Setting the time on a chronometer - how?

[tudorsailor]

It strikes me that having an artificial horizon would be useful for practicing. Since I have a 3D printer I will make one with these instructions from thingiverse. Something to do until I get onto the boat where I have the sextant!

While coastal sailing, is it worth taking sites of landbased objects of known height such as towers and lighthouses to practice ones skills?


Tudorsailor

 

[Refueler]

Bubble Sextant ?? I have my fathers from a B17G he navigated across Atlantic for movie War Lovers.

Sextant use near land ......... brings back memories ... vertical sextant angles for distance off ... horizontal for 2 and 3 point fixes ,, etc.

 

[Skylark]

I’m currently in Barbados, having arrived yesterday, as crew on a friends boat. We took 2 sun sights per day for 16 days, each sight consisting of about 4 readings over about 10 mins. For time, I counted to 10 after taking the sight before looking at my watch, then simply deduct 10 from the reading.

I have 2 Casio’s. A month before departure, I set them both against a Radio time signal and recorded drift over several weeks. Knowing the drift gives confidence of accuracy when comparing the two times each day. The Casio’s are also useful in that they show dual time. They cost around £20 each.

Not a difficult skill to learn and now the skipper and the other crew member of both proficient. Sight reduction process, using Tables, only takes a few minutes and is very rewarding.

For plotting sheets I used A3 plain paper. Lat scale 60mm to 1 degree, Long scale cos mid Lat x 60. All very basic stuff. We also used Imray routing chart 100.

Another very useful skill to learn is Plane Sailing. This is a very easy method to get DR. Either use Traverse Table in Nories or, even easier, simple trigonometry.

We obviously used GPS/Chartplotter but the daily running fixes were never more than 10 miles away from the electickery position.

 

[maby]

...

We obviously used GPS/Chartplotter but the daily running fixes were never more than 10 miles away from the electickery position.

That's quite good going! I was always amused by the description that Shane Acton gave of navigation in "Shrimpy" - long before GPS, of course. He had no formal training in navigation and set out to sail round the world using his school atlas for charts. He arrived in Portugal and made friends with an experienced sailor. When he explained the navigation he had used to get there, his new friend pointed out that, given the scale of the map he was using and the thickness of his pencil, some of the lines he was plotting were close to fifty miles wide!

 

[Skylark]

That's quite good going!

I’ve taken quite a few sights over the years. The “game change” is investing in a high quality sextant. I have a C&P, very similar to the Astra. It has full mirror and is a delight to use. I teach the Ocean Shorebased course and I also have a Davis 15 plastic sextant. It can not compete with the accuracy of a “real” one, but it’s absolutely fine for navigating across an ocean using the sun.

 

[tudorsailor]

Are two non-noon sights more accurate that a single noon sight? I thought a meridian sight was the simplest?

Where can I learn about Plane Sailing online?

Shame you are far from London, as I am sure I will need a shore-based course to truly master astro

TudorSailor

 

[Refueler]

Are two non-noon sights more accurate that a single noon sight? I thought a meridian sight was the simplest?

Where can I learn about Plane Sailing online?

Shame you are far from London, as I am sure I will need a shore-based course to truly master astro

TudorSailor

Accuracy ? It has to be remembered that position based on sun sights is also based on the Running Fix ...

The use of Noon as a standard time point is traditional - and uses the fact of Suns Meridional Passage for Latitude. The longitude is provided basically by the sun sight position line run up by speed / course as in a running fix ... where it crosses the noon latitude - is the Longitude / Latitude position.
Its fact that you could choose any time to run to or back to to create a position.

The Sun sight produces a line (actually an arc) that you are somewhere on that line. It then effectively becomes similar to a bearing line ... to intersect with another line at given time run to.

The noon sight is only a position line and accurate Latitude. Nothing else.

Don't forget that Latitude was never a problem for navigators. It was always the determination of Longitude and Thomas's Chronometer helped sort that.

Online ? Interesting search I reckon !!

 

[GHA]

Are two non-noon sights more accurate that a single noon sight? I thought a meridian sight was the simplest?

Worth the effort to do a handful like half a dozen then a quick graph of time against sextant reading, that way it's easier to see the sights that have big errors and easier to see the max altitude of the sun.
1st offering from google>
NavList: A noon fix inland with an peri-sextant (140941)

 

[Skylark]

Taking a noon sight (to give Latitude) is very straightforward and is a good introduction to using a sextant.

During a 17 day crossing I only took one MP specifically to assess accuracy of Longitude per Bowditch methods. This is controversial and I did it based upon a thread a couple of months ago. GHA knows all about it and I’ll report separately

It’s “normal” to take a forenoon and and afternoon sight, say 4-5 hours apart. The resulting position lines will then give a nice, almost 90 degree cross using a running fix.

It’s all about using transferred position lines. Overnight runs from prior day afternoon to next day forenoon are probably around 120 miles so you need to be accurate with bearing and distance. Log and compass should be calibrated before the crossing. Forenoon to afternoon mileage will be around 40 miles. Just keep these running fixes going twice a day.

Plane sailing is a very simple method, based upon Mercator projection, to calculate a DR from last known position.

Course has to be expressed as a Quadrant bearing, eg 240 degrees is S60W. Use 60 in sin and cos Tables, or in Norries Traverse table.

