Beginner seeks Sextant Navigation advice - books etc.

Thinking about this astral thingy got me wondering about how the rotation/days/hours calcs are affected by the leap year correction.

(365days x 3years)+366days - 1461 days
Dividing this by 4 years gives 365¼ days; 6 hrs anomaly every year until the LY correction. (0.164minutes/day)

Do the tables adjust for this?
Does 0.164 minutes each day really matter as far as position finding is concerned?
Over several centuries this anomaly ever get corrected?

Or am I just being amazingly thick?
 
Robert Wilson said:
Ouch!
I think I'll see what Tom C has to say, then probably put the book away in a cupboard and rely on my plotter, AIS and Garmin hand-held.
Especially regarding the "A level maths" comment
I struggled with "O level Additional Maths" - and failed it....:(

Thanks all of you, but I'd be bighting-off more than I could chew.
Pity
Click to expand...
If you want to understand celestial navigation fully, then yes, it is A-level maths (higher than that if you want to understand how ephemera are calculated - and no, I can't do that!). However, if all you want to do is work out a sight using a pre-determined method, then it is simple arithmetic. That's why you're getting two approaches on it here; those who (like me) can't learn things unless they understand where it is all coming from, and those who are happy to simply follow a method that gives the right answer! There are plenty of the latter around; basically choose one you like and use it - but DON'T try and mix methods; it probably won't work. I'd probably use one of the very useful downloadable javascript web-pages or apps that include ephemera.
 
Robert Wilson said:
Thinking about this astral thingy got me wondering about how the rotation/days/hours calcs are affected by the leap year correction.

(365days x 3years)+366days - 1461 days
Dividing this by 4 years gives 365¼ days; 6 hrs anomaly every year until the LY correction. (0.164minutes/day)

Do the tables adjust for this?
Does 0.164 minutes each day really matter as far as position finding is concerned?
Over several centuries this anomaly ever get corrected?

Or am I just being amazingly thick?
Click to expand...

Wow! That’s a question. Had me reaching for my Bowditch. Section 1521 is “Time and the Calendar”

A fascinating read. It appears that a Tropical Year, the period from one vernal equinox to the next, is 365 days, 5 hours, 48 minutes, 45 seconds (although the length has been changing for many centuries). The Gregorian calendar approximates the tropical year with a combination of common years of 365 days and leap years of 366 days.

The section, having blown my mind and almost given me a headache, then says “see chapter 18 for an in-depth discussion of time.

It seems to me, Robert, that you are already hooked on this celestial malarkey!

The Nautical Almanac shows “Equation of Time” for each day, the time at Greenwich Meridian when the sun passes due south. (with sextant, commonly used to derive latitude from sun’s Meriden Passage) This varies by up to approx + and - 16 minutes with the seasons and is the difference between Mean Solar Time and Apparent Solar Time.

Hope this helps :-)
 
Last edited:
Well, at least it proves I'm not totally thick!:)

So leaving apart the landlubbers who never care too hoots whether the sun is properly over the yard arm and also those sea-farers who rely totally on their plotters and GPS, ;) am I correct in thinking the astral tables used by proper ocean going sea-farers correct for this "slippage"?
 
AntarcticPilot said:
If you want to understand celestial navigation fully, then yes, it is A-level maths (higher than that if you want to understand how ephemera are calculated - and no, I can't do that!). However, if all you want to do is work out a sight using a pre-determined method, then it is simple arithmetic. That's why you're getting two approaches on it here; those who (like me) can't learn things unless they understand where it is all coming from, and those who are happy to simply follow a method that gives the right answer! There are plenty of the latter around; basically choose one you like and use it - but DON'T try and mix methods; it probably won't work. I'd probably use one of the very useful downloadable javascript web-pages or apps that include ephemera.
Click to expand...

That sounds like the best approach for me.:)
Thanks for the encouragement.
 
Robert Wilson said:
Well, at least it proves I'm not totally thick!:)

So leaving apart the landlubbers who never care too hoots whether the sun is properly over the yard arm and also those sea-farers who rely totally on their plotters and GPS, ;) am I correct in thinking the astral tables used by proper ocean going sea-farers correct for this "slippage"?
Click to expand...

Yea. Just as different nautical charts can use different datums (OSGB, WGS etc), the Nautical Almanac has its datum set to Universal Time, aka GMT.
 
Robert Wilson said:
Well, at least it proves I'm not totally thick!:)

So leaving apart the landlubbers who never care too hoots whether the sun is properly over the yard arm and also those sea-farers who rely totally on their plotters and GPS, ;) am I correct in thinking the astral tables used by proper ocean going sea-farers correct for this "slippage"?
Click to expand...

