A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Brad Morris
Date: 2014 Feb 4, 20:51 -0500
Seb has responded to my questions about the navigation privately. I have encouraged him to post his answers here for all of us to see and so that all can participate in the discussion.
Please be patient whilst he finds the time. He is on active duty.
Brad, you wrote:
"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. "
This needs some work, Brad. First, you're using the expression "time sight" much too broadly here. The methodology of the "Old Navigation" (as it was known by the 1930s) consisted of meridian Sun altitudes for latitude, and local time observations or "time sights" compared against the chronometer for longitude. For short, this methodology can be called "Noon Sun AND Time Sights". It's wrong to call this "THE time sight method" unless you're limiting yourself to longitude.
Second, you're setting up a false puzzle here. Navigation by "Noon Sun and Time Sights" was NOT in the slightest way unusual in this period, and it requires no special excuse. There's no need for anyone to "explain" why Worsley navigated by "Noon Sun and Time Sights". In fact, this methodology was normal celestial navigation in this period. Exceptional navigators, especially on faster vessels and most especially on naval vessels, used lines of position in this period, whether labeled as Sumner lines or not.
The idea that Sumner lines changed the world, decades before the Shackleton expedition, is just bad history, but it's a very common misunderstanding even among NavList members with lots of knowledge in other areas. Sumner's book, his guide to navigation by his "lines", was a flop and a failure both commercially and scientifically. Unfortunately, the vast majority of histories of celestial navigation have merely copied from each other, and they have incorrectly implied that Sumner lines were adopted much earlier (and this is not limited to American histories). If you look at almost any navigational calculations from the late 18th century through the early 20th century and right through the 1940s on slower merchant vessels, you will see almost exactly the same methodology and calculations over and over again: latitude by Noon Sun and longitude by time sights (using the same highly standardized logarithmic calculation). And indeed there is nothing wrong with this methodology. It is exactly what I teach in my classes "Celestial Navigation: 19th Century Methods", and it could easily be used for successful ocean navigation today. The "Old Navigation" did not actually become obsolete, at least for slower vessels.. It simply fell out of fashion.
Of time sights, you wrote:
" The sextant is essentially turned into a sundial."
Yes, this is the language that I have been pushing for some years to describe these sights, and I'm happy to see it's catching on. A time sight turns a sextant in a very accurate sundial. It may be useful to consider a geometrically simple case that requires no logarithmic math to work things out. Suppose you're nearly on the equator on an equinox (for example, off the Galapagos Islands on March 21). The latitude is near zero, and the Sun's declination is near zero. You watch the Sun rise from the eastern horizon in the morning. Then you measure its altitude with your sextant, correct it as usual, and then subtract that from 90° to convert it to a zenith distance. If we then convert that angle to hours at the usual rate of 15° per hour, we immediately get the number of hours before noon. That's local time which we can compare with Greenwich local time yielding the longitude. So if we the corrected altitude is 30°, yielding a zenith distance of 60° or equivalently 4:00 hours, then the local apparent time is 4 hours before noon or 8:00am. And that's just what a sundial on land would read at that same location and time. The sextant gives sundial time.
You asked Seb some questions:
"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?"
I'm sure he'll answer as he sees fit, but in the meantime, I have some general thoughts here. Whenever anyone does a historical re-enactment like this, there are certain aspects that matter to the drama and the illusion of the re-enactment, and then there are details that have to be set aside as mere trivialities or features that simply won't interest any intended audience. In my opinion shooting Sun sights would be a requirement for a re-enactment since overcast conditions with a barely visible Sun and few if any other celestial objects visible are an important feature of the navigation. They caught "glimpses" of the Sun and managed to get enough sights during those brief periods to navigate. Naturally, Noon Sun sights (your question 1) would be a reasonable feature to include but not necessarily required. But your question 2 seems to set up a false dichotomy. Modern LOP navigation can certainly take advantage of a set of near-simultaneous twilight star sights, but it can equally well analyze a few daytime Sun sights, and it would be reasonable to substitute Sun LOPs for Noon Sun and Time sights if that suited the re-enacting navigator. Suppose, for example, I shoot the Sun in the morning, then again at noon, and then one more time in the afternoon. It makes little difference to the value of the re-enactment whether I work the sights up as time sights or instead work them as LOPs and cross them on a plotting sheet. All of the issues of accuracy and advancing sights apply in both cases. Only the mechanical calculations are different. As for your question 3, this is one of those aspects of re-enactment that you cannot get around. We KNOW things today that were unknown in the era, and there is no practical way to "un-know" that information. We could "blindfold" our re-enacting navigators and drop them off somewhere unknown (risky!) or we can just accept that the latitude and longitude of the starting point cannot be "un-learned" in a modern re-enactment. Similarly, and really much more important, we cannot re-enact the fear and doubt of a real survival story. If one's life is on the line, does that have an impact on one's navigational expertise? I would think so... Finally, regarding your question 4, using a vernier sextant instead of a micrometer sextant, this is just one of those things that isn't worth bothering over. Micrometers are much easier to use. Yes, one can become a skillful vernier reader with a lot of practice, but why torture yourself over trivia? Only a tiny portion of any potential audience would care in any way about this issue. It all comes down to "picking your battles" --which features of the re-enactment are worth your time? Which features would interest or inform an audience in any useful way?
NavList message boards and member settings: www.fer3.com/NavList
Members may optionally receive posts by email.
To cancel email delivery, send a message to NoMail[at]fer3.com
View and reply to this message: http://fer3.com/arc/m2.aspx?i=126791