Sextant.. Navigation tool or ornament?

[scotty123]

How exactly did he do it?

Here is a chart of his crossing of the Tasman sea via Norfolk and Lord Howe islands. His solution was to aim well off to one side of the target and fly on until a single sun position line through his position also passed through the island then turned onto the bearing knowing it would be ahead. The chart shows the position lines he took.

Looking at the chart, FC took sun sights at 0200, 0300, 0400, giving position lines.

How, was FC able at exactly 0430, to get a position line that passed through Norfolk Island?
Did he, from the 0400 position line, calculate when on his heading/speed a position line which passed through his destination might be found, then took one at 0430, which worked?

I'm just trying to imagine the practicality of knowing how/when exactly, a sight should be taken, to get this coincidence.

Your earlier diagram showing a transferred position line I can understand, but the subtleties of FC's final LOP need's clarification.

 

[snowleopard]

I'm just trying to imagine the practicality of knowing how/when exactly, a sight should be taken, to get this coincidence.

Unfortunately it's many years since I read his book and I don't have a copy so I can only guess and work out how I would tackle the problem. One has to bear in mind that the aircraft would have been doing maybe 80 knots so 10 minutes working out a sight would be far too long. I would work out what the sextant altitude would be when one was on the position line, then set it at that value and wait for the sun to kiss the horizon. It should be fairly straightforward to extrapolate from the previous sight to estimate when he was going to cross the final position line and what the altitude would be then.

Edit: in Wikipedia it refers to flying 'until the sun reached a pre-calculated angle'

 

[mixmaster]

If there ever was the situation where one of my 6 gps's failed or the whole system was shut off for days and days I think it's a reasonable risk to be without a sextant. If on a short passage DR will probably get you there and if something like an Atlantic crossing, keep sailing west or east until you see some land!

Having said that I did carry a sextant when crossing the Atlantic and Biscay (although only had to use it for the pure enjoyment of it).

 

[snowleopard]

if something like an Atlantic crossing, keep sailing west or east until you see some land!

Assuming of course that you arrived in daylight and didn't run up on a reef at night

 

[maxi77]

Assuming of course that you arrived in daylight and didn't run up on a reef at night

There are of course methods for the prudent mariner to achieve this even without a sextant. After all you may need to make a landfall in a period of extended cloud cover.

 

[mixmaster]

yes of course but consider how unlikely the events being described are. You could always heave to/wait at night. You could also signal for help. Not ideal but we're talking about a very unlikely circumstance.
If I was making landfall somewhere off Terra del Fuego I might have a different philosophy of course. I stress again, that I myself do sail with a sextant.

 

[AntarcticPilot]

There are of course methods for the prudent mariner to achieve this even without a sextant. After all you may need to make a landfall in a period of extended cloud cover.

I am sure anyone with enough knowledge of geometry to do celestial navigation could knock one of THESE up pretty quickly. All you need to do is compute the length of the string for the elevation you require. It might not be the most wonderfully accurate means of getting a latitude, but it will work! Those who have time and skills could create something a little more demanding, like a cross-staff, or even a back-staff if you value your eye-sight.

 

[bbg]

There are of course methods for the prudent mariner to achieve this even without a sextant. After all you may need to make a landfall in a period of extended cloud cover.

Yes, indeed. That's why I said in my earlier post that a VHF (even a HH VHF) is a backup navigation tool. Even without GPS / DSC built in. (Something no-one has commented on or picked up on.)

"Container ship Global Navigator, Global Navigator, Global Navigator, this is Three Blind Mice, Three Blind Mice, Three Blind Mice"
"Three Blind Mice, this is Global Navigator. Go ahead."
(on a working channel)
"Global Navigator, would you be good enough to provide confirmation of your position?"

Of course there is no guarantee that you'll see another ship, but if you do there is a pretty good way of getting a reasonable position.

As you approach land, if you have a list of lights you should be able to determine a line of position and maximum range off if you are see the light at night (dipping gives a pretty accurate range; constant gives an outer limit).

So, is a sextant an ornament? Absolutely - a nice one is beautiful and a work of art.
Is it a useful navigation tool? It can be - although the circumstances in which it would be necessary are pretty limited in this day and age.
Is it an essential navigation tool? Not really - the circumstances in which it is your last, best and only hope are even more limited.