straight up in the sky and the stars are very slowly sweeping across your eyepiece your field of view and there are certain stars that are very bright and you

use those for timing purposes you know that this star is going to be at the very center of your eyepiece at a specific time so when that H happens you mark it and you you set your clock and you know that's the specific time right but how do you get the telescope to point exactly upwards and it's use something called a Zenith tube right that's the name for something that points directly upwards and the way you would do it with this is is you would use a mirror made of mercury so at some point the

light that you focus is bounced off a mirror and then you're looking at the light bounced off the mirror and because it's made of mercury flat it's perfectly flat perfectly level that's right yeah so gravity local gravity takes care of leveling the mirror because it's made of a liquid metal and so you know the the normal line to that plane is always going to be vertically upwards according to gravity and the great thing about local gravity famously can't be corrupted no exactly no impact from outside influences other than the varying density of the Earth the

fact of the Earth's a weird lumpy shape well it is a weird lumpy shape but I mean so you can argue about what's the best definition of up right I mean I would say the best definition of up is parallel lines to local gravity no the best definition of up is goes through the center of the earth okay well that's a well that's a very modern defition okay I think it's pretty good and the issue is uh you're up Mercury up Mercury up does not go through the center of the earth yeah so if you

take that line from local gravity and you you you carry it on you draw it down through the Earth it will not pass through the very center of the earth like the geometric center of the earth right or the center of mass of the Earth it won't pass through that oh I would say it's the midpoint of the axis of rotation okay yeah the center of the Earth all right yeah it doesn't pass through that anyway the local gravity line and that causes some problems that's not even the first zero before this telescope there's one

over here so we've actually got a different line this used to I say used to be zero this still is zero for the OS Maps is it so Bradley's meridian line Bradley's telescope yeah cuz they started in the early 1800s before that telescope had been built and when you're building a new better telescope they're like this one's getting a bit old you don't want to stop using it while you're putting in the new ones they don't bother cuz this is the line is it didn't matter doesn't matter doesn't matter and the line is the eyepiece

so they're not going to faf around trying to get the ey piece in the same place they just put in a new telescope over there oh yeah it' be real fat to line it up yeah new line and this isn't the first line so all the way down there this so that was Bradley's Line This is Haley's line of comet Fame right Haley haly no you can see this is getting less and less like a telescope this further back in time you go more rudiment the telesc this is a tube attached to a protractor same

idea yeah and when Haley was using this this is the line that lines up with this is the official reading protractor on this side oh okay so there there's there's this line and people were doing stuff that way as well they every time they get a new better telescope they just move it down a little bit so they got space to build it well in that sense it's nice that the GPS zero keeps going that further that way this is cool look at this because so um we were talking about having like a vertical upwards

you know using a Mercury mirror but another way of doing it which is much simpler is to just hang it you hang a tube so look it's uh hung from up there there's even a plum line uh down here this wire hanging off a uh a mass hanging off this wire you line it up with that and that's how you get vertically up with it's not as good as a Mercury mirror but well this one was early 1700s yeah this was like 1720 they started putting this in and as they got better and better the

line just keeps moving that way to understand the issue now we need to understand how you use this line to work out where you are right because this line is not a line as we keep saying it's just a bunch of observations for exactly what time a particular star a particular star goes overhead and you write that down we know this star goes overhead exactly this time and this star at exactly EX ET ET right so we have all these incredible timings what we actually have the Prime Meridian is just a bunch of reference times

of stars that move relative to the local noon at the prime meridian so if you're over here on your where's your smudge gone it's so easy to see good work so what you're doing is working out what fraction of the day difference is the time local time zones between you and grenwich yeah and that tells you what fraction the planet around you are that's your number of degrees longitude great so all you're really doing and this is why clocks were so important and there's a whole other story here about oh yeah Harrison made some clocks

yeah cuz you need as you're traveling around you can't like phone back yeah this is pre radio waves yeah you've got to know what the time is in Greenwich while you're out and about yes Steve then pointed out that I was not completely correct in what I just said and that solar time zones are not exactly what we want you know so like okay you think about it right the the sun needs to turn through a full turn for a day that's the way we think about it but in that time the Earth has moved

a little bit around the Sun so to get um back you've got to turn a little bit more y so that you're facing the sun again right after after a full turn according to the Stars you need to turn a little bit more because you've gone around the Sun a little bit I should we do it with the globe yeah okay great right so if you start noon's there pointing at the sun means they're pointing at the sun yeah you then do a whole rotation like this yeah but by the time you get back here

this is orbiting the Sun so it's actually over here now yeah and so it's not pointing at the Sun at the sun it's got to go a little bit further yeah and so what we call a day yeah is slightly longer yes than a day and just it's going to have to be something like 1 365th of a day you have to add extra go get and when we're doing things relative to the Stars that's siderial time is that how you pronounce it I called it side real time cuz that I've only ever seen it

