Accuracy/precision of sextant navigation in a small boat

General Questions
#1

I’ve been watching Chasing Shackleton, a 3-part documentary about a group of men who recreated the epic 16-day, 800-mile journey of Sir Ernest Shackleton and five other men in a small boat through one of the roughest stretches of ocean from Elephant Island to South Georgia Island to summon a rescue mission for the rest of their crew they had left behind. The men in the documentary used a similar small boat and clothing/technology of the time, including a sextant and clock for navigation. It’s a testament to skill (and probably a good bit of luck) that anyone - either Shackleton’s group, or the modern recreators - was able to hit South Georgia Island instead of being blown right past it into thousands of miles of empty ocean.

So how much luck was involved? More specifically: how precisely can one navigate using a sextant and a clock while perched on a small boat that’s being tossed on big waves? The Wikipedia page for the sextant describes the accuracy limits of the sextant itself, but what about the mechanical clock that they were using for timekeeping (I’m aware that accurate clocks have historically been a challenge for determining longitude)? And what about the unsteadiness of such a small boat? How much might that have affected sextant readings?

#2

You’d likely do as well for timekeeping if you checked the stars that rotate around Polaris . . .

#3

Not much help in the southern hemisphere…

And, you need the time accurate to minutes, in order to navigate.

#4

Better than minutes. There are 1440 minutes in a day, and 25,000 miles in the earth’s circumference (at the equator). If your clock is off by a minute, then your longitude is off by as much as 17 miles. That’s before even considering sextant measurement error.

#5

to take a reading you need a stable platform or multiple readings.

people rightly give the navigator tons of credit.

less known is the skill of the rest of the crew. they all leaned and moved in unison to minimize the boat movement by the wave.

#6

Yes, that’s exactly why you need the clock. Remember, the goal here isn’t to determine the time; that’s just a step towards the goal. The goal is to determine your location, and you do that by comparing the time you get in two different ways.

#7

That’s of no use. You have three things to find: Position of the stars in the sky, the current time, and your longitude on the globe. You need to know two of those items in order to determine the third.

To answer the OP, when I was in the Coast Guard some 45 years ago, I was able to determine our position within 1/2 mile. In a life boat, and a not-so-accurate clock, I’d venture that Shakelton would be off by 1-2 miles.

#8

I don’t understand how the size of the boat would have much effect on the skills of the navigator. Aren’t most marine chronometers mounted in a gimbaled case?

#9

You have to take measurements with the sextant. Much harder (I assume, I’ve never actually used one) to take accurate measurements when the boat is rolling every which way.

#10

One factor is the observers eye height above the sea. In a small boat with moderate swells, this could be difficult to determine and may result in an error of 1-2 miles.

#11

I’m assuming that Shackleton took along the chronometer from the ship.

#12

I will add that the weather was appalling for Shackleton. There weren’t many times in their journey (17 days IIRC) that a clear view of the sky was possible.
Determining the position is only half the story however. There is also maintaining a course which is pretty difficult in one of the roughest oceans in the world. I am going to give full credit to the navigation skills and sailing skills of Shackleton and his men. No insignificant feat at all.

One thing that worked in their favour is the fact that the prevailing ocean current in that part of the world was propelling them directly towards South Georgia Island. I believe that this was the major factor in Shackleton deciding on this course of action. There were other locations that were geographically closer but just difficult or impossible to get to with their limited sailing vessel. OTOH, South Georgia Island sits in the middle of a swift ocean current and if they deviated from their course a little, that current would tend to bring them back to where they needed to be.

(I think Shackleton made a large number of good decisions of that nature. Compared with RF Scott who had good intentions, good ideas and seemingly sound logic but made large numbers of errors in his decision making, which were eventually fatal.)

#13

Child’s play. Captain Bligh did it without charts, sextant, or chronometer.

#14

I don’t think the two scenarios are comparable.
We are talking different oceans and different conditions.
Also, and I stand to be corrected here, Shackleton had a specific destination that he was heading for. Bligh just needed to be anywhere but here.

#15

I distinctly saw Marlon Brando give him his own sextant.

#16

No, Bligh was headed for Batavia.

But, yes, conditions were different. A large part of Bligh’s journey was coastal navigation.

Still, it’s generally acknowledged as an astonishing feat of seamanship.

#17

The documentary claims otherwise, i.e. that S.G.I. was to the northeast, and the current and winds were moving due east. If they missed the island by being too far north or south of it, once they ended up east of it the winds/currents would have carried them into the open South Atlantic/Southern ocean.

#18

By the by, I love this advertisementShackleton put out for men for his expedition.

ETA: Reading the fine print, that may be apocryphal. Dammit.

#19

This map (green trace) suggests that perhaps 10% of it was.