The Prime Meridian
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Located in East London is the Royal Greenwich Observatory. Passing directly through the observatory is an imaginary line that extends from the North Pole to the South Pole. This line is significant because it's the reference point for every other line of longitude in the entire world. What makes it even more remarkable is that there's no objective reason for this particular line to be so important.

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Before I get into the Prime Meridian and its very strange history, I should first provide an overview of the geographic coordinate system, also sometimes known as the spherical or geodetic coordinate system. The system consists of two sets of imaginary lines superimposed upon the Earth's sphere. The first set of lines are the lines of latitude.

Lines of latitude, also known as parallels, are imaginary horizontal lines that run east-west around the Earth, measuring distances north or south of the equator. They are measured in degrees, from 0 degrees at the equator to 90 degrees at either of the poles, 90 degrees north at the North Pole and 90 degrees south at the South Pole.

Other notable lines of latitude include the Tropic of Cancer at 23.5 degrees north, the Tropic of Capricorn at 23.5 degrees south, the Arctic Circle at 66.5 degrees north, and the Antarctic Circle at 66.5 degrees south. Each of these unique lines corresponds to the tilt of the Earth, which is 23.5 degrees. Lines of latitude never cross, which is why they're also known as parallels.

The distance between each degree of latitude is exactly the same everywhere on Earth. Regardless if you're at the equator or the poles, a distance of one degree latitude is about 69 miles or 111 kilometers. As you move closer to the poles, the length of each line of latitude is smaller than the one before it. When you get to 90 degrees north or south, it shrinks down to a point defined by the poles.

For the purpose of this episode, the equator is a special line that all the other lines of latitude are based upon. We didn't pick the equator. The equator naturally arises from the rotation of the Earth about its axis. If our civilization disappeared tomorrow, thousands of years from now, another civilization would identify the equator for the same reasons we did.

The equator is unique because it's the longest line of latitude, and it's the line that is equal distance from each of the poles. The Coriolis effect is weakest at the equator, meaning hurricanes and cyclones rarely form there. Additionally, due to Earth's rotation, objects at the equator experience the fastest linear velocity, moving at about 1,670 kmph or 1,038 mph.

Early navigators were able to use and figure out lines of latitude quite easily. With the sextant and the stars, it was possible, with a fair degree of accuracy, to figure out your latitude. So to summarize, while lines of latitude are imaginary, they are based on the very real equator, which can be measured. The other lines that make up the geographic coordinate system are lines of longitude, or meridians, which are the more important lines for the purpose of this episode.

Lines of longitude run north-south from pole to pole. Unlike lines of latitude, which never touch, every line of longitude meets at the poles. The distance between each degree of longitude is different depending on which line of latitude you're measuring at. At the equator, each degree of longitude is 69.172 miles or 111.321 kilometers apart.

In a previous episode, I covered the history of determining longitude. This was a problem that stumped humanity for centuries. As easy as it was to measure your location north to south, it was just as impossible to measure your location east to west. This was responsible for numerous shipwrecks because navigators couldn't tell their exact location. Eventually, a system was discovered, and that system was based on accurately keeping time.

An 18th century English clockmaker by the name of John Harrison developed a highly accurate timepiece that could work on sailing ships. The way it worked is that the clock would keep time based on a known line of longitude. When the ship was at sea, they would then measure the time of local noon and compare that to the time on the clock. The difference in time would then determine what line of longitude you were at. Couple that with your latitude as measured with a sextant, and you could know your exact location.

This was a huge breakthrough in navigation, however there was still a problem. In order for the system to work, you needed some known reference line of longitude, aka a prime meridian. And this is the other major difference between lines of longitude and lines of latitude. There is no natural reference line of longitude. There are an infinite number of lines of longitude and there's nothing particularly special about any of them.

While the method of determining longitude occurred in the 18th century, the idea of longitude lines goes back at least to ancient Greece. Eratosthenes and Hipparchus proposed a system of longitude in the 2nd and 3rd centuries BC. The Greco-Roman geographer Ptolemy, who lived during the 2nd century, established a reference line of longitude at the Fortunate Islands, believed to be the Canary Islands in his famous work Geography.

Islamic geographers improved the art of cartography, but oddly enough, they never established a prime meridian. And I say it was odd because Islam has a particular point on the earth that plays a central role in the religion and would have made for an obvious prime meridian, Mecca. European cartographers in the Middle Ages often used Jerusalem or Rome as a reference point for their maps.

In 1493, Christopher Columbus reported that in the middle of the Atlantic Ocean, there was a point where the compass pointed to true north, not magnetic north. Very briefly, it was thought that this might be the natural line of longitude that could be used as a reference. However, it was later found out that the line of zero magnetic declination did not follow a line of longitude, and it was constantly moving as the magnetic pole moved.

During the Age of Discovery, as more countries began sailing around the world in an open ocean, different nations developed their own prime meridians based on national observatories. Spain used the Madrid Meridian. France used the Paris Meridian, established in 1679 at the Paris Observatory. And Britain used the Greenwich Meridian, established in 1675 at the Royal Greenwich Observatory. This system of national prime meridians was fine for a while.

