distance between two fix

[NevisDog]

Originally Posted by cyan

... filling the galley with melting plastic smoke. We are not taking Todd to Tahiti.

Well even if lightning or Todd's baking takes out our timepiece and calculator, we can probably still run down the westing to Tahiti without those - maybe take Todd after all.

[cyan]

Originally Posted by NevisDog

Well even if lightning or Todd's baking takes out our timepiece and calculator, we can probably still run down the westing to Tahiti without those - maybe take Todd after all.

Now that's something I had not considered. If following the trades, say, from east to west from the Americas looking for Tahiti, one could simply stay on 17.6 S with little consideration of longitute. This would not require a clock- just a sextant or even a primitive latitude navigational tool. Has anyone ever tried to use a kamal to determine latitude? A kamal might be the answer to what happens when your sextant breaks.

Ok- maybe Todd can come along. Dude can catch fish.

[StuM]

Originally Posted by cyan

Now that's something I had not considered. If following the trades, say, from east to west from the Americas looking for Tahiti, one could simply stay on 17.6 S with little consideration of longitute.

That was a common way to do it until Harrison came along.

[NevisDog]

Originally Posted by cyan

... stay on 17.6 S with little consideration of longitute...

...except I suspect the 'dangerous archipelago' likely straddles our westing.

[cyan]

Originally Posted by StuM

That was a common way to do it until Harrison came along.

Ah, but the sextant was invented roughly in Harrison's time. How was latitude determined before Harrison? I suppose I could just read some history now...

[Wotname]

Originally Posted by cyan

Ah, but the sextant was invented roughly in Harrison's time. How was latitude determined before Harrison? I suppose I could just read some history now...

Backstaff and before that, a crosstaff.

EDIT: I forgot to mention the Astrolabe.

[StuM]

Originally Posted by cyan

Ah, but the sextant was invented roughly in Harrison's time. How was latitude determined before Harrison? I suppose I could just read some history now...

You're the one that mentioned a kamal

[carstenb]

I bought a used Chelsea spring powered ships clock off e-bay. About $75 bucks. It ahngs on would salon wall and needs winding once per week.

We use it to keep UTC time and this makes our sextant work much easier

[Lodesman]

Originally Posted by NevisDog

And do professional navigators record 'noon' as (a) local as in celestial fix, (b) local as in current time zone, or (c) 24 hr/global as in GMT/UTC/etc?

Back in the day, the navy sent noon position reports (don't know if they still do, or need to) and it was always 1200 UTC(GMT, Z or whatever)

Originally Posted by captmikem

Nowadays the days run is used in several calculations (eta for the pilot, propeller slip, fuel economy etc.) and is normally reported to the company as required.

I imagine the data and calculations work better if they stick with a 24-hour day, otherwise they'd be comparing apples to oranges.

[Lodesman]

Originally Posted by cyan

Does anyone else go full 18th-century with a sextant and a spring-powered marine chronometer?

Skipper on one of my ships had us jr OOWs do this over the week or whatever it took going from Panama to San Diego. The NavO had access to the e-stuff (before GPS, so Satnav Omega). We also had to use the submariners' "Pool of errors" calculations. A lot of cloudy days, and not many fixes quickly led to a massive amount of uncertainty in our position (to us, not the Navigator) - went to show how amazing it was back in the days of sail, that they ever managed to find where they were going.

[GrowleyMonster]

It's pretty straightforward. For accuracy, use this formula.
60 * (90 - Sin^-1(sin(Today's Latitude) x sin(Yesterday's Latitude) + cos(Today's Latitude) x cos(Yesterday's Latitude) x cos(Difference in Longitude between Yesterday and Today's fix))

But if you simply pick the distance off the chart or ocean plot sheet, and compare to the chart's lattitude scale, and use the minutes of latitude spanned by the dividers as the distance in miles, the difference between that and the calculated distance is trivial. As in hundredths of a mile for typical sailboat day's runs.

Any chart plotter, whether hardware or software, will give you true distance, too. Just make two waypoints, and make a great circle route between them. There is your distance. Even an old Loran C machine can do that. (I have even used an old Loran for sight reduction, substituting Assumed Position for one waypoint and dec and LHA for the other waypoint, then subtracting the distance divided by 60 from 90). OpenCPN is certainly capable of giving you a GC distance.

You can also use tabular methods of solving the spherical triangle, if you don't have a calculator or paper chart or dividers. HO229, etc. It's just math. But the simple way is just pick the distance off a chart and measure with the latitude scale, and the next simplest is to use your laptop, phone, tablet, or standalone chartplotter.

[GrowleyMonster]

Oh, and as for Distance Made Good toward next waypoint, simply take yesterday's distance to go and subtract today's distance to go.

[Harry Tate]

Personal best days run of 625nm - Pacific crossing - Great circle with limiting latitude about 50N (IIRC) leaving Yokohama, Japan bound for San Francisco.
Fastest hours run 32.2 kn leaving the English Channel in Ushant VTS.

Ok, it wasn't under sail as you may have guessed, but rather a 295m container ship sucking 250000L fuel per day at those speeds!

Days run usually measured by astro/ celestial nav - noon to noon, but we used to do morning twighlight, upper meridian passage (noon) and evening twighlight.
Good fun and a skill that I still practice today whilst teaching the cadets the dying art.

Nowadays its mostly gps/ecdis based.

[NevisDog]

Originally Posted by Harry Tate

... a skill that I still practice today whilst teaching the cadets the dying art.

Never a dying art, not if we want any valid type of yachtmaster qualification, which may even become a requirement for some jurisdictions/insurance cover. Far too many unnecessary 'plotter-assisted groundings' to continue as we are?

[Harry Tate]

The funny thing is, everyone gets taught how to do the calculations, but very few actually know how to properly use and adjust a sextant.
I say its a dying art because fewer and fewer people keep their hand and eye in by doing regular celestial fixes. Once the exam is done, the skills are never used again because well, plotters.
As a professional mariner, it is a matter of professional pride as well as enjoyment keeping my hand on it.