The book notes how latitude and longitude originally plotted in A.D. 150 by the cartographer and astronomer Ptolemy, with latitude lines running the width of the globe and longitude lines from pole to pole. The parallel of latitude lines are based on the equator so fixed by nature, whereas meridian of longitude lines set from an arbitrary spot, for the past several hundred years Greenwich, England.Sobel notes this difference in how the lines set made it so that sailors could fairly easily gauge their latitude by the sun and length of the day, enabling easy straight east to west travel or vice versa, but it was much more difficult to determine one's longitude.
The two ways to ascertain longitude at sea were via a lunar method, tracking against the stars, but this difficult to do effectively given cloudy nights and the amount of calculation required, and via keeping time aboard ship as well as the time at a separate place of known longitude. From this time difference, one could calculate the degrees traveled and know the location. The problem with this method was having a timepiece that worked as it should, with them rendered unreliable by changes in barometric pressure, temperature extremes, or simply rolling of the ship.
In response to this situation, English Parliament offered the Longitude Act of 1714, stipulating the award of a large monetary prize to anyone that could make possible the accurate determination of one's longitude while at sea. Sobel details how the aforementioned John Harrison accomplished this task via the timekeeping method, building over four decades five revolutionary chronometers, H-1 through H-5. Also noted in the book was how Harrison's had to contend with people who advocated for a lunar solution trying to thwart his superior effort.
