SailNet Community - View Single Post - Working With the StarsóCelestial Navigation
View Single Post
Old 05-06-2003
Jim Sexton Jim Sexton is offline
Join Date: Jan 2000
Posts: 162
Thanks: 0
Thanked 2 Times in 2 Posts
Rep Power: 15
Jim Sexton is on a distinguished road
Working With the StarsóCelestial Navigation

Seemingly old-fashioned, celestial navigation should still be regarded as vital for the prudent navigator.
"And all I ask is a tall ship and a star to steer her by

And the wheel's kick and the wind's song and the white sail's shaking..."

óJohn Masefield, Sea-Fever

These are two of the most beautiful lines ever penned about sailing and the sea. Most of us have seen that first line many times, which brings me indirectly to the subject of this article. Just how do you steer by a star? Many boaters will say: "Why bother? Celestial is old fashioned, not all that accurate, and besides, I have a spare GPS."

Today, the universal use of GPS receivers has relegated celestial navigation to one of those romantic but impractical arts, like fancy knot-tying or building model ships in bottles. Even among professional navigators, celestial navigation is often regarded as a useful-but-cumbersome emergency backup system for the GPS.

Many mariners discard the idea of learning celestial because in their words, "It's too hard." Learning celestial navigation is usually thought of as difficult for several reasons. First, celestial navigation uses a language of its own, which is necessary for learning the subject. Secondly, celestial navigation is a very elaborate system that took centuries to develop into its modern "cookbook" form and few realize just how easy it has become. Lastly, few of us observe the seasonal changes in the celestial sphere and don't follow astronomical events as closely as our ancestors did. As a result, the underlying astronomical concepts of celestial navigation and star identification are unfamiliar to the vast majority of mankind.

"Celestial navigation is an interesting hobby as well as a great starting point for learning astronomy."

However, celestial navigation is an interesting hobby as well as a great starting point for learning astronomy. It gives you a sense of satisfaction and joy in keeping alive the knowledge that connects us with our nautical heritage. Finally, knowing celestial navigation practical rewards, like allowing you to find your way across the ocean without electronics. If your dream is to sail to Bermuda or Bali someday, you must have a working knowledge of celestial navigation.

I want you to know that celestial navigation is not difficult. Take it from someone who has actually done celestial navigation for a living. The work is detailed and lengthy, but anyone who can add and subtract will be able to complete a sight reduction form with just a little practice. Since most of us have had trigonometry in high school, the celestial triangle concepts will be fairly easy to understand. If you don't remember anything about trig, relax as you will not be using it directly.

Anyone who can add and subtract will be able to complete a sight-reduction form.
The method of reducing your sextant observations to a chart position is just as easy as following a cookbook recipe. You do not need to be an astronomer or mathematician to establish a celestial line of position. The ability to use a sextant is just a matter of practice and specially designed celestial tables have reduced previously complex computations to simple arithmetic.

A detailed understanding of navigational astronomy is not essential to establish an accurate celestial position. However, celestial work and celestial lines of position will have more meaning if you understand the basic facts of celestial astronomy, which I will cover in future articles.

The biggest stumbling block to learning celestial navigation for most boaters is the expense. The tools of the trade are expensive. So if you really want to learn and then actually practice celestial navigation, be prepared to spend between $300 and $3000. That's quite a price range and you will see why as we go on. A minimum of three pieces of equipment will be required: a sextant, an accurate time piece, and a copy of the Nautical Almanac for the current year. Additional items may also be necessary. You can use the plotting equipment you already have, but many navigators prefer to use a better grade of plotter and dividers for celestial work and relegate their older set to the emergency navigation kit. I strongly recommend that you also buy a copy of The American Practical Navigator by Bowditch and a good book on star identification such as 365 Starry Nights by Chet Raymo. If you consider Bowditch too technical, then you can get either Dutton's Navigation and Piloting or Boater's Bowditch by Richard K. Hubbard. Dutton's is an excellent book with outstanding coverage of celestial navigation. Boater's Bowditch covers the basic and most practical techniques of navigation found in the original Bowditch, but without the complex technical theory and science coverage that most of us do not need.

The sextant is by far the most expensive of the three required tools for celestial navigation, but having precise equipment can mean the difference between an accurate fix and one that's well off.
The sextant is by far the most expensive piece of required equipment. Even an inexpensive plastic sextant costs almost $200 and a basic aluminum alloy one is about $500. In my opinion, the best sextants are made by C. Plath and their prices range from $1500 to over $3000. Japanese sextants made by Tamaya cost from $1400 to over $2000 and are another excellent choice. Many students of celestial start out with a plastic sextant and after they finish the course, buy a good metal one and store the plastic model in the emergency navigation kit. It can also function as a backup if your good sextant gets dropped.

