Types of Data

Abstract

People have measured time for many years. In Sect. 1.2 we saw how important it was to prehistoric cultures to have systems for denoting the passage of the seasons. A good case can be made that calendar time is the earliest form of data that humanity has analyzed in the abstract. In the age of sail, the accurate measurement of time was of critical importance for trade and military use since the most accurate method then available of estimating a ship's longitude 1 depended on the difference between "local time" and time at a fixed location. Sobel [102] states: To learn one's longitude at sea, one needs to know what time it is aboard ship and also the time at the home port-or any other fixed location. .. Since the earth takes twenty-four hours to complete one full revolution of three hundred and sixty degrees. .. each hour's time difference between the ship and the starting point marks a progress of fifteen degrees of longitude to the east or west. Every day at sea, when the navigator resets his ship's clock to local noon when the sun reaches the highest point in the sky, and then consults the home-port clock, every hour's discrepancy between them translates into another fifteen degrees of longitude. 1 Longitude denotes location around the globe, on the east-west axis parallel to the equator, whereas latitude denotes the location on the north-south axis between the equator and the poles. Latitude is relatively easy to calculate with simple instruments, unlike longitude. In terms of the spherical coordinate system defined in Sect. 2.5.1, latitude is theta and longitude is phi.