Advanced Concepts in Celestial Navigation (Windows on the World)
HTML-код
- Опубликовано: 18 апр 2014
- This video is part of the "Advanced Concepts in Celestial Navigation" film series.
Part 1 of "Advanced Concepts" deals with how to complete 4 sextant adjustments, general celestial navigation accuracy, and manual calculations of computed height and azimuth.
Useful links:
A good video by another RUclipsr regarding sextant adjustments:
• Calibrating and Using ...
An excellent article by Ocean Navigator magazine regarding sextant adjustments:
www.oceannavigator.com/January...
A discussion on a sailing forum regarding celestial navigation accuracy (my view is that 3 miles is excellent, 15 miles is acceptable, and 30 miles indicates you generally are doing the right thing, but probably have some math errors):
www.cruisersforum.com/forums/f...
A book which has a great discussion of manual calculations of computed height and azimuth:
www.amazon.com/Celestial-Navig...
Our Practical Navigator website which hosts these videos and written works:
www.practicalnavigator.org
A cool website which hosts celestial navigation spreadsheets for direct calculations:
www.navigation-spreadsheets.co...
Discussion:
In this episode we begin with a very quick description of where you should be in terms of knowledge. If you are not up to speed, please review the "Getting Started" and "Going Further" video series on the Navigation Training Channel.
From there, we move on to sextant adjustments. Review the links above and the discussion in the video. Remember to use distant objects like sailboat masts when correcting for side and index error. I generally don't mess with my sextant too much (other than for this episode) since it is made of metal and I am very careful with it. If you have a plastic sextant you may need to make these adjustments more frequently. I believe that index error of less than 5 minutes is ok to remove mathematically, and index error is the most important one.
Lastly, we conclude with a discussion of manual calculations. These are useful in academic cases, or if you are more mathematically inclined and enjoy trigonometry. (There is a great line in a C.S. Forester Book - "The rake of the masts was as beautiful as a quadratic equation").
The key thing to remember is that you plot from the DR position when calculating directly. However if you want to practice, solve the sight as you normally would, but then also calculate the Hc and Azimuth directly, so you can compare to the tabular figures. Only in this case would you plot from the AP. Practice makes perfect here, and a programmable calculator helps immensely. John Karls book (above) has a great discussion on manual calcs. Also - as pointed out in the comments by pmh099; the web has several good programs (particularly in MS excel) for direct calculations. Check out their site at
www.navigation-spreadsheets.co...
for some worksheets, as well as a cool plotting tool for laying down LOPs!
Episode title comes from the quote: "Your assumptions are your windows on the world. Scrub them off every once in a while or the light won't come in" Isaac Asimov.
Produced and presented by Christopher D. Nolan, US Coast Guard cutterman and merchant mariner (500 tons Oceans). Currently on sabbatical, returning to service in June 2014.
At 9:32 you hit upon a key detail of LHA that I had previously missed. Thank you for that. My sight reductions are now more accurate.
I've read a number of instructional books on celestial navigation, however your video presentations are the best instructional material that I've ever found. You are a remarkable instructor, thanks very much for your effort to educate us (your students) and for preserving an antiquated art form. Smooth sailing to you.
This is a most helpful series! Reading the books is difficult at best, but watching you do the work is fantastic!
Tried this at home using an artificial horizon and it all works fine!
Very nice! Another way to perform the calculations is with Excel; search the web for Navigation Spreadsheets: sun.xls and intercept.xls are relevant for this example.
Great point! That is a great way to solve the problems as well. I'll add to the video notes - thanks for bringing it up :)
edit - wow I just checked out your site - cool idea with the plotting tool! Love it. For anyone interested:
www.navigation-spreadsheets.com/introduction.php
I'd be interested to hear how many take a computer onboard. I know I don't.
@@SandyGarrity In 1984 Captain Granville had an Apple II on board in The Voyage of the Mimi.
Love this series! Very useful, thanks for sharing this information. By the way, do you know a good place to get all of these tables in csv, or other editable format? I've found a few sources online, but most are pdfs or hardcopy books. I found two .csv files online, but the websites don't look too elaborate so I don't know if the numbers are reliable.
Hi Melvyn, sorry to say I don't have any leads for you on csv files - everything I have is pdf. Good luck!
I made a quick spreadsheet program for the Hc formula you show, and it works for the above example, where the hemisphere is 'same'. How do you adjust the formula when the GP for the Sun and the AP for the observer are in different hemispheres? Is it as simple as putting a negative sign in front of declination? For some reason I'm getting strange results when I input data from a previous example.
+spelunkerd Indeed, one way to do it would be to have latitude vary from -90 to 90, or from 0-180 depending on your program. So opposite hemispheres could either use a negative sign to make the calculations work in the model that you propose. Thanks!
+NavigationTraining Thanks for all your replies, Chris. I've completed your whole video series on celnav and found it to be no less than outstanding. On reading other textbooks on the topic I keep coming back to your tutorials. At this point I'm in the humbling and tedious phase of working out dozens of real examples on pencil and paper. It has been a surprise to see how a slight error with the pencil can distort results by miles. Cheers from Victoria, BC.
Oh I forgot to ask. What sextant are you demonstrating?
I am using the Davis Mark 15 plastic sextant, but would like to start planning for a sextant such as you are using.
thanks
Hello Grandpa - congratulations on the success so far! I use an Astra IIIB from Celestaire...but the Davis is a perfectly good device as well, so don't think you have to upgrade unless you really want to. Keep up the good work!
NavigationTraining
Hi Chris
I decided to get that AstraIIIB and it is spectacular! Thanks for the tip. Also got the bubble horizon you mentioned. Been practicing and now am getting fixes within a mile of my actual location. Very gratifying! Looking forward to doing this on an ocean horizon. Just for fun I went over to one of our lakes and stood on the floating dock to take some sights. The dock rocks in the waves, so simulates being on a boat. A bit more challenging!
Also found that I can use the program Starry Night Pro (my astronomy students use this for their homework) to practice taking sights from anywhere in the world. The Navigator software then is excellent to check my work.
Looking forward to future instruction videos from you. Take care.
Grandpa Jones Awesome Grandpa - it's great to get practice on solid (or semi-solid) ground before heading to sea. And that's a great tip about the Starry Night program - I'll keep that it mind :)
hi sir. lately i've been reading the American Practical Navigator by nathaniel bowdich, particularly on the subject "sight reduction". i see that all you teach here are so far accurate with regards to the book. but there are things that confuses me and they about DR and AP. when computing for the LHA, is the longitude used the actual longitude on the GPS during the sight? In the book APN, the long. being used to compute for the LHA is different from the DR (DR is few minutes advance than the Long used for LHA computation). thanks in advance.
Hi Jun, great question. There are a couple ways to do this. First, if you are calculating the solution directly using the sin/cos formulas, you can use any DR position that you want, including your GPS position (if you are practicing or something). This means the LHA will be something very specific, for instance 123 degrees, 46.5 minutes.
However, if you calculate the solution using the tables, you need to have a round number for LHA in order to use the tables correctly, so you create an artificial "assumed position" so that when you determine LHA, the number is whole, for instance 123 degrees.
In the videos we solve them both ways, but you can pick whichever technique works best for you, and stick with that. Some people prefer tables, and some prefer the direct math. The LHA is just computed slightly different in each way. Hope that helps! -Chris
thank you!