Reference meridian

Longitude is an angular difference from a chosen reference meridian, not an intrinsic reading of a watch. A historical navigational record must identify its reference rather than silently assuming modern Greenwich conventions. Comparing local time with time at the reference meridian provides an angular difference because Earth turns through 360 degrees in 24 hours. Time errors and observational errors remain separate sources of uncertainty. See Royal Museums Greenwich’s longitude explanation and the precision-timekeeping guide.