NavList:

Hello Navlisters,

I’m trying to analyze/follow the method used by James Cook and his colleagues (later back in England) to estimate the longitudinal difference between Burgeo and Oxford based on Cook’s August 5, 1766 eclipse observations from Eclipse Island in Burgeo Harbour. Here’s the excerpt from the Royal Society’s publication of the results that has me puzzled:

“Being, August 5, 1766, at one of the Burgeo Islands near Cape Ray, latitude 49° 36’ 19’’, the south-west extremity of New-found-land, and having, carefully rectified his quadrant, he waited for the eclipse of the sun; just a minute after the beginning of which, he observed the zenith distance of the sun’s upper limb 31° 57’ 00’’; and, allowing for refraction and his semidiameter, the true zenith distance of the sun’s centre 32° 13’ 30’’, from whence he concluded the eclipse to have begun at 0^h 46’ 48’’ apparent time, and by a like process to have ended at 3^h 39’ 14’’ apparent time.”

As discussed in previous posts, Frank’s eagle eye spotted some transcription errors (corrected above in bold text) and a calculation error (not shown above) in the original text.

Here’s my question: what “process” would have been used by Cook to determine the starting time of the eclipse to the level of precision implied by what’s reported (0^h 46’ 48’’)? Was there a standard/generally accepted procedure used by navigators at the time to yield the result shown?

Here’s a link to the Royal Society’s original report: An Observation of an Eclipse of the Sun at the Island of New-Found-Land, August 5, 1766, by Mr. James Cook, with the Longitude of the Place of Observation Deduced from It: Communicated by J. Bevis, M. D. F. R. S. on JSTOR

Cheers,

Jim.