Dear All,
I am a new member here, I have been intrested in Celestial Nav for years. I have a bit of free time at the moment, so i thought I would put more effort in.

I have purchased 2 Mark IXa bubble sextants and I am in the process of trying to get one servisable one at the end. I fly across the Atlantic quite often, usually westbound during the day and eastbound at night, I plan to check the INS/GPS?, and develop my skills.

As this august body has great knowledge, I intend to ask a lot of questions.

In the early day of Astro in aircraft, a dome was used, latter on periscope sextant were stuck though a hole in the roof, neither of these are avaliable to me. I just have a limited field of view through the cockpit windows.

This leads to a couple of problems, celestial body selection, I can’t do much about that, and refraction which probably exists and maybe with some expert help I can correct for refraction.
I know domes had refraction corrections.

Any thoughts would be appreciated.

All the best

Tom

I just added the Polaris- Correction for (Q) (2025) (from The Air Almanac 2025)

Get it here- https://thenauticalalmanac.com/Everythin..._2025.html

PDF direct download- Polaris- Correction for (Q) 2025

Happy 5th day of 2025 to you all. I've looked at a couple sextants in a thrift store recently. One of them is labeled as a 1997 Cassens & Plath Horizon Ultra sextant. It has a cable that is unconnected to anything. Is it a cable that goes from a battery to a light in the optic? Click on this temporary link to see 4 photos of the sextant.

https://quickshare.samsungcloud.com/ukJaaTYd2Zxd

Clear skies,
Frank A.

I have an older Simex Mark I sextant with a 4X star scope and a traditional horizon glass.  I acquired it used off of E-BAY.  The sextant was in good condition, but the box had a rotted leather handle and a balky latch – both easily replaced.

I took it to a known location on land with a good sea horizon to the south and did a few practice sun sights. I came out 2 miles off toward.  I figured it was probably operator error, so on subsequent days I did additional sights. Each time they came out to be just about the same two miles toward from my GPS location.  I have other sextants and I had not run into any similar problems with those, so I figured it wasn't a matter of me “seeing” the sight incorrectly or not getting the right index error or time hack.  It had to be something about that particular sextant with that scope.  Perhaps a collimation issue since the mirrors checked out fine?

I carefully collimated the scope to the best of my abilities since I have no special equipment for the task.  Frankly things didn't change very much so I was unsure if this was the needed fix.

I returned to the same spot and did a series of 7 sun sights using the newly collimated scope.  While I was there, being uncertain if the repair had worked, I did two other series of sights. One of 7 sights with a 0X sight tube and one of  3 sights with no optics whatsoever.

Back at my desk I plotted each of the sight series series on their own graphs. I laid down “best fit” lines by eye to the 7 sight data sets and tossed out obvious outliers. There is no good way of doing that with the 3 sight series.

The remaining “good” sights with the 4X scope were averaged for time and Hs. The average sight plotted directly onto my best fit line, as expected.

For the sight tube sights I picked three sights that both laid directly on my best fit line and which in my notes I marked as particularly “good” and reduced each of these separately

The three no optics sights were averaged for time and Hs.

The 4X star scope LOP fell exactly through my GPS position. (well... after I worked out a couple of blunders...)  Hurrah!

The sight tube sights were surprisingly more difficult to “see” than I would have expected.  A long narrow sight tube, no magnification, and a limited field of view.  One LOP plotted 1/4 nmi away from my GPS; another 1/2 nmi toward; and a third about 4.25 nmi away. I suppose you could find an island with those errors, but since I was standing on terra firma in ideal conditions I was a bit disappointed at the 4.25 nmi away result from a sight that I had remarked as being “good” in my notes.

Lastly the no optics whatsoever sights turned out to be very easy to do. Obviously a super-wide field of view contributed to the ease.  I was pleasantly surprised upon plotting to find that the LOP ran directly through my GPS position. Spot on!  Easier than using a sight tube and, at least in this case, more accurate results.

I was not expecting that result.  But it is good to know and it is a confidence builder that when things get bumpy you can just leave the optics off the sextant altogether and get acceptable results. Maybe not “spot on” as I did this time, but not dozens of miles in error ether.

PeterB

Happy New Year's Eve Eve ??? to you all!

Have you used a 1966 C. Plath sextant, or similar, before? How was it? Would you buy one as your primary sextant?

Sincerely,
Frank A.

Merry Christmas to you all and to your loved ones. We are a week away from 2025. Happy advance new year to you all.

In your opinion, what are the best and most thorough "how to" books on celnav that are still currently in print or otherwise available in 2025? What are the best and most thorough books of all time that are no longer in print or available (such as rare or limited distribution books)?



Sincerely,

Frank A.

