Sextant to Learn Navigation Old School

[barnakiel]

It is also possible to perform basic navigation with a sextant style called now Bris sextant. This one can be home built from plain materials.


https://en.wikipedia.org/wiki/Bris_sextant


Small, cheap, elegant and works.


I wish you sunshine,
b.

[jimisbell]

Originally Posted by barnakiel

It is also possible to perform basic navigation with a sextant style called now Bris sextant. This one can be home built from plain materials.


https://en.wikipedia.org/wiki/Bris_sextant


Small, cheap, elegant and works.


I wish you sunshine,
b.

Looks interesting. Where can one find a clear explanation of the process to use it. The explanation in Wikipedia left me saying, "Wh????"

[barnakiel]

Originally Posted by jimisbell

Looks interesting. Where can one find a clear explanation of the process to use it. The explanation in Wikipedia left me saying, "Wh????"

I believe there is a very fine explanation at Yrvind's site!


He invented this thing not too long ago.


Look there and should you fail to find it, just send him an email and he sure will send you instructions.


Bris mini sextant – Welcome to Yrvind´s website



Welcome to Yrvind´s website – small boat designer, builder, sailor & writer



Cheers,
b.

[barnakiel]

Originally Posted by Zzmeyer

Several posts talked about the importance of accurate time. In addition to my phone I have an SSB receiver, but what I really use is a $10 Casio watch. I track its error over time and it is very small and very consistent.

Yes.


Time is essential IF you want longitude.


As you mentioned, ssb will give time ticks.


Another smooth trick is to check the Casio against gps at periods. If the gps goes S, you have the Casio and you know how much ahead or slow it is.


One other criminal thing is (and I wonder why hardly ever mentioned) that a gps device (say a Garmin 72) even without a constellation still serves as an outstanding Tamaya calculator.


Simply add your EP as one waypoint and the Sun's coordinates as the other. BANG and you have your distance to the Sun. Compare to what you measured and you get intercept. And the angle from your EP to the Sun is your Azimuth.



Simple, elegant and let's you avoid any pen and paper calculations and associated errors. Or else go with paper and then check your result on the gps. (Note that you still need a paper or otherwise almanac!)


There are websites that let you print almanac for your passage too. Free of charge.


Cheers,
b.

[SeanPatrick]

Generally, each four second error in your timepiece will result in a one minute error in longitude. So, if your timepiece is in error by 20 seconds, your longitude will be off by 5'. Of course, this varies with latitude but you get the idea.

[GrowleyMonster]

Originally Posted by SeanPatrick

Generally, each four second error in your timepiece will result in a one minute error in longitude. So, if your timepiece is in error by 20 seconds, your longitude will be off by 5'. Of course, this varies with latitude but you get the idea.

Quite right. Which is why traditionally, navigators have fussed so much over their chronometers.



An ordinary digital wristwatch CAN be used quite successfully as long as it is checked periodically against the time ticks. GPS time is NOT accurate from most ordinary GPS devices. That is, it is only accurate within about a second. It updates on an internal timer which is not triggered by the beginning of each new second. Plus there are other errors. WWV or WWVH are good. I think the BBC broadcast time ticks at the top of the hour may be on the money, too. China's BPM and Canada's CHU also broadcast time ticks. Russia and Spain, Ecuador, Argentina, and I think a couple more countries have time tick stations on HF frequencies 2.500, 3.330, 4.996, 5.000, 7.600, 7.850, 9.996, 10.000, 14.670, 15.000, 15.006, and 20.000 MHz. Probably some more, too. There are also some LW frequencies that you can look up, if you have a receiver that will tune that low. Anyway it is not simply a matter of checking your watch against a time signal. You need to log the difference with the time and date of the check. Do this weekly and keep a running average of the amount of error per day to the nearest .1 second. Now you can quickly and accurately determine the time correction for any given hour and day, even if you can't receive a time signal. Most modern digital watches will only lose or gain within 15 seconds per month, some much better, and the rate of gain or loss will remain pretty consistent. A consistent rate is actually more important than a low error, since you will be applying a correction anyway.

[barnakiel]

Originally Posted by GrowleyMonster

(...) GPS time is NOT accurate from most ordinary GPS devices. (...)

???


Quote, source, reference?


I thought it was. Except for the first seconds that you switch on a cold device.


barnakiel

[GrowleyMonster]

Just did a quickie google.



First make sure your GPS is actually displaying calculated UTC and not "GPS Time" which is another 18 seconds off I think. It is possible that some new machines do not even show GPS time, or do not show "true" GPS time without correction. The default shoult be UT but beware that it could have gotten changed. GPS time is uncorrected from 06 Jan 1980. UT is corrected periodically by "leap seconds" to agree with GMT. GMT is actually less accurate than UT but this is moot since GMT is derived from observation, and celestial navigation is after all, observation based.



