Celestial Navigation
FAQ

Q: What should I do to get started in celestial navigation?
A: The book Practical Celestial Navigation by Susan P. Howell is my favorite. There is a computer course available from: Starpath Navigation. I haven't checked it out, but these are really good people. Lots of stuff on their site.
Q: I need formulas to calculate the bearing from my location, and the distance to an other point on the map. I have the lat, long. for my location and the lat. and long. for the other location.
A: You want to do an internet search on celestial navigation spherical triangle. One hit would be: navtrig. The Law of Cosines is what you need.
Q: What sextant should I get? I saw a good deal on an old one the other day. What about plastic?
A: I'm not an old sextant expert but I have some opinions on the new ones. Barely adequate is the Davis MK 3 at $34. Then there is the MK 15 at $119 and the MK 25 at $185. These are plastic but well made. Even though they look pretty good they are heat sensitive and a fair amount of care is required to get first class results. Check the index correction before, during and after observations, especially at noon when the instrument is spending time out in the sun. My choice is the Astra IIIB at $485. Top quality at a price way below the $1500-$3200 that a German or Japanese model would cost.
Q: I know this is a dumb question but I have no charts or appropriate maps, but I would like to know the geographic location of 23 degrees 0 min N by 160 degrees 10 min W. Failing that, could you direct me to a website that has that information?
A: When I first established this FAQ this question made sense, but by now (in the new world of google) all one has to do is type something like, ( +23.0, -160.1666 ) into "google maps" and in a microsecond you are there.
Q: I bought a four inch sextant with six filters from Castle Navigation marked Stanley London. When the sextant arrived a warning was included that permanent eye damage would result if this sextant was used for sun shots. I called them and was told that this warning was given since they did not know if the sun filters were adequate. Could you advise as to how I could determine whether the filters are alright and if not how they could be corrected?
A: I'd check with a eye doctor if you are concerned. They are probably just covering themselves. I choose the appropriate filter (or combination) without the danger of magnification by lineing them up off the sight path and then holding the sextant up at arms length toward the sun. If the image of the sun is sharp and easy to look at through the filters alone I figure the filters are ok. Then I push them down into the sight path and proceed.
Q: I live far away from the coast ( in Madrid actually). I find it difficult to train myself in celestial navigation because of the lack of a good horizon. I've read somewhere that there is a chance in such cases of building an artificial horizon. I would like to know how to construct it, and what corrections have to be made on the measurements obtain by means of it.
A: Here are a few words on the subject. If some of it is not clear you should consult a textbook. Celestaire sells a bubble level that gives you a crude horizion. That might be good enough. The term artificial horizon usually refers to a dish of some liquid that gives you a perfectly level mirror. Water is poor, dirty oil is pretty good and mercury is super (but hard to come by these days). Instead of using your sextant to measure the angle between the celestial object and the horizon, you measure the angle between the object and its reflection. Then you divide by two. There is no height-of-eye dip correction. That's the idea. There are some details that you can figure out (or ask me) if you really get into it.
Q: I am installing a solar panel to power an entry gate. The instruction book says the solar panel must face True North rather than Magnetic North. My Latitude is 33.3N and the Longitude is 84.6W. What is the variation, E or W, from Magnetic North to determine True North.
A: The variation there is pretty close to zero. (By the way, I hope you're going to aim the solar panel south not north.)
Q: How does one determine when the sun, or any celetial body for that matter, is at its highest point (zenith)? For instance how does a sailor know when to take a noon shot?
A: There is a book (published each year) called the Nautical Alamanac that tells you where everything in the sky is at any given time. It also lists the time of "noon", etc. Not expensive. You might consider getting one. Probably on the web in some form. For the sun, first look up the time of zenith in Greenwich, England. (Close to 12:00 noon.) Add 4 min. per degree of longitude (your best guess)and then you have the Greenwich time of the transit at your location. If you want, you can convert this to local time.
Q: I'm curious, how do you calculate Sumner's lines?
A: I don't remember where I came across the formula. A good place to look is Chap. 20 in "Practical Celestial Navigation" by Susan P. Howell. She describes two calculator methods. She doesn't give them names but the first one is the one I call Sumner's Lines. With an observation and an assumed Latitude you calculate what your Longitude has to be. The other method she describes gives you lat and lon from two sights.
Q: I'm having a problem with ex-meridian calculations. Can you help?
A: My favorate book is Practical Celestial Navigation by Susan P. Howell. She refers to tables 29 and 30 for ex-meridian calculations.
Q: What do I need to know to pass the Coast Guard celestial navigation requirement?
A: I'm not sure. I did an internet search but didn't come up with much except Navigation General (that has thousands of questions on it) .
Q: In reading FAQs from Mr. Burch of Starpath Navigation, he really discourages use of full view as opposed to the traditional split mirror. What do you think?
A: Burch is a sharp guy and there is no question about the limitations of the whole view mirror. That's what I have on my Astra IIIB, but I find stars so difficult that I almost never do them anymore. Pity. If you stick with the sun, moon, planets there is no problem.
Q: I am looking at the Astra IIIB Deluxe model because of the larger index mirror, but was wondering whether or not to go for the full-view Horizon Mirror. Given the other realities of taking sites off a small boat, i.e, rocky platform, low-height-of-eye and a lot of sun lines, what's your opinion...whole or split mirror?
A: At first I thought whole was great, modern technology etc. And it is most of the time. The down side is that it cuts the intensity of everything you look at by 50%. That means that stars (or a faint horizion) can be pretty difficult. Sun, moon, planets, really bright star under good conditions no problem.
Q: Wow..is it really 50%? The guys at Celestaire and elsewhere used the term slight degradation.
A: Think about it. Since the mirror is 50% reflective, 50% of the light is the very best you can hope for. The split mirror is 100% for both sides. It's not a question of quality.
Q: Do you know of any internet mailing lists or newsgroups that discuss celestial navigation? I would like to put some of the stuff I am developing up for review by others more knowledgeable than myself.
A: There used to be a non-electronic navigation listserver that was wonderful. Would be perfect for you. Lots of very sharp people. I've lost track of it and have been unsuccessful in finding it. It may have died out. Give a look. If you find it let me know.
There is a celestial list server at: I'd Rather Be Sailing, but I have not tried it.
Q: What would be the various ways you could determine your location and bearing? How relatively accurate are the methods, and how much expertise might you need for them? I understand the utility of having the combo of a sextant, an accurate clock and a Nautical Almanac; what if there were no almanac, no charts? Or perhaps worse, no sextant?
A: The book Emergency Navigation by David Burch deals with these questions.

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