Click here- Everything you need for 2025


Almanacs
2025 Nautical Almanac- regular format
2025 Nautical Almanac- compact format

2025 Sun only- regular format
2025 Sun only- compact format


Sight reduction
PUB. NO. 249 (download individual Latitudes or Volumes) (Vol 1 courtesy of Celestaire) Epoch 2025
PUB. NO. 249 (download individual Latitudes or Volumes) (Vol 1 courtesy of Celestaire) Epoch 2020
PUB. NO. 229 (download individual volumes)

Stars
2025 Stars- SHA & Declination


Corrections
Increments & Corrections Table (yellow pages)
Increments & Corrections Sun only on 2 pages

ALTITUDE CORRECTION TABLES 10°--90°—SUN,STARS,PLANETS
Altitude Correction Tables for the Moon

Polaris- Correction for (Q) (2025) (from The Air Almanac 2025)


Conversion of Arc to Time
Meridian Passage and Declination of the Sun
Equation of Time curve for the Sun


Astronomical Phenomena


Day of week, Week Number and Day of Year for 2025
ECLIPSES- Solar and Lunar- 2025
Visibility of Planets & Local Mean Time of Meridian Passage- Planets and Sun- 2025

Planet Visibility Charts

Moon Phases for 2025 in graphic form (GMT/UTC time based)
WORLD MAP OF TIME ZONES

Gentlemen,

Using intercept method  I came to kind of  strange  situation.  Let me be  as brief as possible and to the point.

Calculating LHA for observed star.  Western longitude, north latitude. Wrote  formula to calculate it based on grpaphical representation  og GHA Aries, SHA of the star  and longitude ( assumed) .  I got LHA  but depending on the order  I  added and subtracted data  assumed  longitude became different.  But  LHA  was the same.  

Now, specifics:
LHA of the star = GHA Aries +SHA of the star -360- Longitude assumed  = (GHA Aries -  Longitude assumed) + SHA - 360. (  I just made a drawing  to visualize the data  , and from  that drawing  came  those two equations.)

Again,  results are identical,  but assumed longitude in this particular case  is different  in about 25 minutes of longitude.

I think, I asked this question before, but can't find the answer. 

So when I start to  plot  this star position, different  longitudes assumed made   it a little bit difficult to do it right  the first time.  Of course,  I can plot  two positions for the same star and see which one makes  more sense, but maybe  there is a simple way  to go through this.

Thank you.

I was checking out some practice questions for great circle calculations at this site:

https://www.starpath.com/cgi-bin/ubb/ult...6;t=000452

I do not know if the questions are U.S.C.G. or Starpath generated, but in the thread David Burch offers a link to a YouTube video where he shows their solutions by a plotting program called QTVLM  and there he points out that his answers vary slightly from those of the practice questions because QTVLM uses an ellipsoid model of the Earth while the Coast Guard uses a spherical model for these questions -- so I suppose the questions are U.S.C.G.

In the forth question, 5-615 , they list a starting point, an end point, and a distance between the two along a great circle track. They then give you a zone time of the start of the journey and zone time descriptors for both the start and end locations.  You are asked to figure out the estimated local zone time date and time of arrival at the destination based on a steady speed of 13 knots.

In the YouTube video Burch uses the automated program to generate times in UTC and then converts the UTC time of arrival into a local zone time. It seemed to be a fairly involved "work around" with the software to get the software to do what he wanted, but he did get to an acceptable answer. One that was close enough to the "correct" answer that you would definitely pick that one.

When I tried the question (before viewing the YouTube video) I simply took the provided great circle distance and divided it by 13 knots to get the time en-route in hours. Then I converted that into days/hours/ minutes.  This I added to the UTC time of the start of the journey, derived a UTC time of arrival at the end, and finally converted that to a local zone time -- and that didn't work out to be close to any of the provided answers.

I scratched my head, checked my math and it still didn't work out.

Knowing the "correct" answer I was able to back calculate the time en-route for that answer and see how much distance that would cover. The answer was far less than the distance of 4245 nmi offered as a given information in the question.

I then went ahead and solved the great circle problem for initial course and distance using H.O. 208 tables and found the distance by that method to be 4163 nmi.

Lastly I used MarineWaypoints.com great circle calculator to cross check and got a distance of 4164 nmi

So the upshot is the information listed in the question was incorrect. Using that incorrect information there is no way to get the correct answer except by tossing out their numbers and starting from the very beginning.

I attempted to reply to the thread at Starpath to point this out since it seems even David Burch isn't aware of this error, but even though the forum is a "public discussion" I was unable to log into the discussion to post.

PeterB

Hello All 

My first post, time to dip my foot in the water. 

I'm returning to Celestial Navigation after some decades.  The Mary Blewitt book Celestial Navigation for Yachtsmen gave me nightmares for years and years!

Working my way through David Burch's Celestial Navigation: A Complete Home Study Guide and am struggling to find the Polaris (Pole Star) Tables he mentions there are three corrections a0, a1 and a2 (T-22 in the book).  I've spotted a Polaris (Pole Star) Table 2024 A157 that uses a Q correction, then states that you need to add a figure for refraction. 

Have things moved on since David's book or am I looking in the wrong places?

Thanks 

Sandy