The error from being in the wrong place in the leap second cycle is up to a half second, obviously. But the error I was talking about was the actual screen refresh schedule. Listen to a time signal while watching your GPS screen update. It does so about once a second, but not necessarily at the TOP of the second unless it is a very high end unit. If it is a common consumer grade unit, and it is in sync, then it is just luck of the draw that it is so.


some geeky reading for you
https://www.e-education.psu.edu/geog862/node/1736
https://timetoolsltd.com/gps-clocks/...accurate-time/
How accurate is the TIME DISPLAY on my GPS?


in particular, note the last paragraph of the last link.

[barnakiel]

Originally Posted by GrowleyMonster

Just did a quickie google.



First make sure your GPS is actually displaying calculated UTC and not "GPS Time" which is another 18 seconds off I think.



(...) (skipped the rest of the post in the quote but I did read it (...)

Hi,


Thank you for the response.



I started using gps only about 2003. Had the following units onboard: Garmin 72, newer 72, 76, another 76, and 73 (pictured).


Never ever have I seen them display anything but exact UTC time. Down to a second, that is, because their time display format does not show decimal seconds.


None of the units I have ever used (Garmin, Furuno, Ray, etc.) seemed to show anything labelled gps time. They all show UTC + and - the zone.


I am attaching an image taken about 30 minutes ago. This is what I am used to with the most basic devices we own. I tried our acer tablet and a samsung smartphone they to are right to exact second.



Interesting to read out there might be some gps devices that do not show correct time. I was not aware of their existence!


Thanks for sharing!


barnakiel

[GrowleyMonster]

By default they should show UTC within a second or a bit more. The problem is small but nagging... a second equals a quarter minute of longitude.

[Adelie]

Originally Posted by GrowleyMonster

Just did a quickie google.



First make sure your GPS is actually displaying calculated UTC and not "GPS Time" which is another 18 seconds off I think. It is possible that some new machines do not even show GPS time, or do not show "true" GPS time without correction. The default shoult be UT but beware that it could have gotten changed. GPS time is uncorrected from 06 Jan 1980. UT is corrected periodically by "leap seconds" to agree with GMT. GMT is actually less accurate than UT but this is moot since GMT is derived from observation, and celestial navigation is after all, observation based.



The error from being in the wrong place in the leap second cycle is up to a half second, obviously. But the error I was talking about was the actual screen refresh schedule. Listen to a time signal while watching your GPS screen update. It does so about once a second, but not necessarily at the TOP of the second unless it is a very high end unit. If it is a common consumer grade unit, and it is in sync, then it is just luck of the draw that it is so.


some geeky reading for you
https://www.e-education.psu.edu/geog862/node/1736
https://timetoolsltd.com/gps-clocks/...accurate-time/
How accurate is the TIME DISPLAY on my GPS?


in particular, note the last paragraph of the last link.

The time displayed on most or all GPSs is UTC which is +/- 0.6s of solar time. At inconsistent intervals they add leap seconds.

UTC1 is coordinated to Solar time.

UTC is what is broadcast on WWV, WWVH, etc.

GMT is approximately UTC but has some ambiguities in how it was defined and was replaced by UTC for this reason

[sailorman14]

Too much BS about celestial. All you need is a sextant of any type and a watch, an almanac. You can get both your latitude and longitude from a noon sun site. You should be within 5 miles of actual location. That’s plenty good for navigating the oceans because you can see most islands from 12 miles away. It takes you 20 minutes to learn to do that. Don’t bother buying or reading the books and classes unless you want to learn to shoot the stars.

I’m good with a 5 mile error and it’s simple to do plus you can remember how to do it.

[SeanPatrick]

Originally Posted by sailorman14

You can get both your latitude and longitude from a noon sun site. [...]

I’m good with a 5 mile error and it’s simple to do plus you can remember how to do it.

May I ask what technique you use to determine the time of local noon?

[barnakiel]

Originally Posted by sailorman14

(...)



You can get both your latitude and longitude from a noon sun site.



(...)

What method do you deploy to get noon longitude from a noon sight?


b.

[GrowleyMonster]

Originally Posted by barnakiel

What method do you deploy to get noon longitude from a noon sight?


b.

Can calculate Longitude from observed time of LAN, but accuracy is meh. Better than a WAG (Wild Ass Guess) but probably less accurate than the DR position if you have a proper fix of some sort from some time in the last couple of days.


As has been stated, great accuracy is not necessarily necessary. However, to some of us, including me, it is highly desireable, and nothing but the greatest OBTAINABLE accuracy is sufficient. One should always strive to do better than the last time, in something so important. There are folks who really don't care, and that is fine, for them. I will always endeavor to do my best, and present to others the methods that will allow them to do their best, too.