The Greenwich Meridian is in the wrong place

Summary

In this video, the host explores the Greenwich Observatory in London, discovering that the Prime Meridian line, which defines zero longitude for GPS, has moved from its original location. Through a dramatic reenactment and discussions with Steve Mold, they explain the historical and technical reasons behind the shift, including the transition from star-based to Earth-based measurements. The video also humorously addresses the impact on tourists and the complexities of defining 'up' and 'zero' on a spherical Earth, ending with a plug for the host's new book, 'Love Triangle'.

Takeaways

• 📍 The speaker is at the Greenwich Observatory in London discussing the Prime Meridian, which is the reference line for zero degrees longitude.
• ⛲ The Prime Meridian's original location was determined by the placement of a telescope's eyepiece, which has since moved slightly from its historical position.
• 🌧 The script humorously mentions the typical rainy British Spring weather during the visit to the observatory.
• 📱 GPS devices show a small inaccuracy when pinpointing the exact location of the Prime Meridian, with the speaker's device showing -0.1 degrees.
• 🎬 A dramatic reenactment by 'Steve mold' is mentioned to illustrate the confusion of tourists when their GPS does not show exactly zero at the Prime Meridian.
• 🔭 The script explains that the historical method for determining the Prime Meridian involved observing stars and their transit times across a telescope's eyepiece.
• 🌟 The Prime Meridian was originally based on the celestial observation of stars, but this method was later replaced by Earth-based measurements due to inaccuracies.
• 🌍 The Earth's rotation and its orbit around the Sun are discussed as factors that affect the definition of a day and the need for sidereal time measurements.
• 🕰 The importance of accurate timekeeping for determining longitude is highlighted, with a mention of Harrison's clocks and the transition from solar to sidereal time.
• 📚 The speaker mentions a book about trigonometry and the inaccuracies in the historical definition of the Prime Meridian, which was corrected in the 20th century.
• 🎁 The script ends with a promotion for the speaker's book, 'Love Triangle', and an offer for pre-order bonuses, including a virtual bookmark with a personalized touch.

Q & A

What is the significance of the Prime Meridian at the Greenwich Observatory?

The Prime Meridian at the Greenwich Observatory defines the zero point for longitude. It is the reference line from which all other longitudes are measured, and it helps in determining the exact location of a place on Earth.

Why was the Prime Meridian line moved from its original position?

The Prime Meridian line was moved because the original definition of 'up' used for astronomical observations was not perfectly aligned with the Earth's center of mass. As technology advanced, it was necessary to redefine the line to improve accuracy in global positioning systems.

How does the Earth's shape affect the determination of the Prime Meridian?

The Earth's spherical shape makes it challenging to define a fixed point for longitude. The Prime Meridian is determined not just by a physical line, but by a series of observations and measurements that account for the Earth's rotation and its position in space.

What is the role of the telescope in defining the Prime Meridian?

The telescope at the observatory is used to observe stars and determine the exact time they pass over a specific point, which helps in calculating the longitude. The alignment of the telescope with the stars is crucial for accurate timekeeping related to the Prime Meridian.

Why is the term 'solar time' not ideal for measuring the Earth's rotation?

Solar time is based on the Earth's position relative to the Sun, which is not a perfect measure because the Earth also moves around the Sun while rotating. This results in a slight discrepancy, making sidereal time, which is based on the Earth's rotation relative to the stars, a more accurate measure.

How did the historical use of a Zenith tube contribute to the inaccuracies in the Prime Meridian's original location?

The Zenith tube, which was used to point directly upwards, relied on local gravity to level a mercury mirror. However, local gravity does not perfectly align with the Earth's center of mass, leading to slight inaccuracies in the timing of star observations and thus the position of the Prime Meridian.

What was the international agreement in 1884 regarding the Prime Meridian?

In 1884, an international agreement was reached at a conference where countries decided to adopt the Prime Meridian at the Greenwich Observatory as the global zero point for longitude to standardize timekeeping and navigation.

How did the advent of satellite technology influence the redefinition of the Prime Meridian?