A sextant is a very precise instrument for measuring angles in degrees, minutes, and seconds. Sextants are expensive because they are precision instruments, and the best ones are virtually error free. The high-end sextants have errors of less than 10 seconds of arc. Plastic sextants commonly exhibit errors in excess of 5 minutes, even when great care is exercised in using them. If you are not familiar with minutes and seconds of arc, let me explain. We are all are familiar with the course plotter, which is basically a protractor on a long straight edge and is used to measure angles on the chart. These plotters have a division mark for each degree, and most of us would agree that these divisions are pretty small. Now consider taking those degrees and dividing each one into 60 equal parts. Each part is one arc minute. If we then divide each of these minutes into 60 parts, we now have arc seconds. This is just like the minutes and seconds in an hour of time. There are 60 seconds in a minute and 60 minutes in an hour. Just as there are 3600 seconds in an hour, there are 3600 arc seconds in a degree. I think that you will agree that a second of arc is an extremely fine division, and even the best sextants cannot measure to that accuracy. On a sextant, these smallest divisions are displayed as tenths of a minute. The angle that you measure with a sextant is the elevation of a body above the observer's horizon and is called the altitude. We need an accurate sextant because an error of one minute of arc will result in a navigation error of one nautical mile. This time and angle relationship will be explored in detail later on.

"An accurate watch is the next most expensive item."

An accurate watch is the next most expensive item. Today even the least expensive electronic wristwatches can keep reasonably accurate time and they can be updated from the US Naval Observatory clock with the radio. Accurate time is very important because four seconds of watch error can put your celestial observations off by one nautical mile. Nonetheless for most of us there is no need to spend the big bucks on a Rolex when a Timex will do. I would suggest, however, that you buy a spare watch and batteries as a precaution against failure or damage. I also recommend that you keep a separate clock at the nav station and record the watch error every day. This practice will enable you to have an accurate time if the radio should fail and you are unable to "hack" your watch. Buy two fairly rugged, waterproof watches that have a second hand or digital seconds, and which can be "hacked" or adjusted easily. Style and expense is not as important as accuracy.

The Nautical Almanac, another indispensable tool for celestial navigation.
The Nautical Almanac contains the tables used to calculate the exact location of the sun, moon, planets or stars you have to obtain your LOP. But, if you prefer to use another method of sight reduction than the one in the Nautical Almanac, you will need to buy Sight Reduction Tables as well. The commercial edition of the Nautical Almanac is about $20, and if you want them, the additional Sight Reduction Tables are $13 per volume. Most of us can easily get by with the three H.O. 249 tables, or four of the H.O. 229 tables. You will need a new edition of the Nautical Almanac each year, but the Sight Reduction Tables are good for many, many years. I will have more to say on these various tables and sight-reduction methods in future articles.

The last item you may want to consider is a computer program or calculator that solves the sight reduction problem for you. Of course, you must learn how to do this with only pencil, paper, Almanac, and Sight Reduction Tables. But a calculator and computer program is invaluable for checking your work when you are first learning. Later on they speed up and ease the work load tremendously. Several of the computerized navigation and electronic charting programs (The Cap'n is one) have a celestial program built in, or you can buy a handheld celestial calculator, such as the Celestialcomp, to do the tedious calculations. For those of you so inclined, you can use a regular calculator that has trig functions to reduce the sight by using the equations for altitude and azimuth of the celestial body and entering the necessary variables of latitude, LHA (Local Hour Angle), and the declination of the body. These calculators are also handy when doing the angular and time calculations on the sight-reduction form.

"As with any profession, being able to understand and use its distinctive language marks you as an insider."

Perhaps I've already used several celestial terms that you may not be familiar with, such as sight reduction, altitude, azimuth, declination and LHA. This is a good example of the arcane language of celestial navigation I mentioned earlier. But once you know the meanings of these terms it all becomes clearer and much easier to understand. As with any profession, being able to understand and use its distinctive language marks you as an insider, and knowing how to navigate by the stars brands you as a serious navigator among your peers.

I highly recommend that you take a celestial navigation course from either the US Power Squadrons or your local community college and use this series of articles to help you along. Although celestial is not difficult, it does presuppose, and requires, a thorough knowledge of navigation basics, piloting, and log-keeping. Also, it can be frustrating without an instructor to answer immediate questions, demonstrate plotting techniques, and lead you through the sight reduction tables. In addition, the instructor will demonstrate the hands-on fundamentals of sextant usage and take you on class outings where you can practice taking celestial observations and learn star identification. If you can't attend a course, then find another navigator knowledgeable in celestial to help you along.

Suggested Reading List

  1. Navigation Basics by Jim Sexton
  2. Dead but not Deceased by John Rousmaniere
  3. General or Specific Charts by Tom Wood
  4. Suggested ProductóWeems & Plath Tamaya Navigator Computer