Ship’s hourglasses were calibrated to the half hours of the ship’s watch, and if the helmsman didn’t turn the glass too early it actually doubled in defining longitude. Navigators tried to keep count of the hours; the galleons wrecked on the Western Australian reefs remind us of the perils of incorrectly kept time. To appreciate the important symbolism of the hourglass observation, Renaissance painted still lives, also called Vanitas, filled with symbolism and morals. They very often depict hourglasses.