Clocks have always fascinated me. Measuring the passage of time was the high technology of ancient and in fact recent times, allowing feats of navigation and exploration that would have been difficult or impossible without them. The earliest clocks relied on shadows cast by the sun, and hence were not useful in cloudy weather or at night. Mechanical clocks employing the verge escapement mechanism were invented in Europe at around the start of the 14th century, and became the standard timekeeping device until the spring-powered clock and pocket watch in the 16th century, followed by the pendulum clock in the 17th century. During the 20th century, quartz oscillators were invented, followed by atomic clocks.