The line that I'm stradling is 0 degrees longitude, aka the "prime medidian." It is the point from which our clocks are indexed and also all the lines of longitude. How Greenwich got to be the 0 point and how building the perfect clock became THE engineering goal of the era is an interesting story. But I'm not going to tell it here. This is a photo blog. But if you are interested, "Longitude" by Dava Sobel tells it all.