Second Star To The Right – Finding your way with celestial navigation


Marine sextant” by Joaquim Alves Gaspar – Licensed under CC BY 2.5 via Commons.

You’re a long way from home, adrift on an unfamiliar sea where few have ventured before. In every direction you see nothing but sky and ocean. You haven’t spied land for weeks. While you’ve managed to capture some rainwater to drink, food is another matter. Chewing on leather is less than satisfying and the rats trapped aboard ship with you – themselves shockingly thin – are starting to look appetizing. It’s imperative that you determine your position, now, before the rats make the first move.

This was the dilemma facing Ferdinand Magellan in 1519. He needed to establish his position before his food ran out and before his crew mutinied. But how? GPS satellites wouldn’t be launched into orbit for more than four-hundred years. Even the sextant was still a long way off, not to appear until about 1757.

For Magellan, the answer was likely the astrolabe, a simple device that allowed a mariner to ascertain his position by measuring the sun’s altitude above the horizon. It was crude — inaccurate, useless in foul weather, and only able to provide one’s latitude — but sufficient to allow one of Magellan’s captains to successfully complete the first circumnavigation of the globe.

Today, any explorer undertaking the same voyage would likely turn to a GPS unit, able to pinpoint a location within a few meters, any time of day, in any weather. But not necessarily.

Many die-hard traditionalists would rather look to the heavens, plotting their position as Magellan did five hundred years ago, by measuring the altitude of the sun or moon above the horizon and comparing it with the date and time.

Of course, when Magellan set out to find a westward route to the Spice Islands, he had no choice but to use the tools available to him. If he had had the option of using a GPS unit tied to artificial satellites, he almost certainly would have done so.

So why do voyageurs today, who have the freedom of using fast, accurate, and easy GPS, opt instead for the laborious, time consuming and error-prone methods of celestial navigation? What is celestial navigation, how does it work, and how can you learn it?

What is Celestial?

Celestial navigation, usually just called Celestial by those experienced with its use, is an ancient blend of art and science, based on a simple concept. Take your position on the earth’s surface, the current date and time, and the apparent altitude above the horizon of a celestial body; the sun, moon, planets, or stars. If you know any two, you can calculate the third.

We do this all the time with barely a thought.

It’s mid-summer. The sun has just peaked over the horizon. You know it’s early morning, unless you’re very far north or south, in which case it’s closer to noon.

With experience, astronomers know intuitively where to look to find specific constellations at any time of night and in any season.

Determining our location this way may be less intuitive for most of us because we do it so infrequently, but the principle is the same.

The Challenge Is Accuracy

How do you measure the precise altitude of the sun when it’s too bright to safely view? How do you account for the refraction of light through the earth’s atmosphere at different angles, your own height above the horizon, and the inherent inaccuracy of your instruments? A star or planet may appear as a single point of light, but the sun and moon appear much larger. Do you measure their height from the top or bottom, or guess as to the middle? Modern digital timepieces have eliminated one problem that plagued early explorers; we always know what time it is. But do we use our local time or some other standard?

Once you’ve taken your sightings and determined the time, how do you combine those data to produce values for latitude and longitude? The math isn’t difficult, but it can be confusing since you’re working with seconds, minutes, and degrees, not a straightforward base-10 system. Cindy took a few lessons in celestial several years ago. She admits, “the hardest thing I remember was the math. You don’t add and subtract in groups of 10, but rather 6’s since the math is based on 60 seconds to a minute. I made a mistake that could have cost me many many miles! Fortunately it was only a lesson quiz.”

You also have to consult sight reduction tables, either on a computer or, for the purist, in an annual publication like the Nautical Almanac, based on data compiled by the U.S. Naval Observatory. These tables allow you to correlate all the values you’ve measured from two or more sightings and reduce them — thus the name — into a triangle containing your likely latitude and longitude.

If you’re sighting during the day you’ll use the sun. At night you may use the moon, or a star or a planet. Obviously it will help to be able to identify a few constellations, so a basic stellar atlas is also a good idea. A perfect sighting and flawless calculations will just get you lost if you’ve misidentified the star you’re following.

It sounds like a lengthy, convoluted process, and it is when you’re just starting out. But as with any skill, the more you practice it, the easier it becomes.

Learning The Ropes

Assuming the complexity of celestial hasn’t scared you away — perhaps it’s even whet your appetite for more — the next step is to actually learn the process.

Teaching you how to actually take a sighting and how to compute your location is beyond the scope of an article like this, but there are several options available.

You can ask a friend already experienced to show you how it’s done. You can go to the library and borrow a few good books on the subject. Or you can take a course.

Most sailing schools teach courses in celestial navigation. The courses generally last a week and cost a few hundred dollars. Some schools will provide a sextant and site reduction tables; others will require you to bring your own.

At some point, if you decide to pursue celestial, you’ll need to obtain your own sextant. Prices range from less than $200 CDN for basic models in plastic, to $3000 CDN or higher for brass sextants with multiple filters, micrometers, and telescopes. As is often the case, you get what you pay for; a higher quality sextant is likely to cost more, but it will also allow you to take more precise sightings and therefore to plot your position more accurately.

With a good sextant, a little experience, and a lot of patience, you may be able to pinpoint your location within two or three nautical miles. Mikael Pettersson, a lieutenant in the Swedish Navy (reserve), found this to be true when he sailed from Sweden to Japan via Panama and New Zealand in 1987, before GPS was commercially available. His main means of navigation was celestial, using an antique sextant manufactured in 1870.

While there’s no denying that GPS is easier and more accurate, there are good reasons you may choose celestial.

Why Bother?

Keith Gore is an experienced celestial user. But he still recommends GPS. “When you most need a fix it’s likely to be stormy and overcast with nothing visible to shoot. Celestial is a neat trick to learn, but for practicality I’d suggest simply buying a backup GPS or two. Cheap, drop dead accurate, and virtually idiot proof.”

However, opinions vary, and with good reason.

The satellites that provide the GPS service are, for the most part, owned by the United States military. How that sits with your political views is one matter. Another is that, in the event of a security disruption, the Pentagon could decide to switch the system to a less accurate mode known as “selective availability,” or disable it for civilian use altogether.

Even if that doesn’t happen, there’s still the issue of reliability. Badru Hyatt, who’s made transatlantic crossings using only celestial, explains, “GPS’s break all the time. Nautical electronics is notoriously unreliable, what with all the salt water that gets everywhere. Also, it’s not uncommon for a boat to get hit by lightning and have all its on-board electronics fried. If that happens on an offshore passage and you don’t know how to navigate celestially then you will be in a real predicament.”

Mikael, mentioned earlier, agrees. “I would definitely have a sextant with me for any longer voyages. I would never fully depend on electricity on-board. It has nothing to do with romance, just common sense.”

Although celestial navigation takes a certain degree of skill – or perhaps because of this – it tends to provide a greater sense of satisfaction. Badru comments, “I definitely do it for the … I guess you could call it romance, but I’d prefer just to call it pure satisfaction. I find great satisfaction in being able to accurately determine my position anywhere on the earth using only ‘old fashioned’ methods.”

While Magellan’s voyage may have ended in disaster, that was due more to his failing as a diplomat than from the ‘old fashioned’ navigational tools he used. Certainly there was no failing on the part of the stars. With a little practice you can use those same stars to guide your way today.

Jonathan Mycroft is a freelance writer and author. His latest novel, Trial by Fear, is now available as a pocket book and as a downloadable e-book from Visit his website at