Hello Shipmates,

Forgive me if this information has been posted previously.

Here is a sextant index correction sheet that is as simple and elegant to read as they come.

Captain Melas sent this short, understandable, and efficient page to me in an effort to teach an old salt new tricks. 

Captain Diego Melas has also created a web page both lovely to look at and generous in its method at teaching navigation both off shore and inshore.  I appreciate his kindness and tireless efforts to beat new information into my stubborn head.

This morning I used the instructions to correct the index error on a borrowed sextant and came away with a better understanding of the process and the machine.  As we used to say in the Navy - "A piece of cake...!"

Give your sextant the best accuracy its capable of by carefully aligning it's mirrors; but more importantly, have fun.
https://sidaerum.com/
joe

Thomas Dolby wrote the song Oceanea which is a rather nice song.

In various portions of the video a sextant is shown.

Here's a link- https://www.youtube.com/watch?v=7STUTxwdC5A

Dolby was popular many years ago.  His "office" or studio is in an old lifeboat named "Nutmeg".

He's a rather interesting man in his portrayal or attire of the 1920's or so.

The Morse code at the end reads, Atomic Sakhalin, which is an atomic powered lighthouse in Russia...and I assume Sakalin island.


Paul

Within a day or two I will post another CN Challenge with a twist.

I and my captain friend reduced the observation data to come and its good... so you guys should have little problems ... However there is something new (if you dont know already) in the mix.  The natural horizon was obstructed but I took the sun shots anyway.  I learned about DIP-SHORT - a reasonably accurate method of compensating Hs when you cant see the horizon.  This is sooooo cool.  I had no idea dip-short even existed.

Read up on it, shipmates...  the knowledge might come in handy.

joe

I thought you might like to read a short interesting history (with photos) of the Sextant.  This talk was given in 2000, but unless history has changed the information is still good.

Download here

I usually only read books that are recommended by like-minded folks I trust - especially books which explore the realms of sociology and political science.  However, the book I'm recommending is of an entirely different nature.  It's written for those curious cerebral gentleman with remembrances of age-old mechanical computers - SLIDE RULEs.

This book endeavors to explain simply and concisely the slide-rule as it applies to air and sea navigation problems.  Ive studied about 90 percent of this example-packed book with delight while slipping and sliding my slide-rule until it gets warm in my hand.   Although there are some minor errors in the text it is still fantastic good fun.

Download and check out The Slide Rule for Air and Sea Navigation 1974

Bowditch 2017 has just been uploaded to the site.

2017 Bowditch- American Practical Navigator

It's organized sequentially by each volume's chapters and tables.

National Geospatial doesn't provide individual chapter download.

Hope you like it. 

Johannes

This post has several parts so bare with me please.  If these subjects came up before I have not seen them.

1.    I just learned that the 2017 PRACTICAL NAVIGATOR (Bowditch) is available.  I downloaded it here and looked over a few chapters - it's really a wonderful piece of work..  If a young person was only interested in the sea this pub would give him everything he needs to function as a well rounded sailor in several disciplines.  There is so much information the format now includes a second volume.

2.    I know this forum is about CN, but forum members might also be interested in the other arts of navigation as well - I am, anyway.  However, I'm having trouble finding good examples on how to use the Haversin tables.  I know what a haversin is and the actual formula to find distance between two points, but an example of table use has not come to my attention... Has anyone a clue??

3.    I dont know how well I can explain this, but the calculator in my cell phone (CP), which includes trig functions, does not arrive at the same answer as my electronic calculator or trig function tables.  I have been learning, for the first time, how to use these tables as well as log tables with great satisfaction and fun, but my CP calculator really threw me for a loop when the Sin, Cos, and Tan functions spit out different numbers than what the purpose-built electronic calculator or tables say. 

For example:  Sin 40

Scientific Calculator - .64279  (rounded up)
Trig function Table -  .64279
Cell Phone calculator - .74511

I can find no options in the Cell Phone calculator which might give me a clue as to why the numbers are not the same...  Any Thoughts???

4.    I would appreciate if anyone could give me a few plain examples on how to use the traverse tables.  Meridional parts and conversion, Log of trig, and Amplitude tables also give me trouble...  I have no idea as to their use.

I may not be much of a navigator or mathematician, but its not for want of trying... :) My goal is to acquire the same skills as those navigators who plied their trade before the advent of batteries and electronics. Dont get me wrong - I appreciate the modern convenience of GPS and all the other cool navigation stuff, but give me a Gunter scale and some tables to occupy my free time and Im as happy as a sailor with a beer in one hand a cute girl in the other.

