NavList:

This is somewhat related to Jim Wyse's interesting Captain Cook subject but I don't want to hijack that.

We have talked before about the rich resources to be found within Archive.org.  One such resource is "The nautical almanac and astronomical ephemeris" which can be found for many years in the 18th and 19th century.  I find the explanations very interesting.  It is fun to look at a volume for a particular year that we know would have been used by Cook, Bligh, Vancouver and so on. Also interesting are the changes in content over time.

The first year covered was 1767, the year after the eclipse that Jim references.   A good clean scan of it can be found here...

https://archive.org/details/bim_eighteenth-century_the-nautical-almanac-and_great-britain-commissio_1766

The explanations start on scan page 152 and were penned by Neville Maskelyne (Astronomer Royal).  It is exciting reading when we remember that at the time it was written, 1766, there was much happening in the world of navigation at sea and timekeeping.  Harrison's H4 chronometer was still being tested by Maskelyne that year and the fallout between Harrison and the Board of Longitude was unresolved.  Harrison's H4 had met expectations during the Barbados trials (1763-64) but new conditions were imposed which limited the reward for an accurate marine timepiece (to solve the longitude problem) and angered Harrison.

This extract from Maskelyne's explantion to the almanac is interesting:

"The Eclipses of Jupiter's Satellites are well known to afford the readiest, and for general Practice the best Method of settling the Longitudes of Places at Land; and it is by their Means principally that Geography has been so much reformed within a Century past, and the Position of the mot distant E Places determined to equal Accuracy with the nearest. It was E hoped that some Means might be found of using proper Telescopes on Shipboard to observe these Eclipses, and could this be effected, it would be of great Service in ascertaining the Longitude of a Ship from Time to Time."

Maskelyne continues with a statement that feels as political as it does scientific, given the context of the time.  His statement remained in the alamanac for many years.  When Vancouver referenced his almanac in the summer of the year 1792, anchored not too far from where I sit now, he would have found the same statement. By that time his expedition had no fewer than four chronometers.