Yes they do..... there are a couple of extra corrections in the 'Air Tables' for Precession and Nutation but that only applies to stars. A couple of other little corrections in the almanac.....

You can overthink this stuff...

Napier did all the hard work... you just have to accept that it works.... although understanding the derivation of the Haversine - and why we use the Haversine Formula - comes in handy if you want to bore the tits off your watchmate on the 12 to 4.....

I would still recommend that you look at this .. https://www.dropbox.com/s/a5blh1rgvi...ation.pdf?dl=0 written by a simple sailorman for simple sailormen.....
Published in Cruising Helmsman magazine over 3000 people signed up to do the course... met one 12 year old who had succesfully completed it....
 
Robert Wilson said:
Gotcha, thanks.
Looks good, but a bit late to be trawling through 170+ pages!
I'll have a go when wide awake in the morning!!
Click to expand...

Well as published you were meant to spend nine months trawling through it.... 20 pages a month... less than a page a day....
Ramp that up to 2 pages a day and you should be through it by the spring thaw... :)

Ran over 12 months.... the last 3 months a/ have been lost b/ involved a lot of 'filler' and c/ had stuff about Loran and the new fangled GPS etc....
 
Robert Wilson said:
Thinking about this astral thingy got me wondering about how the rotation/days/hours calcs are affected by the leap year correction.

(365days x 3years)+366days - 1461 days
Dividing this by 4 years gives 365¼ days; 6 hrs anomaly every year until the LY correction. (0.164minutes/day)

Do the tables adjust for this?
Does 0.164 minutes each day really matter as far as position finding is concerned?
Over several centuries this anomaly ever get corrected?

Or am I just being amazingly thick?
Click to expand...


Just over thinking it a bit.
There are a lot of people who post here who are much smarter or better educated than I am. Some of them clearly know a great deal more about astronomy Astro physics and mathematics than I do.

The world is not a perfectly round sphere does not take exactly 24 hours to complete 1 spin on its axis or take exactly 365 days to go round the sun.
These are just some of the reasons why the Nautical Alminac is published every year. The data good for the year.

As a Navigator we work on a presumption Ptolemy version of the universe is the correct on Capernicus,Galeio and Kepler versions just screw it all up. Making it to complex for our simple minds. Never mind the Big Bang theory. And constantly expanding universe.
As navigators. We treat a number of things as constant. Which are not. The Stars do move just very slowly. The first point of Aries. Was in the constellation of Aries about 2000 years ago. Not anymore.
Even the short method star tables have a life expectancy of about 15 years or so. They go out of date.
The rate of change of the suns position is constant enough for it to be calculated over the course of a year straight from the published alminac. For longer periods more has to be taken into account.
Actually last years alminac can be used if you apply a correction it even includes the correction.

But it’s simpler just to use a fresh alminac every year.

A good book which I found was an light enjoyable description of the basic concept is Longitude about Harsison .
It’s not really about navigation. But it’s a good read.

Most of the complex stuff has been figured out by astronomers and mathematicians to make it easier for navigators to use.

Franks page mentions the haversine formula. It is a complex formula developed by a brilliant mathematician so not particularly bright navigators would only have to do addition and not have to remember when to add or subtract.
The Alminac tables actual use Napier’s rules and a much less complex formula which requires rules about when to add and subtract.

There is practical navigation.
Which is just the ability to use the tools provided to determin where you are. It’s actually surprisingly straight forward.

There there is the principles of navigation. Which gets into why it works and some of the mathematics behind it. Which is about my level of understanding. Some of the math and theory behind why it works.

Then there is astronomy, the royal observatory and NOAA who deal with the complicated stuff. Reproduce it in simpler formats so we can use it.

I can’t say for sure but I have heard the haversine formula and the use of logarithms is not even taught or only mentioned in passing in marine schools today. I am told the pocket calculator has taken over. Even so I have found a few of the younger generation we’re quite interested.
 
Now that makes a clearer understanding of what initially sounds an horrific complexity :applause:

Way to go man, way to go !!:encouragement:

"the haversine formula and the use of logarithms is not even taught or only mentioned in passing in marine schools today. I am told the pocket calculator has taken over."

Those pupils don't know they're born ;)
 
Robert Wilson said:
Now that makes a clearer understanding of what initially sounds an horrific complexity :applause:

Way to go man, way to go !!:encouragement:
Click to expand...

I was thinking the same. If that post doesn't put you off, nothing will :sleeping:

It really isn't difficult to grasp the concept. The epiphany is only a few hours of study away, honestly :encouragement:
 
You must log in or register to reply here.

Share this page

Top