Developing ways to pinpoint one's latitude and longitude was one of the greatest scientific endeavors in history, a quest that ultimately took centuries and was a matter of life and death. These astronomical calculations depended on navigators knowing how their instruments might be tilted with relation to the positions of the moon and stars, explained study co-author Ken Seidelmann, an astronomer at the University of Virginia in Charlottesville. The act of "determining the vertical," or knowing which way was straight down, in turn depended on watching a basin of li...