Sextant wanted and other questions

[pabrides]

My Astro IIIb and Davis mk3  were stolen in the US a number of years ago.  Who ever stole them probably wouldnt even know what they are let alone know how to use them.  I really cant afford one now and wouldn't trust a good deal on Ebay- even shipping would be cost prohibitive.      

I am in the hopes that one of you wealthy sailors (if there is such a thing) will have pity on a poor old expat sailor living on the economy in South-East Asia and perhaps donate a reasonable sextant.    

I currently practice taking observations with my own altitude creations; a back-staff-like carpenter's square and a modified 45 that acts like a Gunter quad.  As I mentioned to the forum admin I even made a few Bris sextants.  The carpenters square is not marked in degrees so I use the cm scales and  some trig to get the altitude.

I promise to the best of my ability the sextant will receive great care, and that any donated sextant will never be sold in my lifetime, and upon my death I will have my wife return the sextant to society by way of donation to a likely individual or school with the same stipulation (or perhaps returned to the donator).

Part 2...  I also need some info relative to the HP correction tables.  I could use it before but now I am a bit confused about how to use it properly.  I know what HP is, but the table is a bit confusing.  Perhaps the instructions are in a paper almanac, but my almanac was obtained from this forum's host site without it. 

Part 3...  Then there is the matter of a typical refraction error correction..  If refraction error is combined into the Sun altitude correction table, then why is there a separate refraction table?  Im a bit confused and worry that I am mis-reducing my Gunter observations.  I actually do pretty well, but I wonder if I can do better.  When taking a Gunter sun shot I usually use star alt correction or Ha+LLc-SD+-T/P.  There is no DIP or IC correction with the Gunter.

By the way - when using my Gunter like instrument I never look into the sun.  I drilled a small hole at the front which allows a few rays of light to project onto an aligned white card at the back where the image of the sun is centered... I then rotate the Gunter locking the weighted string onto the degree scale. I then instantly stop a stopwatch which has been coordinated to GMT (UTC).  I usually do best if i wait a minute or two for the string to center on a whole or half degree mark.  Of course I am not taking the observation in the wind or on a rocking boat - I might not then be so lucky with my accuracy... :)  Moon, planet, and star shots seem to work out pretty well although they are a bit more difficult to align on the Gunter.  One of these days Im going to make a marine astrolab.  Ive also been experimenting with permanently positioned mirrors in the tradition of the Bris... and developing some ideas on how to make a Bris act like a sextant or octant (bris glass is self aligning, unlike sextant mirrors)...  necessity IS the mother of invention.

Any help would be greatly appreciated.

Joe

[Rdutton]

Joe,

As to your question number 2:

HP is particular to Moon observations.
Using the Altitude Correction for the Moon here-
https://thenauticalalmanac.com/Altitude%...20Moon.pdf

Date- June 8, 2018
GMT- 01:00
Moon HP= 56.2 (found in The Nautical Almanac)
Example- Ha= 52° 32' (LL- Lower Limb)

Step- 1
In the table find your Ha range using the top row (for degrees) and side column (for minutes).
Ha (App. Alt) is found to be 52° 45' (located in the column- 50° - 54°) and the row 30'

Step- 2
-While in the column- 50° - 54° drop down to the HP correction area. (the lower portion of the page)
- Look to the left side of the HP correction area and find the HP closest to the HP of 56.2 in the L column.
- In the row of 56.1 you'll find a correction of 3.6 (3.6 minutes of arc) where that row intersects the 50° - 54° column.
- Add the 52° 45' to 0° 03.6' and get- 52° 49' (rounded up)

Now you have the correction for HP of the Moon.

Concerning question 3-

Why is there a combined refraction figure for the Sun (and planets, stars...etc) in the Altitude Correction Table but there is also a separate correction table for refraction found elsewhere?

Here's the Altitude Correction Table-
https://thenauticalalmanac.com/Altitude%...0Table.pdf

- The Altitude Correction Table has all of the necessary corrections (Refraction, Parallax, Semi-diameter) in one figure so you don't have to find them individually. Dip is also included in the Altitude Correction Table but that's one of those figures that specific to your physical position above the water and therefore a variable that can't be included in the, for example, Sun correction figure on the sheet.

The Altitude Correction Table makes it a lot easier to arrive at Ho and saves time.

Does that make sense?

Roland

[pabrides]

The Altitude Correction Table makes it a lot easier to arrive at Ho and saves time.

Does that make sense?

Roland

Thanks Roland,

Your explanation made HP as clear as the ocean blue - thanks so much. 

However, I am still curious as to why someone might want to use the separate refraction table https://thenauticalalmanac.com/Increment...0Table.pdf  (last page left). 

My thinking is that perhaps the separate refraction table might be better than the formula I wrote of in my post for a Gunter or back-staff sun observation: Ha+LLc-SD...  If the separate Refr table applies, using it might be faster than using the star refr or the formula.  I just dont know which is the best method or which would be the most accurate, disregarding my instrument or equipment operational errors. Maybe I should have mentioned that taking a Gunter or back-staff shot does not involve specific limbs, thus one of the reasons for my concern... the sun and moon tables imply accurate observation of a specific limb with a sextant. Parallax, LLc, or ULc may not even apply to a Gunter or back-staff shot because the whole disc is used.

I wish I was smart enough to establish a reliable and accurate procedure for observation tools other than sextant.

joe

[CelNav57]

Joe,

The addition of the HP correction chart for the moon in Increments & Corrections-
https://thenauticalalmanac.com/Increment...0Table.pdf

...is helpful but the Altitude Correction Tables for the Moon are easier to use.

I included the HP correction table in Increments & Corrections just for convenience.
However, I didn't include the HP correction table in any of the yearly Nautical Almanac's Increments & Corrections tables on the site.

Thank you for you ideas and questions.

CelNav57 (Johannes)

[pabrides]

I included the HP correction table in Increments & Corrections just for convenience.
However, I didn't include the HP correction table in any of the yearly Nautical Almanac's Increments & Corrections tables on the site.

CelNav57 (Johannes)

It was not the HP tables that I was referring to, those were easily found on your site, but the instructions for the HP tables.  I seem to remember that the last paper almanac I bought many years ago had instructions, more so than what you see in your PDF of the HP tables.  I thought I remembered an explanation with example in the paper Almanac - this was about the year 2002 or so when I had my Astra IIIb.  Just A few years ago when I took up CN again I didnt bother applying HP to Gunter moon observations.  Recently I re-examined Bowditch and wished to clarify my knowledge of HP - thats when I had the brain fart and couldnt remember how to use the table....   an explanation was not forthcoming.

Anyway, I easily found your HP tables in an appropriate PDF - no problems there...  I just couldnt remember how to use them and couldnt find an example or instruction... :)

Roland clarified the procedure flawlessly.... 

Thanks
joe