If you read the first part of the story, you will know that Uncle Ahab is teaching his nephew, Ernest how to sail. But rather than getting wrapped up in halyards and sails, Ahab started at the start, giving Ernest a quick overview of charts, what they are called charts and a three-point fix (catch up here!)

Ernest had asked some great questions, one of which was about longitude and latitude. Sitting watching the sun go down and the harbour water lap gently against the hull, Ahab explained the concept to Earnest.

“So, we know the earth is round Ernest” began Ahab, “but we still need to find our way around it, remembering of course that in the middle of the ocean there is nothing to see for miles around, no land, no handy reference points”.

“Very simply put, latitude and longitude are angles that uniquely define points on a sphere, like our round planet”.

Ernest looked none the wiser, but Ahab pressed on.

“Latitude at its very simplest is a line that runs around the earth horizontally, but it is a bit more complicated than that,” continued Ahab. “ Imagine that deep in the centre of the earth if point C. We’ll call latitude P and this sits on the surface of the earth, like a hoop. The latitude of point P is the angle of where P passes through C.”

More stunned silence. “It will help to have a cross-section diagram of the earth, with all these angles highlighted,” said Ahab “but stick with me, Ernest.”

“There are a few latitude hoops and these are measured by angles. The value of the angles range from positive, also known as Northerly and up to 90° to Southerly latitudes that are negative, falling as low as -90°. This is to do with the North Pole and the top and the South Pole at the bottom.”

“So, longitude runs length ways…?”

“Well spotted Ernest! Longitude is defined in terms of things called meridians. There are half circles if you like that run from pole to pole.

To helps us make sense of it all, there is a prime meridian, a bit like a benchmark or a starting point. The one that was chosen for earth was the one that runs through Greenwich in London.

All the meridian measurements and the like refer to how far from the prime meridian the new meridian is located.”

“Lost me…”

“Ok, to use the right terminology the longitude of point P is the angle that the plane containing the meridian passes, with respect to the prime meridian. Anything in front of prime meridian is positive, anything behind it or to the west is negative. And like latitude, there are maximum angles too although these are doubled. Positive longitude to the east can go up to 180° and longitudes to the east can reach -180°”

“So, it’s like a grid laying over the whole of the earth?”

“Yes, in a way” replied Ahab

“But how does it help with navigation?”

“That’s a great question and one that a trainee sailor should be asking! I’ve found a really great article that explains longitude and latitude even more, and how sailors in the 18^{th} century without GPS and other tools navigated their way around the globe using the invisible grid we have just talked about. “

“Can we go sailing now?”

“Not yet, the tide is not in our favour and we’ve a bit more to do yet. Next time, I want to talk about a few more bits to do with charts and about the true north and magnetic north”

“But there’s only one North Pole…”