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Basic practical astro navigation

CPD

Active member
Joined
20 Sep 2006
Messages
2,902
Location
Hampshire
I am reasonably confident at being able to take a sun sight and understand the importance of timing. What I would like to be able to is to take that reading and plot it. The several books/videos I have looked at all start from the theory and quite honestly, I become lost quite quickly. I understand that it is a complex subject, and that to be able to read from different stars must be an amazing thing to do. But what I am after at the moment is how to take a reading and plot it, without too much confusion and long calculations. I dont want to use a computer to do the calculations, but want the very basics so I can take figures from tables and use them without getting confused. Once I have mastered sun sights, then I would be interested to move on, but for now, simple, simple, simple. I am coming from the basic practicalities first, theory secong angle. Work commitments make the chance of a September evening class on the subject look remote. Any suggestions for websites, books etc ?? Thanks as usual.
 

bumblefish

Active member
Joined
22 Dec 2002
Messages
1,572
Location
Brighton
I have signed up for this course, the coursework all comes on a CD, still struggling to find the time to complete it though!
 

capnsensible

Well-known member
Joined
15 Mar 2007
Messages
29,791
Location
Atlantic
I get lucky enough to do a bit of astro from time to time and my favourite reference book is Tom Cunliffe's Ocean Navigation. Small, well written and even I can understand it. Absolutely the best. hope this helps.
 
G

Guest

Guest
When taking a noon sight, the experts often tell you to take the sun's elevation ten minutes or so before anticipated local noon, then wait until the sun drops back again to that elevation, then record the time and divide 'em to calculate local noon, then add or subtract the equation of time to find longitude.

The problem with this technique is that the cloud cover may not play ball, and the method is 100% reliant upon taking accurate sights (from the cockpit of a small boat !), and getting the second one without a hitch.

A tip I picked up ages ago (sorry, forget the source ...) is to plot several sights before and after anticipated local noon on graph paper (or make up a suitable 'graph tablet' from white formica), then plot the resulting curve onto this from which it will be pretty clear what the max altitude was, then simply bisect the curve to get an accurate local noon. Although this method requires more sights to be taken, it's relatively easy to spot any 'whoopsee' sighting errors, and enables you to compensate if a cloud obeying Murphy's Law obscures the sun at a critical moment.

Admittedly, your calculated position will always be 30 mins or more in arrears, so it will only tell you where you were the best part of an hour ago - but that should be good enough for an approximate daily position away from land.

If you Google for "Davis Mk15" you should be able to download their user .pdf file for that sextant which includes a pro forma suitable for calculating a basic noon sight.
All you need to practice this is an el cheapo plastic sextant, a clear view of the horizon (or make-up an artifical horizon), Declination and Equation of Time tables (or make up your own), a watch set to Greenwich, ... and a pencil.

Colin
 
G

Guest

Guest
Is there anybody reading this who is ok at programming in Javascript ? I've located one source for obtaining a daily Sun's Dec and EoT through to 2050, and another source for obtaining a month's table listing of the Sun's data - but which doesn't include Dec and EoT (Sod's Law !).
Both are HTML pages with the clever stuff written in Javascript. The authors are happy for the source to be modified - but I'm not up to the job. Tried and failed.
What I'd like to achieve is a monthly (or yearly) listing of Sun's Dec and EoT to publish in the public domain so that people can practice noon sights and carry this data onboard for emergency use. My thinking is that combining elements of the two Javascripts will do the job.
Thanks, Colin
colinpowell(AT)freeuk.com
 
G

Guest

Guest
Yup, plenty of stuff on the web - but as you comment, requiring a working laptop to drive 'em might prove tricky.

http://www.tecepe.com.br gives a 3 day listing with good Dec, but lousy EoT resolution. Could extrapolate, I suppose ...

ICE (old dos program from the US Naval Observatory - public domain - download from http://www.celnav.de ) gives a comprehensive array of astro data (but no EoT !), and then only one day at a time, and changing the date to generate an extended list would be tedious.

http://www.celnav.de/longterm.htm is about the best I've found so far to extract Dec and EoT, but 'as written' will only output one day's figures at a time. What I'd like to do is modify the Javascript of this program to output a monthly or yearly table. If no-one is willing or able to assist, then I'll just have to teach myself yet another cryptic language ...

