NavList:

Reading a book review in The Economist just now I was reminded of a diary entry from Samuel Pepys (for those unfamiliar, that's PEEPS), the famous English diarist whose writings comprise some of the best resources on the day-to-day history of government and life in England in the Restoration period in the 1660s.

It was 358 years ago on Saturday, 13 May 1665 (O.S.) and Pepys wrote:
"So home and late at my office. But, Lord! to see how much of my old folly and childishness hangs upon me still that I cannot forbear carrying my watch in my hand in the coach all this afternoon, and seeing what o’clock it is one hundred times; and am apt to think with myself, how could I be so long without one; though I remember since, I had one, and found it a trouble, and resolved to carry one no more about me while I lived."

In other words, although he told himself he would never get drawn in by this seductive technology again, he now sees himself [my 21st century comparison] as being like one of "these kids today" checking his phone every few minutes. Imagine how obsessed Pepys would have been if his watch could get text messages. Or better yet if his watch had a keyboard to let him record notes for his diary entries. He never would have put it down! Of course this watch was really quite primitive and prohibitively expensive, but that changed quickly.

For navigation, the importance of pocket watches should not be underestimated, and it began much earlier than many of us imagine. For dead reckoning navigation, a pocket watch was invaluable, and for officers who could afford them, watches made sandglasses instantly obsolete. As scientific navigation advanced, the ability to carry celestial observations forward in time, at least for a few hours, was normal a century after Pepys chided himself for his lack of self-control. So in 1775, for example, a navigator could shoot the Sun for local time at 3:00 in the afternoon. Then perhaps three and a half hours later, a navigator might shoot a lunar for Greenwich time. A common watch could bring the local time forward and match it up to compare against that later Greenwich time, yielding longitude.

Pocket watches also trained "thinking" navigators that longitude difference is time difference, decades before this knowledge could be put to practical use. As early as the 1730s, navigators wrote about the delay in sunrise when they sailed west. Chasing the Sun, they could check their change in longitude from one morning to the next by dead reckoning and then, comparing with the time from a common watch, they could see that the difference matched up, more or less. For every degree of travel west in longitude (in a fairly constant latitude, especially at times of year with relatively unchanging Sun declination), the Sun would be delayed by four minutes of time, as measured by a watch. So clearly if you could make a very fine watch, you could turn the tables and determine longitude directly by time difference without worrying about dead reckoning. Easier said than done, but they could see it even then!

Frank Reed
Clockwork Mapping / ReedNavigation.com 
Conanicut Island USA

File: