NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Patrick O'Brian characters discuss time and longitude
From: UNK
Date: 2016 Jun 26, 16:06 -0700
From: UNK
Date: 2016 Jun 26, 16:06 -0700
Peter,
You give 3 reasons why Jupiter moon observations would have yielded more exact GMT than the Moon around 1800:
- no sextant error (the eclipse of the Jovian moon involves no angle measurement)
- the inherent high precision of the eclipse event (lasts only 10 seconds or so)
- here I'm guessing, but low perturbations of the Jovian-moon orbits, so the almanacs weren't so limited by orbital models on the theory side? The NA gives data for these eclipses, so it would be possible to check how well they did.
Unfortunately, however,
- Sextant error is replaced by telescope error. There is a characteristic time offset (typically 10 to 20 seconds) inherent to each individual telescope by which the observed event will differ from the theoretical one (however the latter be defined). On top of that, there can be significant personal error, especially for emersion or egress, where one has no warning. (E.g. Cook saw an emersion on average 20s after Green at Venus Point.)
- the duration of ingress / egress depends on which satelite we are talking about. It's minutes, not seconds.
- The theory of of the Jupiter satelites was more elusive than that of the moon because of the resonances that were not immediately understood. Wargentin's method was more or less to empirically fit orbital parameters to the observed data without using much analytical theory. Almanac error at Cook's time was in the order of half a minute of time. I can't say at the moment how much that had improved in the few decades until the Napoleonic Wars.
In sum, the timing of Jupiter moon events was most useful when used with reference to control observations with calibrated instruments at known locations. No ship's captain set his chronometer by them.
Herbert Prinz