The introduction of satellite technology allowed for more precise measurements of the Earth and its features. This led to the realization that the original astronomical observations used to define the Prime Meridian were slightly off, prompting a redefinition based on Earth's center of mass.

What is the current method for defining the Prime Meridian?

The current method for defining the Prime Meridian is based entirely on Earth's center of mass, using geodetic measurements from satellites and other advanced technologies, rather than astronomical observations.

Why is the discrepancy between the old and new Prime Meridian locations only about 100 meters?

The discrepancy is minimal because the original definition was already very close to accurate. The slight adjustment of about 100 meters was made to align the Prime Meridian with the Earth's center of mass, improving the accuracy of global positioning systems.

What is the significance of the book 'Love Triangle' mentioned in the script?

The book 'Love Triangle' is a publication by the same author who discusses the complexities of the Prime Meridian and other related topics. It provides a deeper understanding of the subject matter and is promoted as a resource for further reading.

Outlines

00:00

🌍 Greenwich Observatory and the Prime Meridian

The video script begins with the narrator standing at the Greenwich Observatory in London, discussing the significance of the Prime Meridian, which is the reference line for zero degrees longitude. Despite the GPS showing a slight deviation from zero, the narrator explains that the true Prime Meridian has moved from its original location due to changes in the definition of 'up' and the use of a different telescope for its measurement. The script includes a humorous reenactment with a friend, Steve Mold, to illustrate the confusion tourists face when their GPS does not align with the marked Prime Meridian. The video promises a thorough exploration of the topic, including the history and technical aspects of longitude measurement.

05:02

📡 The Evolution of Longitude Measurement

This paragraph delves into the historical and technical aspects of determining longitude. It explains how the Earth's rotation and equator provide natural references for latitude, but longitude requires an agreed-upon reference line. The narrator discusses the international decision in 1884 to standardize the Prime Meridian, which was previously contested by various countries using different lines. The script also covers the method of using stars to determine latitude and the challenges of defining a universal zero longitude, highlighting the importance of accurate timekeeping for celestial navigation.

10:04

🕰 The Importance of Time in Longitude Measurement

The narrator explains the crucial role of time in determining longitude. By observing stars and noting the exact time they pass overhead, astronomers can calculate their longitude relative to the Prime Meridian. The script describes the use of a zenith tube and a mercury mirror to achieve a true vertical line for accurate celestial observation. It also touches on the limitations of local gravity and the historical shift from using stars to satellites for Earth-based measurements, which led to a redefinition of the Prime Meridian in 1984.

15:09

🔍 The Shift from Celestial to Earth-Based Measurements

This section discusses the transition from using celestial bodies for timekeeping to relying on Earth-based measurements. The narrator explains how advancements in technology, such as satellites and radar, allowed for more precise location tracking. The decision in 1984 to cease optical measurements and rely solely on Earth-based data led to a slight adjustment in the Prime Meridian to maintain consistency with previous measurements. The script also mentions the complexities of aligning the new definition with the old system due to the Earth's uneven gravity.

20:14

📚 The Release of 'Love Triangle' and a Special Offer

In the final paragraph, the narrator shifts focus to the release of their new book, 'Love Triangle,' which includes a discussion on the Prime Meridian among other topics. The script promotes a pre-order campaign with a sweepstakes for a chance to win a Q&A session and a personalized bookmark. The narrator thanks the viewers for their support, acknowledges the contributions of past and future versions of themselves and their friend Steve, and invites viewers to check out Steve's channel for more content.

Keywords

💡Greenwich Observatory

The Greenwich Observatory, also known as the Royal Observatory, is a historical site in London known for its significance in astronomy and navigation. In the video, it serves as the backdrop for discussing the concept of the Prime Meridian. The script mentions the observatory as the location where the Prime Meridian, or 'line zero,' is supposedly located, which is central to the theme of longitude and GPS accuracy.

💡Prime Meridian

The Prime Meridian is the meridian (line of longitude) defined as 0°, from which all other longitudes are measured. It is the reference line from which global time zones are established. In the video, the concept of the Prime Meridian is explored, noting that its actual position has shifted over time, which is a key point in the narrative about the accuracy of longitude measurements.

