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When Columbus sailed the ocean blue in 1492, he didn’t have access to a very important navigational tool that not even Toscanelli’s grid could supply. It was easy to know where you were going North and South. You could calculate your location based on the angle of the sun or the North Star – reliable objects in the sky whose angles never changed relative to the other information every navigator had. That’s because latitude, the lines that go around the world east to west, are the same distance apart going north to south. The world is about 24,900 miles around. There are 360 degrees in a circle. The one-degree lines of latitude are about 24,900 miles divided by 360 degrees apart. About 66.6 miles. Do you remember Ptolemy’s map? There was not much of the southern hemisphere on the map? That’s because they used the North Star to navigate north and south and the star disappeared behind the curvature of the earth when you got too far south. If you went there, you had no idea where you were. No one was afraid they'd fall off the edge of the earth because no one really believed the world was flat. But they were afraid of getting lost.
Longitude is very different. The lines go from one pole to the other so they are wider at the equator than they are at the poles. So there is no reliable solar body that can be used to calculate your longitude – except the sun. It travels from the east and goes to the west every day. Remember, to the medieval mind, the sun went around the earth and what’s interesting is that all these calculations work even with the erroneous world view in mind. It takes 24 hours to go 24,900 miles. That means, even though the distance between the lines of longitude vary from north to south, the time it takes for the sun to travel across the sky doesn’t. Let’s say the earth is 24,000 miles around (just to make the math easy). Each hour the sun travels a thousand miles around the earth at the equator. Those are the time zones. Go north along a time zone, and it gets narrower, but it still takes an hour.
Greenwich is zero degrees longitude. If you’re on the ocean at high noon (you can tell this by looking at the sun) and you know that it’s 3PM at Greenwich, you know you’re three time zones away. Calculate your latitude by looking at the North Star. And you know exactly where you are.
Once again, this technology is all about relationships, perspective and how different elements work together. Longitude and Latitude are a direct result of the grids used in perspective painting. How do we put a three-dimensional spherical globe on a two-dimensional surface in the form of a map?