Why was longitude determination such a difficult problem compared with finding latitude? A very basic answer is that latitude is measured north or south and so is independent of the Earth's east-west rotation, whereas longitude's determination by celestial means is affected by that rotation. Latitude can be found, in principle, from angular measures alone—say the angle of the midday sun above the horizon—but longitude requires knowledge of time. Thus if a mariner had a clock keeping Greenwich time and found that it read 2 p.m. when the sun was at its maximum angle above the horizon—the local noon—he would know that his longitude was two hours west of Greenwich. The whole problem lay in finding a clock that would keep time with sufficient accuracy over the long voyages of the 17th and 18th centuries. The best timekeepers of the age were pendulum clocks, but these were useless on the heaving deck of a small ship, whereas spring-wound clocks were relatively crude and hopelessly inaccurate for voyages over many weeks or months.