Longitudes and latitudes

A ship on the ocean. A crew member is ill, and they need an ambulance helicopter from the main land. Our position is … umm … on the Atlantic between Nigeria and Brasil. Between Nigeria and Brazil? How is anyone supposed to find that vessel, if all they know is that it’s drifting around somewhere on a huge ocean?

Is there any other way to give the position? Yes, there is. Look at the globe. It’s divided into a grid, covering the whole earth. Let’s have a closer look.

The earth spins around an axis. The earth’s most northern point, the North Pole, and the earth’s most southern point, the South Pole are on this axis. Some of the lines in the grid start at the North Pole. They spread out in all directions, but in fact they are all going south! And they meet again at the South Pole.

These lines are called meridians, or longitudes. One longitude in particular goes through the city of Greenwich near London in England. This is the first meridian, or the prime meridian. If Kim moves east from Greenwich and the prime meridian they will soon arrive at the next meridian, 1 degree east. A bit further east is 2 degrees east.

Then 3 degrees east, 4 degrees east, and this goes on all the way to the other side of the globe, in the middle of the Pacific ocean. Here is 180 degrees east. Philip also starts in Greenwich but moves west. He crosses the meridians 1 degree west, 2 degrees west, 3 degrees west, all the way around the earth to 180 degrees west. And there is Kim waiting in their dinghy.

180 degrees west is the same meridian as 180 degrees east. Kim travelled over the eastern hemisphere, and Philip over the western hemisphere. Altogether, the earth is divided into 360 meridians, all the way around the earth. But to make that grid, we need lines in another direction. They circle around the axis of the earth.

These lines never meet. They are parallel, and the lines are called parallels, or latitudes. The longest of these lines is the equator. All the way along the equator the latitude is zero. If Kim moves north from the equator they soon cross the latitudes of 1 degree north, 2 degrees north, 3 degrees north.

When they arrive at 90 degrees north, they are exactly at the North Pole. Philip moves south. He crosses the latitudes one degree south, 2 degrees south, 3 degrees south, all the way to 90 degrees south. This is the South Pole. So, there are 180 latitudes, or parallels, from 90 degrees north to 90 degrees south.

So, longitudes and latitudes are those lines that form that grid on the earth. Along the equator, there’s about 111 kilometres between two meridians. Between two parallels there is always 111 kilometers. That’s a pretty big box, so finding a ship in distress will still be difficult if all you know is a latitude and a longitude. Therefore, each box is divided into 60 distance minutes, in both east-west direction, and north-south direction.

At the equator, a side of one of these smaller boxes is about 1852 metres. To make it even more accurate, these boxes are divided into 60 distance seconds in both directions. At the equator, a side of one distance second is about 30 metres. And now, the captain of the ship can give his accurate position on the ocean. Our position: Two degrees, 49 minutes, 25 seconds south.

15 degrees, 24 minutes, 18 seconds west. Over. Copy that! Help is on the way. Over and out.