 Do you know what an astrolab is? Astrolab? It's a navigational instrument. Yeah, it's a kind of a circle thing with a pointer like a protractor, and it can point up and you can tell how tall or how far above the horizon the North Star is. And it wasn't marked in latitude. It was marked in cities, okay? So as you travel down the coast of Africa under Henry the Navigator's new invention called a caravel,