NavList:

David C, you wrote:
"Worsley used lunar occulations of stars to determine time. Would typical marine navigators of the time know anything about such things? It is something that a land surveyor would have learned. I note that there was a physicist on board the Endurance.  Did the ship have a good library of surveying/astronomical/navigation books that they could spend a year studying? Should the physicist share some of Worsley's kudos"

Yes, the "physicist" (name please?) contributed to the success of the occultation work, too. Typical marine navigators mostly would have no idea what an occultation was. This was certainly a land-based method almost exclusively, and since they were nearly motionless on the ice, the Shackleton team could apply methods more suited to land exploration. 

The 'kudos' heaped on Worsley is over-done. He did his job, and he did it competently. Most importantly, he continued to work competently under deteriorating conditions that might have caused many navigator to become despondent and careless. Also, when people say Worsley was a "great navigator", I think it's important to remember that the word "navigator" has two somewhat different definitions. A navigator can be someone who 'navigates' an ocean, or in other words a skilled professional mariner. A navigator (and this is the sense for most of us) can be someone who has the skills to determine a vessel's position (and also speed, but often that's a given).

"Is a lunar occulation of a star a special case of a lunar i.e.  lunar distance=0?"

In principle, yes. The key principle in lunars and occultations, too, is that the Moon moves relative to the background stars (ignoring parallax effects for the moment) at an average speed of half a degree per hour, which is equal to half a minute or arc per minute, which is also equal to half a second of arc per second of time. The math used to work occultations is significantly different from the math for lunar distances, but in my opinion that should be seen as a "mere technicality". The big difference between lunars and occultations is the method of observation. For lunars, we measure angles with sextants, and it's not possible to do better than 5-10 arcseconds even under the best circumstances with excellent instruments. Occultations on the other hand are visually timed. It's certainly possible to time an occultation to the nearest second of time, and on land they can be reasonably timed to the nearest quarter of a second. Timing an occultation to the nearest second of time is comparable to measuring a lunar distance to the nearest half a second of arc, which is ten or twenty times better than the capabilities of manual sextant observations. Unfortunately there are some interesting problems that arise when we get down to seconds of arc, so it's not a foolproof improvement.

Note that the only reason Worsley et al. needed occultations was to test the chronometer(s). With that done, and under the assumption that the chronometer did not change its rate in the months after the occultations observations, the rest is really just common celestial navigation and dead reckoning analysis. They saw Endurance drop through the ice on a cloudy day. They knew how far they were from that spot. The next day Worsley took Sun sights which, properly done, could fix the camp's position to the usual accuracies of celestial navigation, which would be on the order of one mile. The most important catch in this analysis was that the camp was migrating. They were shooting sights on a moving ice floe, much like the condition of a vessel adrift and becalmed. Sights have to be adjusted for motion, like in a running fix, and assumptions have to be made to extrapolate that motion backwards to the time when Endurance began the last two miles of its voyage ...straight down.

What's missing in all the media coverage is that the team that found Endurance, and all of us riding along virtually with them, got very lucky. It easily could have happened that the motion of the ice overnight back in 1915 was less predictable and even at one knot in some odd direction could have misplaced the calculated position by over 20 nautical miles. We don't actually know where they found Endurance. So far they're keeping that secret. And even if they tell me, which they might, I can't tell all of you! Was it 4 miles from Worsley's position, which we have been calling the "Endurance Memorial Position" (given with added and indefensible precision, a bit like listing the time of a patient's death to the exact minute)? Was it 8 miles? But we do know, I think, that it wasn't 20 miles even though that could have resulted just from random and unpredictable motions of the ice.

Frank Reed

Frank Reed