NavList:

Hello Seb

Firstly, congratulations on surviving the boat journey.  I dearly loved the footage and the reenactment.  Brilliant!

In reference to Worsley's log you make mention of the copy by James.  It may astonish you (maybe just a little bit) but I managed to track down the original log, now reverentially preserved in a museum.  So its Worsley's navigation, in Worsley's own hand.  George and I were frankly astounded.  George and I struggled with how to publish this information without descending into the navigational terminology weeds.  As George Huxtable is no longer with us, I have refrained from publishing such that a suitable period has passed.  It may finally be time.

Frank Worsley navigated by the Time Sight method.  Even though Sumner had discovered the modern method of Line of Position (LOP) nearly 50 years earlier, Worsley did not bother with that graphical method when a much more traditional and numerical method was available. 

The Time Sight method begins with the noon sun in the determination of latitude.  Used for hundreds of years prior to the heroic journey, this method finds the greatest altitude of the sun at meridian crossing.  Then, knowing the declination of the sun (for the day of the year), one can discover the latitude.  The longitude may be found by taking the altitude of the sun in early morning or late afternoon.  The sextant is essentially turned into a sundial.  Through a calculation that depends upon knowledge of the latitude and the (for the period) GMT, the numerical value of the longitude is found.  In combination, daily knowledge of the latitude and longitude may be had.

The Time Sight method 'advances' each previously found latitude to discover the longitude.  

Worsley's log can be summarized by the following (1) Time Sight Navigation, when the sun was available (2) Dead Reckoning, when the sun was not and (3) calculation of the distance to South Georgia Island.

Worsley was most concerned about Cape Belsham.   Cape Belsham is a prominent location on Elephant Island and was noted in several Sailing Directions well prior to 1914.  The latitude and longitude of Cape Belsham was given in those Sailing Directions, but which land mass was it?  From the camp, it was hard to tell. Why was Worsley so concerned?  Because this would provide a reset to his imperfect knowledge of longitude, specifically with the rate of chronometers exposed to cold temperatures.  Knowing the location of the camp to Cape Belsham would allow him to provide a reference for the chronometer, a well respected tradition when ashore.

Worsley was also concerned with Bird Island, a small islet near South Georgia Island, because they intended to navigate around it on the way to the whaling station, a plan abandoned in the face of peril.

That's the summary, for explicit details, I'm afraid you'll have to wait.  The above should be enough of a summary for a layman to get a sense of how Worsley navigated.

I have some questions for you about the recreation:
1) in regards to the navigation, did you perform noon sun (meridian crossing)?
2) was the navigation the modern LOP fix which uses multiple bodies around civil twilight to simultaneously determine latitude and longitude? 
3) did you start out with perfect knowledge of your position on Elephant Island (latitude and longitude)?  Shackleton and Worsley did not!
4) why did you use a micrometer sextant when Worsley navigated with a vernier sextant?

As to the markings on "the" sextant, I have been in correspondence with Pippa Hare (Secretary of the James Card Society) who has promised to look into it for me.  I am patiently waiting.

Brad