It’s simple to work out 1. dLat, 2. Departure and 3. dLong. Simple formulas, happy to explain when i’m back home later next week. Feel free to let me know.

 

[Skylark]

Plane Sailing.
Very simple trigonometry. Likely to be more accurate than drawing course and marking off distance travelled on the Plotting Sheet.

Convert bearing to quadrant notation. NtowardsE or W. StowardsE or W. So 100 deg is 80. 240 is 60 and so on.

From last fix, change in Lat (dLat) is cos course x distance from log. Answer in Nautical miles or minutes of Lat (same thing).

Departure (new term) is sin course x distance. Answer is Nautical miles.

Distance travelled in Long, minutes, is Departure/cos midLat. MidLat is simply last fix lat - half dLat.

If travelling SW, DR is last fix Lat Lat minus DLat. Long is last fix long + dLong.

Easy. If you don’t have Log Tables (from school) or a simple electronic calculator, Traverse Table in Norries does it by tabulation.

 

[Refueler]

I always preferred Burtons ... still got my set somewhere ..

Funny thinking back to my time as Cadet training to navigate etc. When you went into exams - the adjudicator checked your Burtons / Nories for cheat sheets !! Some even put clips on to lock you out of the explanation pages ... or they supplied a suitably 'pages removed' version !

 

[tudorsailor]

The noon sight is only a position line and accurate Latitude. Nothing else.

Don't forget that Latitude was never a problem for navigators. It was always the determination of Longitude and Thomas's Chronometer helped sort that.

Gosh! I mistakenly thought that by noting the time that the sun was highest that one could work out the time difference from the Greenwich meridian and hence one's longitude.

Back to Mary Blewitt...

TudorSailor

 

[Refueler]

Gosh! I mistakenly thought that by noting the time that the sun was highest that one could work out the time difference from the Greenwich meridian and hence one's longitude.

Back to Mary Blewitt...

TudorSailor

Never said you couldn't ... but if you are a seasoned noon taker ... you do not 'time' the sight as with any other. You go out few minutes before and follow the rise of sun to get max and through to see its reduction. You are not actually watching / recording the time as with a morning or star sight.
You calculate Noon time purely as a reference to make sure you bracket that max altitude sight.

If you were to be able to record the time of max - altitude - then of course you have Longitude ... but it aint that easy to do ... because you are sextant UP .. read .. Up read ... Up read ... etc etc.

I did spend 17 years as a ships navigator many of those before Satnav ...

 

[tudorsailor]

Never said you couldn't ... but if you are a seasoned noon taker ... you do not 'time' the sight as with any other. You go out few minutes before and follow the rise of sun to get max and through to see its reduction. You are not actually watching / recording the time as with a morning or star sight.
You calculate Noon time purely as a reference to make sure you bracket that max altitude sight.

If you were to be able to record the time of max - altitude - then of course you have Longitude ... but it aint that easy to do ... because you are sextant UP .. read .. Up read ... Up read ... etc etc.

I did spend 17 years as a ships navigator many of those before Satnav ...

Please do not misunderstand. I was not doubting your expertise. I am simply admitting my ignorance. I also appreciate the replies that are helping me on this path

TudorSailor

 

[Refueler]

Please do not misunderstand. I was not doubting your expertise. I am simply admitting my ignorance. I also appreciate the replies that are helping me on this path

TudorSailor

Sorry if my reply comes across badly - not intended. I also misunderstood your post - apology.

All sights apart from Meridional Passage are strictly recorded time to be able to calculate from astro tables.

But Noon sight is taken as the calculated local noon and then run back to ships noon. You assume the calculated time is the actual time you got max altitude.

I have to admit that during my seagoing career - I bought a Texas Ti59 programmable calculator that had a full navigation module inserted. Amazing machine ........ knocked the Tamaya Sight Calculator for 6 ... for less than half price.

I still have my Ti59 ... but battery is dead and power supply I lost ... shame. I know it still works.

 

[emandvee44]

I still have my Ti59 ... but battery is dead and power supply I lost ... shame. I know it still works.

There you go
AC Adapter and Battery Charger-Charger Texas Ti 58-Ti 59 | eBay
M

 

[Refueler]

There you go
AC Adapter and Battery Charger-Charger Texas Ti 58-Ti 59 | eBay
M

Didn't happen to see a battery pack as well ??

Only thing I was not keen on - the click keys. But a brill bit of kit.

 

[Refueler]

Just seen there are a number of Ti 59's still for sale ..... was one of the best offerings TI ever made.

But no Navigation Modules ...... sorry !

 

[tudorsailor]

I have to admit that during my seagoing career - I bought a Texas Ti59 programmable calculator that had a full navigation module inserted. Amazing machine ........ knocked the Tamaya Sight Calculator for 6 ... for less than half price.

Is it OK to ask if there is an android app or a program for my PC that will do the maths for me - or is this cheating?
For the purposes of YM Ocean I assume that one has to show all the working and not use an electronic shortcut?

TudorSailor

 

[TLouth7]

I still have my Ti59 ... but battery is dead and power supply I lost ... shame. I know it still works.

Is it OK to ask if there is an android app or a program for my PC that will do the maths for me - or is this cheating?

My uncle has an astro navigation cartridge in a programmable calculator but on a recent trip he also had the software emulated on his phone. I believe it was made available by the guy who created the original calculator software. I realise that is not terribly helpful but the point is it's out there somewhere.