How to Use a Sextant

The First Time I Used a Sextant (and Why You Should Try It Too)

I remember my first successful “sight.” I was standing on the deck of the Hawthorne, a sturdy cutter used by a navigation school off the coast of Maine. The sun was just dipping toward the sea. My hands were clumsy, the brass arc cold. Captain Ellis, the instructor, looked over my shoulder and muttered, “You’re about to find yourself.”

That was the moment I realized a sextant isn’t just a tool. It’s a bridge between mathematics and the horizon? a way to know your position on Earth using nothing but light, angles, and time. In an era where GPS does it all silently, learning how to use a sextant connects you with centuries of seafarers, explorers, and aviators who relied on this elegant instrument to navigate vast oceans and uncharted skies.

If you've ever been curious about celestial navigation, this guide is for you. I’ll walk you through how to use a sextant, in plain English, no jargon. We'll keep it simple and hands-on, with a summary tutorial at the end. But first, a bit of context.

What Is a Sextant, Really?

A sextant is a precision instrument used to measure the angle between a celestial object (like the sun, moon, or a star) and the horizon. This angle, known as the altitude, allows you to determine your latitude, and with additional information and a bit more calculation, even your longitude.

It works using two mirrors. When you align the reflected image of a celestial body with the visible horizon, you can read the angle off a graduated scale. That angle is your key to knowing your position on the planet.

The beauty of the sextant is its independence: no batteries, no electronics, just physics and optics.

Tools You’ll Need Before You Begin

To take a sight with a sextant, you’ll need:

1. A sextant (obviously)? preferably one with a micrometer drum and adjustable index mirror.
   
   

2. A marine chronometer or accurate watch? to note the exact time of your sight.
   
   

3. A nautical almanac? which gives celestial positions for specific times and dates.
   
   

4. Sight reduction tables or a navigation calculator? to turn your angle and time into a position.
   
   

5. A clear view of the horizon? sea or artificial horizon if on land or in flight.
   
   

Now let’s walk through the basics of how to use a sextant.

Step-by-Step: How to Use a Sextant

I spoke with marine navigator Elena Rivas, who trains cadets in traditional celestial navigation. Here’s the method she teaches:

1. Set the Sextant to Zero

“First thing, always check your index error,” Elena told me.
To do this, align the horizon using the horizon mirror and reflected view. If the line isn’t level, note the difference? this is your index error, which you’ll need to subtract or add from your final angle.

2. Point at the Horizon

Hold the sextant vertically with the telescope aimed toward the horizon. Keep your eye on the horizon line.

3. Bring the Celestial Object Down

Using the index arm, move the mirror so that the image of the sun, star, or moon drops down to the horizon line. You should see both the horizon and the reflected celestial object in the same field of view? lined up perfectly.

4. Rock the Sextant

Rock the instrument slightly side to side. The lowest point of the arc (the dip) is the correct reading. This ensures you’re measuring the true altitude.

5. Read the Angle

Once aligned, lock the arm. Read the angle on the arc and micrometer drum. This is your observed altitude.

6. Record the Time

Note the exact UTC time of the sight using your watch. Accuracy is crucial, especially for longitude.

7. Correct and Calculate

Apply corrections:

• Index error
  
  

• Dip (based on height of your eye above sea level)
  
  

• Refraction (from standard tables)
  
  

• Sun's semi-diameter (if measuring the sun’s lower limb)
  
  

Now you have the corrected altitude. Use the nautical almanac and sight reduction tables to plot your line of position.

A Simple Summary Tutorial: “Sun Sight at Sea” in 60 Seconds

If you're short on time, here’s the ultra-simplified version of how to use a sextant to find latitude with the sun at local noon:

1. At noon, take a sight of the sun at its highest point (when it stops rising).
   
   

2. Record the angle and exact time.
   
   

3. Look up the sun’s declination for that date in the almanac.
   
   

4. Latitude = 90° – observed altitude + or – declination
   
   

• Add if the sun is in the same hemisphere
  
  

• Subtract if it's in the opposite hemisphere
  
  

That’s it? you’ve got your latitude. It won’t be perfect, but it will be close enough to get you home.

Final Thoughts: Why Learn This Now?

I asked Captain Ellis recently why he still teaches sextant use when every phone has GPS.

He said, “Because knowing how to use a sextant means you’ll never be lost, even when everything else fails. And maybe more importantly, it teaches you how to see.”

He’s right. Learning how to use a sextant isn’t just about survival. It’s about learning to observe, measure, and understand your place in the world through your own hands and eyes.

So go ahead? try it. Aim for the sun, line up the horizon, and find yourself the old-fashioned way.