The goal/objective is to generate accurate hard copy tables of Dec and EoT, these being the absolute minimum almanac data required to get an approximate noon sight, so this can then be carried for emergency use - just in case the gps, computer and/or any other electronic gear fails - for imho, that's the only justification for learning basic astronavigation these days.
'best
Colin
 
G

Guest

Guest
Ok now - I've cracked it.
Spent a few hours learning enough Javascript to do the job - only to discover just how poor this language is at formatting output, so I've had to resort to extensive use of leading zeros.
Will be compiling several year's worth of tables over the weekend, so if anyone has a need for this stuff, perhaps they'd let me know what format (.PDF, .TXT etc) is preferred ?

Right now I'm thinking 6 months to an A4 page, so that 2 pages back-to-back can be laminated together ?
Colin
 

PacketRat

New member
Joined
20 May 2007
Messages
170
Location
Merseyside
You beat me to it, Colin!

Using the last web page you referred to, I added a table, so that when you enter the date etc and press calculate, the table lists all the Sun DEC and EOT figures for each day at noon for the month in question. The additional code is in VBScript, by the way.

So far so good, but my plan to copy and paste the results to a spreadsheet didn't work out. That aside, the revised web page seems to work OK. Needs double checking though if it's going to be used by anyone.

If you like, I could email you the file.
Robin
 
G

Guest

Guest
Well batted, Robin !

Yes I'd very much like to see your code, especially as http://www.celnav.de/longterm.htm is the routine I'd have preferred to have worked on. But - with only a smattering of Javascript under my belt it wasn't immediately obvious how to easily add a table, so I judged it quicker/easier to modify the existing Javascript 'table output' routine js listing over at: http://www.jgiesen.de/astro/astroJS/rsTable/index.htm

I simply replaced the Sunrise/Set output string stuff with Dec and EoT instead. Then converted the existing decimal format to angular degrees/minutes/seconds and chronological minutes/seconds, and tidied up the formatting. Job done.

But with this routine it's not obvious (to me !) how Delta-T is calculated, and I've also found a small bug in the existing Javascript (a need to input 1300 hrs local time to get 1200 UT - even when sat on the Greenwich meridian in winter !).

I fully agree with you about the need to double-check any routine which may be used 'for real'. If you're willing, I'll create some identically formatted yearly files with 'my' modified routine, same with yours, then binary compare 'em which should flag up any serious discrepencies. Then I'll manually compare a couple of dozen random sights with those from ICE (Interactive Computer Ephemeris) from the U. S. Naval Observatory, before releasing this stuff.

All the best,
Colin

colinpowell(AT)freeuk.com
 

tgpt21

Member
Joined
4 Apr 2007
Messages
104
For good old steam navigation I can recommend "Basic Principles of Marine Navigation" by DA Moore. It got me through my 2nd Mates 34 years ago. A slim volume and full of meat. You'll find a secondhand copy somewhere. I lost mine and got one off the internet from a dealer. It's KANDY publications SBN 85309 011 4. It's all in there. All you need is a nautical almanac and Nories tables and a good watch. Hours of harmless fun.
 

Gargleblaster

Well-known member
Joined
16 Dec 2003
Messages
1,143
Location
Medway, Gillingham Reach
I really think the answer is to do a celestial navigation course. I did the RYA Yachtmaster Ocean several years ago. And I must admit every so often I do a voyage using only celestial, although when I get worried about hitting certain bits of land like Bermuda or the Scillies I do turn the GPS on.
I find a laptop useless as it drains my batteries and never has a charge when you want. But I do use a scientific calculator with deg, minutes, seconds, which eliminates the need for sight reduction tables. I always plot onto graph paper, and find two sights a day using the sun run sun process the most effective.
On the boat I have a little pencil and paper calculator which makes the process using the scientific calculator a breeze. Somewhere at home I have a cdrom from which I produce the pencil and paper calculator which at some time in the future I will try and email to you. But may not make much sense unless you do a course or read lots of books. Celestial navigation is fairly easy as long as you have a process to follow which ends in an appropriate result.

Glayva
 
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