A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Position-Finding
From: Don Seltzer
Date: 2018 Dec 1, 20:02 -0500
On Sat, Dec 1, 2018 at 5:55 PM Charles McElhill <NoReply_McElhill@fer3.com> wrote:
...did they just note the chronometer difference or did they change it. I’m guessing they just logged the observed difference and pressed on
That is my understanding, and Bligh's entries suggest it. On 19 Dec 1787 at Spithead he wrote that the chronometer had been ashore for about a month being periodically checked against observations. The 'rate' that was established was -1.1" per day, with an offset from Greenwich time of +1' 52.5". This offset would have been left alone, rather than stopping the chronometer to reset to zero difference. Upon sailing, Bligh would have kept an adjusted accounting of the chronometer reading. Ten days later on 29 Dec he would likely have read the displayed time on the chronometer, subtracted 1' 52.5" and then added in 11" to get Greenwich time.
The next time he had a chance to carefully check his chronometer was in June 1788 ashore for a month at Simon's Bay at the Cape. Multiple observations showed that his chronometer now had an offset of -3' 33.2" from Greenwich time. That was a change of -326" from when he left Spithead over 193 days. During the previous six months the rate of the timekeeper had increased from the initial -1.1" per day. From his time at Simon's Bay, Bligh determined that the new rate was about 3" per day. His subsequent reckonings would have used the new offset of -3' 33.2" corrected by -1.1" per day.