Cruisers Forum
 


Join CruisersForum Today

Reply
 
Thread Tools Search this Thread Rate Thread Display Modes
Old 12-03-2019, 10:46   #1
Registered User

Join Date: Feb 2019
Posts: 19
Celestial Navigation Qs. Simulated sights, long term almanac

Hi, I'm working my way through Celestial Navigation 2nd edition, and enjoying it a lot. I have two questions for anyone who might be able to help:

1) Can someone point me toward a website that can generate simulated sextant sights (or at least Ha or Ho) given GPS coordinates and a time? I found one a few weeks ago and can't find it again.

2) What are favorite "lifeboat" long term almanac and site reduction tables? My priority is evergreen over precision. Bonus point for being physically compact (but I want paper, not electronic), another bonus point for easy reduction procedure.
__________________

ohthetrees is offline   Reply With Quote
Old 12-03-2019, 14:23   #2
Registered User

Join Date: Dec 2018
Posts: 64
Re: Celestial Navigation Qs. Simulated sights, long term almanac

Quote:
Originally Posted by ohthetrees View Post
Hi, I'm working my way through Celestial Navigation 2nd edition, and enjoying it a lot. I have two questions for anyone who might be able to help:

1) Can someone point me toward a website that can generate simulated sextant sights (or at least Ha or Ho) given GPS coordinates and a time? I found one a few weeks ago and can't find it again.

2) What are favorite "lifeboat" long term almanac and site reduction tables? My priority is evergreen over precision. Bonus point for being physically compact (but I want paper, not electronic), another bonus point for easy reduction procedure.

You have long term almanac for sun and stars valid till 2050 with concise sight reduction and altitude correction tables by Dr. Geoffrey Kolbe.
__________________

Gruz is offline   Reply With Quote
Old 12-03-2019, 15:52   #3
Registered User
 
SeanPatrick's Avatar

Join Date: Dec 2012
Location: Norfolk, VA USA
Posts: 68
Re: Celestial Navigation Qs. Simulated sights, long term almanac

Quote:
Originally Posted by ohthetrees View Post
1) Can someone point me toward a website that can generate simulated sextant sights (or at least Ha or Ho) given GPS coordinates and a time? I found one a few weeks ago and can't find it again.

Stellarium is a good, free solution for generating simulated sextant sights. Admittedly not a website - but a good solution, nonetheless.


If you want Ho (or Hc, more properly), try the USNO website.
SeanPatrick is offline   Reply With Quote
Old 12-03-2019, 16:13   #4
Registered User

Join Date: Feb 2019
Posts: 19
Re: Celestial Navigation Qs. Simulated sights, long term almanac

Quote:
Originally Posted by Gruz View Post
You have long term almanac for sun and stars valid till 2050 with concise sight reduction and altitude correction tables by Dr. Geoffrey Kolbe.
Just what I'm looking for, thanks. I actually saw this a while ago, and it somehow slipped my mind again. I wonder if there is even a perpetual almanac available. You know, in case the world ends I've heard of them, but not sure if they really exist in the wild.
ohthetrees is offline   Reply With Quote
Old 12-03-2019, 16:22   #5
Registered User

Join Date: Apr 2011
Location: SoCal
Boat: Ericson 35-3
Posts: 42
Re: Celestial Navigation Qs. Simulated sights, long term almanac

I've been meaning to work through this websites sight reductions:
https://celestialnavproblems.wordpress.com/

Also, I'm part way to Hawaii with the Starpath book from David Burch Hawaii by Sextant, it was worth the few dollars it cost me, but get the print edition not the e-edition.
brownr377 is offline   Reply With Quote
Old 12-03-2019, 16:35   #6
Registered User
 
SeanPatrick's Avatar

Join Date: Dec 2012
Location: Norfolk, VA USA
Posts: 68
Re: Celestial Navigation Qs. Simulated sights, long term almanac

There are too many variables involved - such as the slowing of the Earth's rotation - for a perpetual almanac to be realistically possible. Programs like Stellarium and AstroNav will produce data far into the future. But the accuracy of this data becomes more and more questionable the further out you go.


We can model the apparent motions of the heavenly bodies fairly well, but this is just a model. The Moon is the most difficult body to model accurately. Almanac data for the Sun for a particular year can be used again every four years - but even this will become less accurate over time.
SeanPatrick is offline   Reply With Quote
Old 12-03-2019, 16:46   #7
Registered User

Join Date: Feb 2019
Posts: 19
Re: Celestial Navigation Qs. Simulated sights, long term almanac

Quote:
Originally Posted by SeanPatrick View Post
Stellarium is a good, free solution for generating simulated sextant sights. Admittedly not a website - but a good solution, nonetheless.


