NavList:

Dale L, you asked:
"What are "station keeping orbits"? A technical term?"

No, just my term, meant to be descriptive. If you look at the AIS tracking, the vessel appears to be visiting certain pre-selected locations in the search area (I'm guessing these are "best bet" positions). The Agulhas II is apparently just station-keeping for periods of twelve hours or more while, I assume, their robots are surveying the ocean floor below. The vessel is making lazy "orbits" with speed around 0.1-0.2 knots. The smooth, roughly elliptical shape suggests motion under the action of the local tidal current, but that's just a guess.

You wrote:
"I read the preprint article. I was not aware of the use of occultations for CN. They are no longer listed in the NAs, are they? Where can they be found?"

They're no longer used for celestial navigation, and they were hardly ever used for navigation per se, meaning finding the position of a vessel underway at sea. Instead Worsley and the Shackleton expedition had a different problem more similar to the problems facing exploration and survey teams months away from time checks in places like the interior of Africa and Central Asia. Checks on GMT using occultations are fundamentally similar to checks on GMT using lunar distances. They both depend on the snail-slow motion of the Moon, about one second of arc in two seconds of time, on the celestial sphere relative to stars and other celestial bodies. The biggest difference is that occultations require no angular measurement. When a star vanishes behind the Moon's limb, the lunar distance is effectively zero. Also since the observation is a visual disappearance, occultations, if properly observed and analyzed, are potentially much more accurate than lunars. Unfortunately, much of this potential accuracy was unavailable historically since the lunar limb must be taken into account, which has undulations from mountains and highlands as well as craters and depressions on a scale of +/-3 seconds of arc. These limb corrections were unknown historically, but can be analyzed using modern lunar digital elevation models which have become available just in the past 25 years. 

If you want to observe a lunar occultation yourself, there are lists prepared by IOTA, but you can do this yourself in Stellarium. Just set the focus on the Moon and run it in fast mode until you see the Moon's dark limb run over a star brighter than magnitude 6.0. You can observe that with binoculars yourself, and you will find that the time in Stellarium (which does not include limb corrections) is accurate to within a few seconds. 

And you wrote:
"I reread Worsley's account of the James Caird voyage last evening. Lots of interesting details about his only four sun sights and his surprisingly detailed navigation efforts, even wind directions like "South 'by' West" (boxing the compass, anyone?). All very fascinated!

Of course, everything he did was second nature and even "basic" navigation back in 1915. Time sights instead of lines of position is the most obvious case. Certainly Worsley knew about lines of position and probably understood that a time sight calculation "really" gave him a single point on an LOP for a selected latitude, but like most navigators in this time period, he apparently found little benefit from lines of position and stayed with the methodology that had served navigation for almost 150 years. Also the seemingly exotic styling of compass directions, like "S by W" was normal then.

Frank Reed

PS: As long as we're here, how do we generate and read these "by" directions? I have found that it can be convenient to translate "by" as "and a bit". The "bit" is one compass point, which is 11.25° (one-eighth of 90°). The direction "S by W" means "due South and a bit (11.25°) toward West)". The general form of the "by" directions is "XX by Y" where XX can be any of the cardinal points as well as the two-letter intermediate points, N, NE, E, SE, etc., and Y is always one of the four cardinal points, N, E, S, W. This means that both the Hitchcock film title "North by Northwest" and the popular festival/conference name "South by Southwest (SxSW)" are, technically, improper compass directions. Notice that the first portion of the naming should never be an intermediate "three-letter" compass direction like SSE. Although we can form directions like "SSE by S" and SSE by E", these are already covered by "S by E" and "SE by S". If you want to build a list, pick any quadrant of the compass, like from North to East. That's two directions: N and E. Then fill in three the normal intermediate compass directions: NNE, NE, ENE. And last fill in the four "by" directions: N by E, NE by N, NE by E, and E by N. And again, if you read the "by" as "and a bit", I think it's a lot easier to visualize the direction's meaning. So "NE by E" means "aim exactly northeast and then a bit towards east" where "a bit" is approximately 11.25° which is about the width of a 3x5 index card held at arm's length.