💡Longitude

Longitude refers to the geographical coordinate that specifies the East-West position of a point on the Earth's surface, as opposed to latitude which specifies the North-South position. The script discusses the importance of longitude in navigation and how the Prime Meridian serves as the baseline for measuring it. The video highlights the historical and technical aspects of defining longitude.

💡GPS

GPS stands for Global Positioning System, a satellite-based navigation system that provides location and time information in all weather conditions, anywhere on or near the Earth. The video script mentions GPS in the context of how it uses the Prime Meridian as a reference point and how its readings can be affected by the actual position of the Prime Meridian.

💡Zenith

In astronomy, the zenith is the point in the sky directly above an observer. The script discusses the use of a zenith tube, which is an instrument that points directly upwards to determine the local vertical. This is relevant to the video's theme as it relates to the historical methods of determining the Prime Meridian through celestial observations.

💡Sidereal Time

Sidereal time is a timekeeping system used by astronomers, which is based on the rotation of the Earth relative to the stars, rather than the Sun. The script explains the concept of sidereal time in the context of how it was used to measure the positions of stars and thus determine longitude in the past. It is a crucial concept in the historical understanding of celestial navigation.

💡Harrison Clocks

Harrison Clocks refer to the series of marine chronometers developed by John Harrison in the 18th century to solve the problem of precise timekeeping at sea for accurate navigation. The script briefly mentions Harrison in the context of the importance of accurate timekeeping for determining longitude, which is a key aspect of the video's exploration of navigation history.

💡Meridian Line

A meridian line is any line of longitude on the Earth's surface, running from the North Pole to the South Pole. The script discusses the historical movement of the Prime Meridian line, which is a central theme of the video. It explains how the line's position has changed over time, affecting the accuracy of longitude measurements.

💡Tide

In the context of the video, 'tide' refers to the gravitational forces exerted by the Moon and the Sun that cause the periodic rise and fall of sea levels. The script mentions tides in the context of how they affect the Earth's shape and gravity, which in turn influenced the accuracy of the Prime Meridian's position.

💡Earth-based Measurements

Earth-based measurements refer to the use of satellites and other technology to measure the Earth's dimensions and features without relying on celestial observations. The video script explains that in the 1960s, the decision was made to stop using optical measurements of stars and rely solely on Earth-based measurements, which marked a significant shift in how the Prime Meridian was defined.

💡Pre-order

The term 'pre-order' is used in the script to refer to the act of reserving a copy of a book before its official release date. The video mentions a pre-order campaign for the book 'Love Triangle,' with incentives for early buyers, which is a way to engage the audience and promote the book's upcoming release.

Highlights

Visit to the Greenwich Observatory in London to see the Prime Meridian, which defines zero longitude.

Discovery that the actual zero longitude is not at the marked line but has moved to a location in the park.

Explanation of GPS inaccuracies and the difficulty of finding the exact zero longitude with a phone.

Dramatic reenactment by Steve Mold to illustrate the confusion of tourists regarding the zero line.

The historical shift of the zero line due to the use of different 'up' definitions in astronomical observations.

The role of the telescope and its alignment with the eyepiece defining the true zero longitude.

The challenge of defining a universal zero longitude due to the Earth's spherical shape and lack of a fixed origin.

Historical agreement in 1884 to standardize the Prime Meridian for global navigation and timekeeping.

The method of determining latitude by observing stars and measuring angles with a telescope.

The distinction between solar time and sidereal time, with the latter being crucial for accurate star observations.

The importance of clocks in synchronization with Greenwich Mean Time for navigation before modern technology.

The transition from star-based measurements to Earth-based measurements in the 1960s due to technological advancements.

The decision in 1984 to cease optical measurements of stars and rely solely on Earth-based measurements for defining the Prime Meridian.

The practical implications of the Prime Meridian's shift on global GPS systems and the effort to minimize discrepancies.

The release of the book 'Love Triangle' by the author, discussing various topics related to trigonometry and the Prime Meridian.

Involvement of the audience through a pre-order sweepstakes for the US edition of 'Love Triangle' with exclusive rewards.

A humorous conclusion reflecting on the nature of time and the video's content being in the past, inviting viewers to engage with the content.

Transcripts

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