Kelly Prime

They didn't know their longitude because at that point, no one knew how to calculate longitude at sea.

That is until the 18th century, when the disaster in Scilly helped inspire Britain to action.

But before we tell you about all that, we need to explain what longitude actually is, which is a lot harder than it sounds.

This is Alexi Baker, manager of the History of Science and Technology Collection at the Peabody Museum.

And see?

I told you this was hard.

I'm going to try to do this as simply as possible.

So latitude and longitude came about when some Greek guy from a very long time ago decided to throw some lines on the globe.

The idea being that if we put lines on a map, they'll form a grid and then people can use that grid to navigate the world with more precision.

The trouble with this imaginary grid system is that it's imaginary.

These values were great for understanding the world from a distance, for looking at a map from the comfort of your own home.

But if you actually wanted to use the system to navigate, like if you were out in the middle of the ocean, you needed to be able to calculate your latitude and longitude in real time.

That made latitude pretty simple.

It's just based on your distance north or south of the equator.

Easy enough to figure out by looking at the sky.

If you can see the North Star and it's 10 degrees above the horizon, you're at 10 degrees latitude.

Using the sun takes a little more calculation, but it's still very doable.

The Earth rotates west to east, which means even while the north-south values, like the poles, equator, and lines of latitude, all stay fixed, the sides of the Earth, the east-west values, are in constant motion.

What all this means in practice is that you can't get longitude just by looking at the sky.

None of those useful celestial bodies will hold still.