20.1 Requirements and Definitions

Throughout most of history, people have had the need to track the events that affect their everyday life. The movement of the earth with respect to the stars provided an annual cycle. The rotation of the earth with alternating periods of light and dark provided a daily cycle. The position of the sun or the length of a shadow provided an indication of the current part of the day. As the need for more precision grew, time was expressed in terms of a burning rate, as with a candle; a material flow rate, as with water or sand through a hole; or the oscillation of a mechanical device, such as a pendulum.

None of these was sufficiently accurate for navigational needs. Latitude (angular distance from the equator) could be measured by knowing the angle of elevation of the North Star above the horizon. Longitude (the east-west angular distance from the Greenwich, England, zero meridian) was determinable only if the navigator knew the time in Greenwich. The sun travels across the sky at a predictable rate of about one degree in every four minutes. Thus, for every four minutes that the clock showing Greenwich time differs from the time determined by local noon of the sun, the observer is one degree of longitude away from Greenwich. In 1713, the British government offered an award of 20,000 for a clock that...