written so that's what I said yeah I said side real time and my wife as an astronomer was like you mean siderial time I suppose she knows yeah I'm like okay I'm going to have to consult an independent astronomer yeah yeah yeah so yeah the reason you look at the stars and you get your timing is you want siderial time okay so actually if you're out in the middle now I don't know the practicalities of how they did this yeah I'm sure some people will know this in great detail I feel like we're getting too

far off the math path though okay fine that you want to get your local time somewhere else and then compare it to this one okay great and you're right solar time is terrible yeah we can all agree so here comes the problem if the telescope that was used to get the timing of this line was off a bit I can't help but geometrically thinking of about that in terms of it not going through the center of the earth yeah so if you're if you're you know you've got a line pointing upwards just continue that line

downwards it's going to ideally go through the center of the Earth but um but not so I'm going to try and label that this is what I imagined when you first told me all about this and this is what I think a lot of people are picturing and for me this is what makes the most sense when you say they got up wrong yeah okay there it is so so the plane now defined by okay so what I've done we'll do we'll do a spin yeah what I've done is if if they're here imagine this

overlaps perfectly If instead of looking straight that way you're a little bit off you're going to Define This Plane which is no longer a great circle which is off to the side yes right and that's what I was picturing when you were saying they're using the wrong up yeah but actually this doesn't matter yeah because it's all about timing the only difference was instead of looking directly above they were looking ever so slightly too far ahead yeah in the future yeah so all their timings were just off by a tiny tiny amount that's right so

they were actually timing when the stars go overhead of a point about 100 met that way yeah so they didn't actually do anything with this line yeah they did did another great circle which is just ever so slightly tilted let me get let me get the exact number okay so there's the number of degrees that's out divide that by 360 that's the the fraction of a day okay that we're off so we just go with 24 hours times 24 is that many hours off it's that many minutes off it's that many seconds off a third

. 35 seconds so that's how long it takes that's how long it takes yeah so for a third of a second we're over there it's so close so CL and for all purposes previously it didn't matter no like it was so it was it was so incredibly close so in 1884 we voted in this line yeah and we all agreed it was good enough which it was it was within the accuracy of everything we were doing fine time advanced technology got better we had satellites we had radar we could work out where things are much

much more accurately eventually I think it was like in the 1960s we started to notice there was a problem yeah and by then the tide was turning so when this line was defined it was all defined by looking out at the stars yeah gradually over time we did less looking out at the stars and more just looking back at the Earth Earth measuring the Earth using satellites around the earth and they decided to draw a line at some point and the name that the official body was called kept changing but at some point the powers

that be yeah had to just say stop looking at the stars and they made that final decision in 1984 oh wow 100 years later they stopped all Optical measurements of stars and made it 100% earth-based but when they when they did that they try and match it up as best they can with the old looking at the stars version and you think well why don't we just put zero here yeah you could just shift it all around and decide that's where it's going to be well you'd have to convince every other location on Earth to

change their their GPS by 102 M or whatever it is yeah to solve one issue and the one issue is tourist complaining they made the new system fit the old system as best as possible yes because you've got a different lumpy gravity everywhere and you can shift this kind of new definition around to try and make it fit here but then it's not going to fit there if here it's not going to fit there you're kind of doing a weighted average of minimizing all the errors yeah and gr lost 100 m i talk more about

this and cover loads of other triangle Topics in my new book love triangle which is out now in the USA you can see look it's very future mat with a copy in Central Park that's exciting and thanks to everyone who pre-ordered the UK version we were a number one best seller in the Sunday Times but what about the New York Times so if you pre-order the US version of Love Triangle before it comes out on the 20th of August the fine people who are publishing it at Riverhead uh they're running a sweep Stakes if you

pre-order it absolutely anywhere you can enter into to maybe win a Q&A with me online and definitely I will send you a customized virtual bookmark I mean you got to print your own bookmark and buy your own book but the point is I will send you a bookmark half of it will be me looking disappointed on the old line the nonzero line and the other half of the bookmark will be me on the real line and I be holding something with your name on it I feel like I can write some terrible python code to

automate that I reckon I can get that down to to ones of seconds per name so unless thousands of you sign up I'm starting to realize the floor on the plan here you can contribute to the problem by pre-ordering the book absolutely anywhere I will link to the site below where you can register your pre-order you will enter the sweep stakes and you will definitely get a bookmark participants must be us residents 18 or older other details on the website and yes if you did previously pre-order on Mas gear I will send you the bookmark

as well just cuz I appreciate your support that's pretty much it for the video I mean we should you know the real heroes we should thank past Matt and Steve past Matt and Steve we just want to say thank you so much for making this video possible yes thank you so much we're still here on the prime meridian having a great time very much enjoyed visiting the Royal Observatory Granite who very kindly let come in filming the line look at the telescopes had a great time oh and thanks to Steve thanks to P Steve for

joining here in the past at the observatory and uh we're all in the past aren't we and people watching this video it's all in the past yeah that's true we're all in the past some of us are less in the past than others and so thanks to Future Matt and future Steve for joining in and that's it thanks for watching and W wave wave wave all right okay you you check out Steve's Channel what should we watch now the algorithm thinks you might enjoy this video next