Absolute location: how the equator, meridians, and our system of positioning work

Defining absolute location

Where are you? Seems like a pretty simple question to answer, but think again — where are you, on the surface of the Earth? Are you inside your house, on the street, or maybe at work? Those are all relative locations because they describe your position relative to something. If you want to offer a more precise answer, you need to use absolute location.

Absolute location is a way of explaining where on Earth you are by using a scientific system of coordinates — typically, latitude and longitude. Latitude is essentially an angle that ranges from 0° at the Equator to 90° (North or South) at the poles. Conversely, longitude measures where you are east or west of the prime meridian in Greenwich, England — which, by convention, has a longitude of 0°.

So if we were to give a simple definition, absolute location refers to the exact geographic coordinates of a place, person, or any other object on the Earth’s surface. Most commonly, this is represented using a map projection, which implies transforming a 3D position on the globe to a 2D representation of the position. These coordinates are most commonly given in terms of latitude and longitude and most recently, in terms of global positioning systems.

It’s a simple concept, but with complex implications and far-reaching applications. Let’s take it step by step, in more detail.

Describing absolute positioning: the equator

We live on a spherical planet — well, it’s not exactly spherical, the Earth is rather an irregularly shaped ellipsoid, but for our purpose, let’s assume it’s a sphere.

If you want to explain where you are on a sphere, you have to define some system of coordinates. Thankfully, although the Earth isn’t exactly a sphere, and it’s not exactly symmetrical, there’s a useful frame of reference you can use for this: the Earth’s axis.

Our planet rotates daily about its axis. From this axis, we can define two poles (the north and the south pole) as the points where this imaginary axis ‘enters’ and ‘exits’ the earth. You draw a line halfway between the north and the south pole and boom: you have an equator. Already, you can use this equator, which has a physical significance, as it’s determined by the axis of rotation.

With this equator, you can define how close you are to the equator and the poles — by convention, one of these is the north pole, and the other is the south pole. This is called latitude.

But you can’t define a position using just the latitude, you need the longitude as well.

History of latitude and longitude

You may be tempted to think that absolute location is a relatively novel concept, but the notion was well-known by the Ancient Greeks. For instance, in 200 BC, Eratosthenes first discussed absolute positioning using latitude and longitude, as did several other ancient scholars, like Hipparchus.

But the ancient Greeks ran into a problem: you have a physical reasoning for drawing an equator, but you don’t have a physical reasoning to draw the other line (let’s call it a ‘meridian’).

Since you don’t have a better alternative, you can just set one meridian by convention. The Greek geographer Strabo did this over 2,000 years ago, consistently using a meridian for a world map.

But then, for centuries, there was no international convention.

It was only in 1884 that the International Meridian Conference took place in Washington DC, and 22 countries voted to adopt the meridian we use today. The meridian passes through the town of Greenwich just north of London, which is why we call it the Greenwich Meridian (or prime meridian).

Then, in time, other countries gradually accepted this and it became the norm we’re still using to this day.

There were several reasons why Greenwich was chosen for this. A part of it was history, but a much bigger part was the fact that the USA and most maritime companies were already using Greenwich as the Prime Meridian.

So then, with an equator and a prime meridian, you have a coordinate system where you can have an absolute position for every point on Earth.

The math of sphere positioning

Every site in the world can be described using its latitude and longitude. For instance, the place where the equator meets with the prime meridian has 0° latitude, and 0° longitude. We do it in degrees because remember — the Earth is a (sort of) sphere. So how does all of this work?

Well, you could look at it in multiple ways, but technically speaking, both latitude and longitude are angles. A circle has 360 degrees, a half-circle has 180 degrees. Since we measure it from 0 degrees, latitude varies between 0° at the equator, +90° (or 90° North), and -90° (or 90° South). Latitude is the angular distance of a place north or south of the Earth’s equator. Analogously, the same thing happens for meridians.

Each degree of latitude covers about 111 kilometers on the Earth’s surface. You can divide one degree into 60 minutes, and one minute into 60 seconds. A second of latitude covers 30.7 meters. It can be confusing using minutes and seconds, but in this case, these are not a unit of measure for time.

Since this can all get a bit confusing, people commonly use lines of latitude, also called parallels, and lines of longitude, called meridians. Parallels run east-west as circles parallel to the equator, with four of them being particularly notable:

Arctic Circle	66° 34′ (66.57°) N
Tropic of Cancer	23° 26′ (23.43°) N
Tropic of Capricorn	23° 26′ (23.43°) S
Antarctic Circle	66° 34′ (66.57°) S

Meanwhile, meridians are imaginary lines that run around Earth north-south (up and down) and meet at the North and South Poles. They’re not exactly parallel, because they meet up at both poles of the Earth.

Finding your absolute location

You can determine the absolute location of any point on Earth using latitude and longitude. This can be done in several ways. Let’s take New York, for instance: one way of determining its position is to say that it lies at a latitude of 40.730610 and a longitude of -73.935242. If you input those coordinates into a map service (40.730610, -73.935242), it will take you to New York.

But you could do the same thing by expressing the coordinates in minutes and seconds and using North/South/East/West notations instead of + and -. For instance, the above coordinates equate to 40° 43′ 50.1960” N and 73° 56′ 6.8712” W — input that into the map service, and you’ll get the same location.

So going back to the original question about an absolute location, absolute location describes the location of a place based on a coordinate system — most commonly, the latitude and longitude of the place. There are alternatives like geolocation that have become somewhat popular, but the absolute dominant standard is currently latitude-longitude.

If you use your smartphone where you are in the world, it will let you know your absolute location — if you’re enabling it to track itself. Nowadays, this is done with a satellite network, most commonly GPS. The phone sends out a signal to multiple GPS satellites and then tracks back the signal, georeferencing the position.

Up until a few years ago, GPS-type systems were restricted to the military, and when they became available to the public, they were inaccurate, with significant errors. But thanks to great improvements in technology, GPS tracking now offers precision in the range of one or several meters in most cases. So while your phone may not be able to know your exact absolute location, it will be pretty close.

Similar coordinate systems have been defined for other solid celestial bodies, such as the Moon, Mars, Venus, and even moons like Europa or Triton. Having a system like this is not just convenient, it’s very useful and important for efficient communication and travel.