If you want Ho (or Hc, more properly), try the USNO website.
Wow, Stellarium is amazing! What a world where things like that are free! I think I'll be able achieve practice sights and reductions with that tool. Thanks.
ohthetrees is offline   Reply With Quote
Old 13-03-2019, 10:43   #8
Registered User

Join Date: Oct 2007
Posts: 82
Re: Celestial Navigation Qs. Simulated sights, long term almanac

Why not take your own sights in your back yard for practice?
Take a flat pan, something with low sides, something like a rectangular cake pan, and put about one inch of used oil that is very black.
Be sure the pan is on a flat surface with lots of space around it, not near trees or bushes, so you can see the sun easily.

Move around the pan until you can see the sun reflected in the pan of oil and then begin taking sextant sights.

Flip down enough shades over both viewing areas, the upper and the straight ahead 'horizon' area to protect your eyes.

Look directly at the sun with the arm set at 'o'degrees, and gradually slide the arm forward as you bring down to the reflected sun in your artificial horizon, the pan of oil.
Align the upper limb of the lower sun reflected in the oil with the lower limb of the upper sun and read the angle.
If using a plastic sextant you can get the index error also by using the zero setting and looking at the reflected suns in the oil.
Paul Howard is offline   Reply With Quote
Old 13-03-2019, 11:03   #9
Registered User

Join Date: Apr 2011
Location: SoCal
Boat: Ericson 35-3
Posts: 42
Re: Celestial Navigation Qs. Simulated sights, long term almanac

^^^ This.

I'm no expert i'm just starting to learn cel nav, I think it's quaint and interesting, but I've no mastery of it. Anyhoozle, taking sights out on the water is a whole different thing that needs lots of practice. I need lots more practice according to my fixes.

Also the reason I like the Starpath book is it's actual sight data taken during the last transpac the author did using celestial alone. So you get bad sights, marginal sights when that's all there marginal star fixes cause that was what was available and lots and lots of plotting.

I have found in my learning that the actual sights and the plotting need much more work than grinding through the sight reduction math, which is more tedious than difficult after you've got the process under your belt. That's for me anyway.
brownr377 is offline   Reply With Quote
Old 14-03-2019, 09:56   #10
Registered User

Join Date: Feb 2019
Posts: 19
Re: Celestial Navigation Qs. Simulated sights, long term almanac

Quote:
Originally Posted by brownr377 View Post
I've been meaning to work through this websites sight reductions:
https://celestialnavproblems.wordpress.com/

Also, I'm part way to Hawaii with the Starpath book from David Burch Hawaii by Sextant, it was worth the few dollars it cost me, but get the print edition not the e-edition.
Cool link, I'll work through some of those problems. Thanks for the suggestions all!
ohthetrees is offline   Reply With Quote
Old 14-03-2019, 10:36   #11
Moderator
 
Adelie's Avatar

Join Date: Feb 2010
Location: La Ciudad de la Misión Didacus de Alcalá en Alta California, Virreinato de Nueva España
Boat: Cal 20
Posts: 7,928
Re: Celestial Navigation Qs. Simulated sights, long term almanac

Kolbe has the longest term almanac for Sun and stars. It also has Davies/Concise/NAO sight reduction which is very short but a pain to use.

There is a 3-5pg long term almanac for Sun and stars also available.

I’m working on a combined almanac/sight reduction/plottting system for Sun,moon&stars based on Ageton but it’s several years from completion based on current progress.

Right now Kolbe is the best combined system.
__________________
Num Me Vexo?
SailboatData
A house is but a boat so poorly built and so firmly run aground no one would think to try and refloat it.
Adelie is offline   Reply With Quote
Old 14-03-2019, 10:59   #12
Registered User

Join Date: Feb 2019
Posts: 19
Re: Celestial Navigation Qs. Simulated sights, long term almanac

Quote:
Originally Posted by Adelie View Post
Kolbe has the longest term almanac for Sun and stars. It also has Davies/Concise/NAO sight reduction which is very short but a pain to use.

There is a 3-5pg long term almanac for Sun and stars also available.

I’m working on a combined almanac/sight reduction/plottting system for Sun,moon&stars based on Ageton but it’s several years from completion based on current progress.

Right now Kolbe is the best combined system.
I'll pick Kolbe up! Would it be possible to use the Kolbe almanac with something like Henning Umland Ageton tables? Or any other arbitrary tables or even direct calculator solution? Or is the almanac layout specific to the provided reduction tables? Forgive the naive questions, I'm very new to this stuff.