In the 18th to 19th century, after the British figured out the key to determining longitude, they set their ship clocks to the time at the Greenwich Observatory, which became known as Greenwich Mean Time. The reason why Greenwich was used by the British was because they were able to make more accurate calculations regarding time via astronomical observations than any other observatory in the country.

By the late 18th century, the nautical almanac used Greenwich as its reference point, making Greenwich Mean Time widely adopted by British mariners. By the 19th century, however, increased global trade and standardization, such as the development of the metric system, demanded a common basis for time and longitude, which were highly intertwined. New technologies such as the telegraph and railroads could cross international borders and also necessitated the need for common standards.

With the expansion of the British Empire and its dominance in naval and commercial activities in the 19th century, Greenwich Mean Time naturally became the preferred reference time for global navigation. The International Geodetic Association, during their meeting in Rome in 1883, brought forward the need for an internationally agreed-upon prime meridian. They proposed three criteria that any prime meridian must meet to be accepted internationally. First, it must have a first-rate astronomical observatory.

Second, it had to be directly linked by astronomical observations to other nearby observatories. And finally, third, it had to be attached to a network of first-rate survey triangles in the surrounding country. According to a report published by the association, there were only four observatories in the world that met the criteria. Berlin, Paris, Greenwich, and Washington, D.C. It was their recommendation that Greenwich be selected as it was the best choice for a host of geographic and astronomical reasons.

In 1884, the International Meridian Conference was held in Washington, D.C., where 22 of 25 countries voted to adopt the Greenwich Meridian as the Prime Meridian and use Greenwich Mean Time as the world's standard time. If you visit the Greenwich Observatory today, which I highly, highly recommend you do if you ever visit London, you can stand right on the Prime Meridian. They have a line in the ground that you can walk on or over.

To be extremely technical, the prime meridian corresponds to the eyepiece of the telescope which was used at the observatory. If you visit, I also suggest you do the following. Take out your smartphone and open up a GPS app which gives you the exact latitude and longitude. And what you will see is that the prime meridian line on display is not in fact the prime meridian.

The prime meridian line at the Greenwich Observatory does not exactly correspond to the GPS-defined prime meridian because of differences in how they were determined. The GPS-defined prime meridian is based on the International Terrestrial Reference Frame, a more precise system that uses satellite data to measure the Earth's shape and rotation from its center of mass rather than from a fixed point on the surface.

This geodetic system eliminates local gravitational influences and provides a globally consistent reference. As a result, the modern geodetic prime meridian used in GPS is located about 102 meters or 334 feet east of the historic Greenwich Line. This shift represents a difference between older astronomical positioning methods and the more accurate Earth-centered coordinate system used today.

There are a few other places in France, Spain, Algeria, Mali, and Burkina Faso where you can see the Prime Meridian. However, one of the most interesting places to pass us through is just outside of Accra, the capital of Ghana. When I was in Ghana, I remember passing by a Prime Meridian sign. I pulled out my phone to verify it, and sure enough, we drove right over 0 degrees longitude.

If you go further south of Ghana into the ocean, you will eventually reach a point in the open water which is known to geographers as Null Island. This isn't an actual island. It's simply where the Prime Meridian crosses the equator. It has zero degrees latitude and zero degrees longitude. I want to close this episode by bringing something up which applies to astronomers. On the Earth, the Prime Meridian is a totally arbitrary choice.

If you want to create a coordinate system for other planets, how do you go about selecting a prime meridian? For most celestial bodies, the process is just as arbitrary as it is for the Earth. On Mars, for example, researchers have arbitrarily defined the Martian prime meridian to be the line of longitude that passes through the crater Erie Zero. Mars also has an equator, so with those two features, you can then create a full latitude and longitude system on Mars.

If the Earth and Mars prime meridians are both arbitrarily chosen, does that mean that it has to be that way on every celestial body? The answer, surprisingly, is no. It is possible for some objects to have a natural prime meridian. Perhaps the best example of this is the Moon. The Moon has an equator because the Moon rotates around its axis. However, the Moon's rotation happens to be the exact same as the time it takes to orbit the Earth.

That's because the moon is tidally locked to the earth, which means that only one side always faces the earth. So the point which is always closest to the earth can serve as the point through which the line of longitude would run for a natural prime meridian. If you are looking at the moon, its prime meridian runs right down the center of the side that you can see.

The next time you look at a map online or determine a route somewhere on your smartphone, take a moment and think that it's all possible because of an arbitrary decision to place the prime meridian at an observatory in East London. The executive producer of Everything Everywhere Daily is Charles Daniel. The associate producers are Austin Oakton and Cameron Kiefer.

Today's review comes from listener TheNugget1227 over on Apple Podcasts in the United States. They write, Pineapple on pizza. This podcast is amazing. Hi Gary, I love listening to this podcast. You can literally make anything so interesting from rogue waves to the history of pasta, just to name a few. My question for you is, do you like pineapple on pizza? This is something that I will endlessly debate people on. Team pineapple for the win. I hope you're having an amazing day.

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