The Marine Tack Blog
A Seamanship Essay.
Finding Your Way by the Stars: The Sextant Today
For centuries, sailors crossed oceans without GPS, electronic charts, or satellite communications. Their guides were the sun, moon, stars, a good nautical chart, and an instrument that could turn the sky into a navigation tool: the sextant.
Today, most recreational boaters will never need to navigate an ocean using celestial observations. GPS can tell us our position within seconds, chartplotters can display our course, and smartphones can provide an impressive amount of navigational information.
So why should anyone still care about the sextant?
Because learning celestial navigation provides something modern electronics cannot: a fundamental understanding of where you are and how you can determine your position when technology isn't available.
Looking Beyond GPS
Modern navigation has made boating safer and more convenient, but it has also created a certain dependence on electronics.
A GPS receiver depends on satellites, electrical power, antennas, and functioning electronics. A chartplotter depends on power and software. Even a smartphone requires a working battery and compatible equipment.
A sextant needs none of those things.
With a sextant, a navigator can measure the angle between a celestial body and the horizon. Combined with the correct time, a nautical almanac, and some calculations, that observation can be used to determine a vessel's position.
It may sound complicated—and initially, it can be—but the basic idea is remarkably simple.
You are using the sky as a reference point.
What a Sextant Does
A sextant is a precision optical instrument designed primarily to measure angles.
When navigating by the sun, for example, the navigator uses the sextant to measure the sun's altitude above the horizon. The instrument's mirrors allow the navigator to bring the image of the sun down toward the horizon while reading the angle on the sextant's graduated arc.
That angle becomes the starting point for a navigation calculation.
The sextant itself doesn't tell you, “You are here.”
Instead, it gives you a measurement that can be combined with known information about the celestial body and the exact time of the observation.
The result is a line of position. Take several observations, or observations of different celestial bodies, and those lines can intersect to provide a navigational fix.
The Fundamentals
Celestial navigation is based on a fascinating relationship between the observer and the heavens.
The apparent position of the sun, moon, planets, and stars changes predictably as Earth rotates and moves around the sun. Navigators can use those predictable movements as a giant natural clock and reference system.
The basic process involves:
- Measuring a celestial body's altitude with the sextant.
- Recording the precise time of the observation.
- Using astronomical information and calculations to determine a line of position.
- Combining observations to establish a navigational fix.
This is one reason celestial navigation remains such an interesting subject. You aren't simply reading information from a screen. You are observing the natural world and turning that observation into useful navigational information.
Why Time Matters
One of the most important parts of celestial navigation is accurate time.
Earth rotates approximately 15 degrees of longitude every hour. That means even a relatively small error in time can produce a significant positional error.
For traditional celestial navigation, an accurate marine chronometer or reliable time source was essential.
Today, a navigator can obtain extremely accurate time from several sources. Ironically, modern technology can therefore make celestial navigation easier without replacing the sextant itself.
The navigator can still take the observation manually while using modern tools to help with timekeeping, calculations, or reference information.
A Backup That Works
Perhaps the most practical reason to learn celestial navigation today is redundancy.
Good seamanship has always included backup plans. If your primary navigation system fails, you should have another way to determine where you are.
A sextant can become part of that backup system.
It doesn't need batteries, internet access, or a satellite signal. It can continue to function in situations where electronic navigation equipment cannot.
Of course, carrying a sextant doesn't automatically make someone a celestial navigator. The skill requires practice.
Knowing how to identify stars, take accurate sights, correct observations, perform calculations, and plot a position requires training. But even learning the fundamentals can give a boater a much deeper understanding of navigation.
More Than a Backup
There is another reason people continue to learn celestial navigation: the experience itself.
Standing on deck at sea, raising a sextant toward the sun, and taking a sight creates a connection between the navigator and the environment that is difficult to replicate with a digital display.
The ocean becomes more than a route on a chart.
The sun becomes a navigational reference. The stars become landmarks. Time becomes a positional measurement.
That perspective can change the way a person thinks about boating.
Sextants Still Matter
Celestial navigation is no longer the primary method of navigation for most recreational boaters, and there is no reason to pretend otherwise. GPS is faster, easier, and extraordinarily useful.
But the sextant has not become irrelevant.
Instead, it occupies a different role.
It represents navigation at its most fundamental level—using observation, mathematics, time, and the natural world to determine position.
For today's boater, learning to use a sextant can provide an invaluable backup skill while also building a better understanding of how navigation actually works.
Technology tells us where we are.
A sextant teaches us how we can figure it out ourselves.
And on the open water, knowing both is a pretty good way to stay on course.
Enjoy and be safe out there.
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