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A sextant is a handheld optical tool that helps you find your position at sea by measuring the angle between the horizon and a celestial body, usually the Sun or a bright star. That single angle – combined with an accurate time reading – draws a line on your chart and tells you exactly where you are without GPS.
Why learn a centuries-old skill in the age of satellite chips?
Electronics fail – batteries die, lightning strikes, and saltwater can creep into every connector. A sextant needs none of that. It works on bright days, star-filled nights, and even during partial power outages on board. Modern merchant ships still carry them by law, and many skippers practice a daily sight so they can back up the plotter in seconds.
Sextant 101: what it does in plain English
A sextant measures angles. Held upright, it tells you how high a body sits above the horizon. Held sideways, it measures the gap between two shore markers so you can fix distance off a beach or gauge whether another vessel is closing in on you. Think of it as a super-accurate protractor with built-in mirrors.
How the magic works (minus the math)
- Double reflection. Two tiny mirrors bounce a ray of light so that the angle you read on the arc is double the swing of the index arm. That’s why a 60-degree arc measures up to 120 degrees.
- Horizon reference. Because you sight against Earth’s visible horizon – an optical “level” that never lies – wave motion matters far less than you’d think. Even on a rolling deck you can spot the instant the Sun’s lower rim kisses the water and lock in a clean number.
- Time stamp. Note the exact second on your watch. Feed angle + time into sight-reduction tables. The tables spit out a line of position. Cross two lines from different sights and you have a fix.
Result: latitude, longitude, and peace of mind when every screen on board goes dark.
Understand the parts (and why each matters)
| Sextant Part | Plain-Language Role |
|---|---|
| Frame | The stiff “pizza-slice” skeleton; keeps angles honest even when metal expands in the sun. |
| Arc | Graduated 0–120° scale; more than enough for any marine angle you’ll shoot. |
| Index arm + micrometer | The movable pointer and fine-tune knob; lets you stop exactly where you want. |
| Index mirror | First bounce. Mounted on the arm; aims the celestial body. |
| Horizon mirror | Half-silvered. Shows real horizon on one side, reflected body on the other, so you can “snap” them together. |
| Telescope | A short scope (1× to 3×) that brightens the view and trims wave jiggle. |
| Sun shades | Smoky filters. Protect your eyes and stop glare when shooting the Sun. |
Knowing these parts makes troubleshooting easy when a captain shows you a new instrument.
How to take a Sun sight
Prepare the gear
- Set filters. Clip on the darkest shade – you can always lighten later.
- Zero the index arm. Check the mirrors line up true; adjust if needed.
Find the Sun
Hold the sextant upright. Sweep the sky until a glowing disc drops into view on the mirror. Keep the horizon in the clear half of the horizon mirror.
Bring the Sun down
Slide the index arm so the Sun’s lower edge (called the lower limb) just touches the sea. Use the micrometer for the final feather-light nudge.
Rock and lock
Gently tilt the sextant side to side. The Sun will trace a tiny arc. Take the angle at the lowest point – this cancels any slight tilt of your body.
Record the data
Note the angle and exact UTC time to the second. You’ve captured one of the oldest, still reliable pieces of navigation data on Earth.
Common sights beyond the Sun
- Polaris (North Star). Easy night latitude: its altitude ≈ your latitude in the Northern Hemisphere.
- Moon or planets. Handy when clouds block stars at twilight.
- Horizontal sextant angle. Measure the angle between two lighthouses to find distance offshore – a fast coastal position check.
- Mast-head angle. Old racing trick: compare the target boat’s mast height to the horizon to see if you’re gaining.
Reading the scale: on-arc vs off-arc
The main arc runs left of zero. Readings there are called “on the arc.” A few degrees to the right – “off the arc” – are used for detecting and correcting tiny instrument errors. Remember to read the micrometer backwards when you’re off the arc. It sounds odd, but after two tries it’s second nature.
Keeping your numbers honest: error checks
- Perpendicularity. Is the index mirror 90° to the frame?
- Side error. Is the horizon mirror 90° to the frame?
- Index error. Are both mirrors truly parallel at 0°?
- Collimation. Is the telescope aligned with the frame?
Run through these quick tweaks before a long passage. Most take less than a minute once you’ve done them a couple of times.
Care and feeding of a sextant
- Keep fingerprints off the arc. Oil equals rust and pitted graduations.
- Rinse salt, pat dry. Fresh water and a soft cloth will stop corrosion.
- Grease the worm gear. A dab of Vaseline keeps the micrometer silky.
- Store in its box. Out of direct sun, away from damp lockers.
Treat it like a camera lens and it will last a lifetime – and still fetch top dollar if you ever upgrade.
Metal or plastic? Picking the right tool for your boat
| Feature | Metal sextant | Plastic sextant |
| Accuracy | ≤1 NM typical | 3–5 NM typical |
| Weight | Heavier – steadier in waves | Light – easy on the wrist |
| Cost | $$$ | $ |
| Durability | Excellent; resists flex | Good but can warp in heat |
| Best for | Offshore passage makers, professional skippers | Coastal cruisers, training, emergency backup |
Large mirrors and a whole-horizon viewfinder simplify sights on a bouncing deck. If you sail lakes or trailer a skiff, a quality plastic model stashed in a dry bag may be all the backup you need.
Small-boat tips captains swear by
- Stand on centerline. The farther off-center you lean, the more hull roll you feel.
- Brace your elbows. Tuck them against your life-jacket straps for a human gimbal.
- Practice at anchor. Ten sunrise sights in calm water build muscle memory you can trust when seas pick up.
- Log every sight. Write angle, body, time, and conditions. Patterns teach more than perfect numbers.
FAQ’s new users ask at the dock
Q: How accurate is a handheld sight?
A well-practiced boater routinely nails ±1 NM; that’s inside visual range of most land features. The world record with a sextant is under 0.1 NM.
Q: Do I need math skills?
Not anymore. Phone and tablet apps crunch the sight-reduction tables. Still, learning the manual method is wise, paper tables never run out of battery.
Q: Can I use a sextant on a lake?
Yes, if the lake is big enough to show a clean horizon. Alternatively, use an artificial bubble horizon – supplied with many aviation sextants.
Q: Will it work under overcast?
No sight needs a clear view of both horizon and body. But skies often open up for a minute. Have the sextant ready and grab the gap.
Where the sextant fits in a modern safety plan
- Primary nav remains GPS. No argument – the little blue dot is unbeatable for speed and convenience.
- Redundancy matters. A sextant and paper chart are your “lifeboat navigation” kit if lightning pops every circuit.
- Training builds confidence. Knowing you can navigate blindfolded to the satellites keeps stress low when the weather turns and autopilot goes on strike.
- Safety culture. Practicing sights pushes the crew to check chronometers, update paper charts, and review emergency comms – habits that pay off in every mishap.
Final thought
A sextant is more than brass and glass. It’s self-reliance you can hold in one hand – an all-weather lifeline that doesn’t care about batteries, cell towers, or government satellites. Spend a Saturday morning learning its dance, and you’ll join generations of mariners who could always find home when the horizon looked the same in every direction.
Sea Tow Team
Sea Tow has been the premier leader in on-water boating assistance since 1983. We want to share news, press, tips and all things boating.