What do you think of his Ageton tables by the way? I haven't tried them yet, having a hard time because he asks for 't' which he describes as meridian angle, which seems like it should be similar to LHA, but is apparently something different?! He doesn't explain how to calculate it.
ohthetrees is offline   Reply With Quote
Old 14-03-2019, 11:26   #13
Moderator
 
Adelie's Avatar

Join Date: Feb 2010
Location: La Ciudad de la Misión Didacus de Alcalá en Alta California, Virreinato de Nueva España
Boat: Cal 20
Posts: 7,928
Re: Celestial Navigation Qs. Simulated sights, long term almanac

Any almanac of sufficient accuracy can be used with any sight reduction method provided the use the same units.
Generally units are degrees, arc-minutes and decimal arc-minutes.
If something uses right ascension, decimal degrees or, heaven forbid, gon then you are going to have extra steps requiring conversions and extra potential for error.

Everything readily available is standardized on degrees, arc-minutes and decimal arc-minutes.

I started with Ageton reproduced in Bowditch 1984, then got a copy of the actual Ageton.
I now have copies of Pepperday and Bayless which are modifications to Ageton.

I have no opinion of Umland’s variation. Can you link to it?

If I recall correctly for t and LHA one is the absolute value of the other which is signed (+/-).
__________________
Num Me Vexo?
SailboatData
A house is but a boat so poorly built and so firmly run aground no one would think to try and refloat it.
Adelie is offline   Reply With Quote
Old 14-03-2019, 11:44   #14
֍֎֍֎֍֎֍֎֍֎

Cruisers Forum Supporter

Join Date: Apr 2006
Posts: 15,057
Re: Celestial Navigation Qs. Simulated sights, long term almanac

"What are favorite "lifeboat" long term almanac and site reduction tables?"
If you can pick up an old Palm Pilot (PDA) there is an app, I think called simply "Navigator" by John Mason (Manson?) that is absolutely priceless. The Palms run about two or three months on two AAA batteries, and the app used to be dirt cheap. Rather robust and reliable, much more so than screwing around with a computer or any tablet today.
FWIW.
hellosailor is offline   Reply With Quote
Old 14-03-2019, 13:13   #15
Moderator
 
Adelie's Avatar

Join Date: Feb 2010
Location: La Ciudad de la Misión Didacus de Alcalá en Alta California, Virreinato de Nueva España
Boat: Cal 20
Posts: 7,928
Re: Celestial Navigation Qs. Simulated sights, long term almanac

One of the problems with Ageton is that accuracy degrades as k or t or LHA approach 90d.

The commentary in Ageton talks about errors of 2-3’ but really median error is about 10’ and worst error is about 30’. On NavList the have discussed this. A member there did a MonteCarlo simulation of 1,000,000 random GPs and EPs and that was what resulted. Navigators in the 30s knew there was a significant problem which is why HO-249 was developed. This kind of testing was only possible once computers were developed and the true magnitude of the issue established.

At some point I’m going to ask that member for the data set or if he would develop error stats for bands of k/t/LHA rather than for the whole data set.

There is an ancillary reduction technique called Stadler that gives much more accurate results near 90d but it really only improves things from 87-93d.

I’m still a fan of Ageton despite the precision problems due to it’s relatively small size, the fact that there is only addition and subtraction involved (fewer mistakes than multiplying and dividing) and because it is the only modern method that uses EP rather than AP.

In Pepperday the is a form for working a sight backwards to to identify a star that you’ve shot but weren’t sure its identity.
__________________

__________________
Num Me Vexo?
SailboatData
A house is but a boat so poorly built and so firmly run aground no one would think to try and refloat it.
Adelie is offline   Reply With Quote
Reply

Tags
celestial navigation, navigation

Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes Rate This Thread
Rate This Thread:

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Long term sailboat storage near Long Island Sound Joe500 General Sailing Forum 3 29-08-2016 12:14
Bowditch Long-Term Almanac / Free Download OErjan Navigation 8 12-01-2015 12:12
Painting simulated lapstrake hull? sailing_jack Construction, Maintenance & Refit 13 05-10-2012 16:37
Celestial Navigation Help Needed Loose Ends Navigation 67 10-11-2011 12:19
Celestial Navigation SkiprJohn Navigation 45 29-12-2008 22:15



Copyright 2002- Social Knowledge, LLC All Rights Reserved.

All times are GMT -7. The time now is 03:58.


Google+
Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2019, vBulletin Solutions, Inc.
Social Knowledge Networks
Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2019, vBulletin Solutions, Inc.

ShowCase vBulletin Plugins by Drive Thru Online, Inc.