Thanks in advance
joe

Ok.... so this is not a humorous or funny post, but I wanted to tell you a true story about opportunities and conveniences that we sometimes lose from misuse or disuse.

Sometime back I was walking by a local neighborhood store with my wife when I stopped and asked her to wait a moment...  I went in and bought a coke and some gum.  When I came outside my wife asked why I bought those items when we could have bought them for a few pesos cheaper at the grocery store.

I told her that she was absolutely right...  However, I told her that I sometimes spend a few pesos at that corner store just to support them because someday when Im in a hurry I might need something quickly when I dont have time to run all the way to the big grocery store, and that she should sometimes spend some money at that local store too.

My wife then proceeded to tell me about a little seldom used store around the corner from where she lived when she went to college... that one day she quickly needed some women's supplies, but when she went to that corner store it was already out-of-business.  

I said, "you just made my point, honey." Her mouth twitched side to side... I added, "You took that store for granted."

I look at this forum the same way...  I feel lucky to be a part of it... and I know the administrators care about the site and the quality of the membership.  I know because they asked me to explain my interest in CN before they let me be a member.  I like that.

I feel as though I would lose something precious if this forum ever disappeared never to return.  I know there are good people here who have the same interest in Celestial navigation - and that makes me feel a little less lonely in the world. 

I hope you see what Im driving at.  We are all busy, but if we want to keep the friends we have and the site where we know we can find them, then we need to work at it - or we're gonna lose our opportunity.  I suggest that we use this site, and contribute to it, or it may not one day be here... and that would be a very tragic loss.
By contribute I dont necessarily mean money... I mean contribute your thoughts, ideas, experiences, and life's lessons.
respects
joe

On July 7th we took a trip to the beach and I was able to use the natural horizon on Lake Michigan for a series of afternoon sun sights. This is the first time I've been able to use other than an artificial horizon or a bubble sextant. It was exciting to actually try it at last. I used a Davis Mark 3. 

Good fun and the results were interesting.
 I varied between upper and lower limb sightings, took six different sightings with multiple individual sights for each one. I threw out the sixth set of numbers because they were all over the place. They were taken in a rush and by then I was hot and tired.

The other five sets however I was able to work up. I had three sets that were basically the same, though the LOPs were all out about 8 nm to the west of my actual position. One set was dead on and a fifth set was about 10 nm east.
UL and LL didn't appear to make any difference, the set of three sights contained both.
 I adjusted the mirror between each set of sights. I had only the horizon for this which I wasn't used to. When practicing at home I use a distant tree which I'm able to use to correct both horizontally and vertically. With the horizon all I could do was get things lined up one way and turn the sextant on its side to adjust the other way which is far more difficult. I'm assuming the errors were a combination of how my eye perceived the horizon and not getting the index correction quite right. 

Hopefully I'll get another opportunity soon and can work on my technique more. Possibly I can nail down that elusive IC for this sextant.

The following paraphrased excerpt is from the 2018 Nautical Almanac:

Determine Hc using a calculator
Hc = asin[sin(Declination)*sin(Latitude) + cos(Latitude)*cos(Declination)*cos(LHA)]
As it would be entered into the Casio calculator
Sin-1(Sin(Ap Latitude) x Sin(Declination) + Cos(Ap Latitude) x Cos(Declination) x Cos(LHA)

 
Instantly one sees these two formulas are not the same.  The second formula is missing an operation bracket and one set of operations are transposed.  But that's only half the problem.  It is unclear which operation comes first.  Is cos lat added to sin lat before being multiplied to the first sin dec... is one question. 

I think you understand the operational problems here.  Some calculators dont take anything for granted, while some operate to the normal order of operations - as in multiply first then add.

Which of the following is right???  (I removed some brackets for clarity)

Hc = Sin-1 ((SinDec x SinLat) + (CosLat x CosDec x CosLHA))
or
Hc = Sin-1 (((SinDec x SinLat) + CosLat) x CosDec x CosLHA)
or
Hc = Sin-1 ((SinDec (SinLat + CosLat)) x CosDec x CosLHA)
or
Hc = Sin-1 ((SinLat (Sin (Dec + CosLat))) x CosDec x CosLHA)

Or something else???  My calculator shows an error using the original formula

Please be clear if you offer a formula in response.

joe