# Antipode Calculator

Digging a tunnel through the center of the Earth would bring you to your **antipode**: let's find out where it lies with our **antipode calculator**! Here you will learn a little bit of history, what the antipode of a point is, and **how to find the antipode** using a tiny bit of math.

## What is an antipode?

The antipode is the **diametrically opposite point of a given location on a sphere**. In the past, the question "what is an antipode?" had many interesting answers. The Greeks knew the Earth was spherical, and the polymath Erathostenes calculated its diameter with an error of only about $1000\ \text{km}$: impressive! Knowing the size of the planet, they were enthralled by the idea of landmasses on the other side of the globe, where people would live in the opposite direction of them: "anti" means "opposite" and "pous" means "foot".

In the middle ages, people had wilder imaginations, and instead of a land where people were standing upside-down, the term antipode came to mean upside-down people, with feet in place of their heads and vice versa!

*Terra Australis*, the hypothetical continent on the other side of the globe, was depicted in various ways, attracting explorers until it was clear that it was not much different to the one they came from. In later years, antipodes lost their mystical connotation and settled more as a curious feature of our planet.

## How to calculate the antipode of any point?

A point on the surface of a planet is uniquely identified by the pair of coordinates $(\theta, \varphi)$:

- The
**latitude**($\theta$) indicates the north-south position of the point, divided into 180 degrees, from +90° at the North Pole to -90° of the South Pole, passing through 0° at the equator. - The
**longitude**($\varphi$) gives information on the east-west position. There is no geographical indicator for $\theta = 0\degree$; it was arbitrarily fixed in 1884, choosing the Royal Observatory in Greenwich, London, as its reference point — you can go there and take a picture claiming to be in both hemispheres at the same time! If the point is located in the Western Hemisphere, then the longitude varies from $0\degree$ to $-180\degree$; in the Eastern Hemisphere, the value is positive between $0\degree$ and $180\degree$, for a total of $360\degree$.

And here is how to find the antipode $(\theta_\text{a}, \varphi_\text{a})$ to a point $(\theta,\varphi)$!

- The
**latitude**of the point, with a**change of sign**: $\theta_\text{a}= −\theta$ - The
**longitude**of the point transformed according to:- $\varphi_\text{a} = \varphi + 180\degree\ \textrm{if}\ \varphi \leq 0$
- $\varphi_\text{a} = \varphi - 180\degree\ \textrm{if}\ \varphi \>0$

That's how we calculate the antipode of a point! Easy, right?

Coordinates can get messy; do we use decimals or minutes and seconds — and weren't those time units?! If you want to find out more, try our coordinates converter!

## Some facts about antipodes

Earth is a blue planet: landmasses cover less than $30\%$ of the surface of the globe. In consequence, it's highly likely that the antipode of a given point lies in the middle of an ocean! **Only** $15\%$ **of the land has a land antipode**, and most of the rest falls into the Pacific Ocean.

The Pacific Ocean itself is so big it contains its own antipode! And speaking of containing the opposite point: the Sun never did set over the British Empire, as Britain and New Zealand are almost antipodal.

In 2020, two men (one in New Zealand and the other in Spain)

by placing bread slices on the exact coordinates of two antipodal points. They didn't manage to finish eating it, apparently.Another interesting fact! At every moment, there is at least one pair of antipodal points on Earth with the same temperature — we know this thanks to the $a$ and $b$. Now move around the globe following the **great circle** (the biggest line you can draw on Earth connecting $a$ and $b$, which would then contain pairs of antipodal points). After half a turn, $a$ and $b$ would switch position. This means that at a certain point $c$ of the great circle, the pair of antipodal points had the same temperature. Don't believe us? Take a look at the picture representing the variation of temperature from $a$ to $b$ (red line) and the variation from $b$ to $a$ (blue line)!

## How does our antipode calculator work?

In our antipode calculator, we have a list of countries and cities with more than 100,000 inhabitants (fewer than that, and you wouldn't find yours!):

- Choose your
**country**. - Choose the closest
**city**to you (they are sorted by population). - Let the antipode calculator work — you will see the result in no time!

Before trying to find an antipode, take a look at the map above the calculator: it will help you avoid ending up in the water too many times! In it, two maps of the globe are overlaid, pairing together antipodal points. As you can see, the overlap between land is really small.