It’s relatively easy to calculate how far away the horizon is if you know two things: the height of your eye above the ground, and how big the Earth is.

Because we know *r*, the radius of the Earth, and we can measure *h*, the height of the eye above the ground, we can use Pythagorus’s theorem to calculate *d*, the distance to the horizon.

Which gives an equation for the distance to the horizon *d* as a function of *r* and *h*:

For the average person’s height of 1.62 m and the average radius of the Earth of 6367.5 km that gives a distance to the horizon of 4542 metres or 2.8 miles.

From the world’s highest public observation deck, on the 100th floor of the Shanghai World Financial Centre at a height of 474 metres the distance to the horizon is 77.8 kilometres, giving a viewable area of nineteen billion square metres, over seven thousand square miles.