A sextant is a navigation instrument that measures the angle between two visible objects, usually the horizon and the Sun, Moon, a star, or a planet. Navigators combine that measurement with accurate time and astronomical data to help determine their position at sea.
Using a sextant involves taking a sight, reading the scale, applying corrections and calculating a position line. The instrument works without a GPS signal or electrical power, but successful celestial navigation still requires suitable visibility, reliable time and the right reference information.
In this guide
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What Is a Sextant Meaning and Purpose
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How a Sextant Works
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Main Parts of a Sextant
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What You Need Before Using a Sextant
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How to Use a Sextant to Take a Sun Sight
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How to Read a Sextant
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From a Sextant Reading to a Position Line
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Common Sextant Mistakes
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Other Uses of a Sextant
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Practicing Celestial Navigation
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Frequently Asked Questions
What Is a Sextant Meaning and Purpose
A marine sextant measures angular distance. In celestial navigation, the angle between a celestial body and the horizon is called its altitude. Altitude here means an angle above the horizon, rather than a height measured in feet or meters.
The name sextant refers to an arc spanning one-sixth of a circle, or 60°. Its two-mirror arrangement allows that arc to measure angles of approximately 120°. A compass indicates direction, while a sextant measures an angle that can support a position calculation.
How a Sextant Works
A sextant brings two views together: a direct view of the horizon and a reflected view of the celestial body. Moving the index arm changes the reflected image’s position until it reaches the horizon. The graduated scale then indicates the measured angle.
The instrument uses double reflection. Light from the body reflects from the index mirror to the horizon mirror and then toward the observer. Different models use split-field or whole-horizon viewing arrangements, so the appearance through the telescope can vary.
Main Parts of a Sextant
|
Part |
Function |
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Frame |
Supports the mirrors, arc and moving components. |
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Index arm and index mirror |
Move together to change the reflected image’s position. |
|
Horizon mirror |
Combines the reflected celestial image with the horizon view. |
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Telescope or sighting tube |
Provides the viewing path for making contact. |
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Graduated arc |
Shows the measured degrees. |
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Micrometer drum or vernier |
Provides the finer angular reading, depending on the model. |
|
Solar shades |
Reduce sunlight through the appropriate optical paths for solar observations. |
Scale design and reading precision vary by model. A fine scale division describes reading resolution; it does not guarantee the same accuracy in a sight or position fix.
What You Need Before Using a Sextant
Prepare your equipment and observation record before taking a sight:
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A serviceable sextant and suitable manufacturer-approved solar shades.
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A reliable time source checked against UTC, with any known time error recorded.
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A Nautical Almanac covering your observation date.
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Appropriate sight reduction tables or a suitable calculation method.
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A notebook, chart or plotting sheet, plotting tools and an approximate position.
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Your height of eye above sea level and a clear, usable sea horizon.
Sun safety: Use the correct, undamaged solar shades before bringing the Sun into view. Follow your instrument’s manual. Ordinary sunglasses are not a substitute for sextant solar shades.
How to Use a Sextant to Take a Sun Sight
The following sequence describes a lower-limb Sun sight using a visible sea horizon. Moon, star and artificial-horizon observations require different details and corrections.
Step 1 Check index error
With the sextant set near zero, view the sea horizon and adjust the fine control until the direct and reflected horizon images form one continuous line. Read and record the index error. Follow the manufacturer’s instructions for mirror checks and adjustments.
Step 2 Position the solar shades
Select the appropriate shades before observing the Sun. Check that the image is comfortable to view and that the horizon remains visible. Stop if the shades or optical components are damaged.
Step 3 Bring the reflected Sun toward the horizon
Hold the sextant by its handle and use the index arm to bring the reflected Sun into the horizon view. Use the fine control as the image approaches contact. The horizon remains your reference while the reflected image moves.
Step 4 Establish lower limb contact
Adjust until the bottom edge of the reflected Sun just touches the visible sea horizon. Record that you observed the lower limb; the correction procedure depends on which edge you used.
Step 5 Check verticality and time the contact
Gently rock the sextant so the reflected Sun traces a short arc. Judge contact at the lowest point of that arc. Fine-tune as needed, then note the exact UTC time of contact. The Sun continues moving, so avoid delaying the time record.
Step 6 Read and record the sight
Read the instrument after timing the contact. Record the date, UTC, body, limb, raw sextant altitude, index error and height of eye. Record any clock correction separately so you can reconstruct the observation.
Step 7 Repeat and apply corrections
Take several timed sights and check their consistency. Apply the appropriate corrections before sight reduction. A repeatable reading is useful, but it does not eliminate a shared timing or instrument error.
How to Read a Sextant
On a micrometer sextant, read the whole degrees on the arc and the additional minutes on the drum. A vernier model uses a different fine-reading arrangement. Consult your model’s instructions before interpreting subdivisions.
|
Reading component |
Illustrative value |
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Whole degrees from the arc |
35° |
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Additional minutes from the drum |
24.6′ |
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Raw sextant altitude Hs |
35°24.6′ |
There are 60 minutes of arc in one degree. Therefore, 35°24.6′ equals 35.410°, not 35.246°. Angular minutes describe angles and must not be confused with minutes of time.
Hs is the raw sextant altitude. It still needs correction before it becomes the observed altitude used for sight reduction.
From a Sextant Reading to a Position Line
Correct the measured altitude
Apply index correction, dip and the appropriate celestial-body altitude corrections using the Nautical Almanac and your chosen method. For a visible sea horizon, dip is subtracted and depends on height of eye. Use the dip table rather than a fixed correction per meter.
For index error on the arc, subtract the error; for index error off the arc, add it. Distinguish the measured error from the correction, which has the opposite sign.
The body corrections account for effects such as refraction, semidiameter and parallax as applicable. Select the correct body and limb. When using combined correction tables, follow their instructions so an adjustment is not counted twice.
Use time and the almanac
Use the corrected UTC and date to obtain the celestial body’s Greenwich Hour Angle and declination from the almanac, including the required increments and corrections. Those values describe the body’s astronomical position at the observation time.
Calculate and plot a line of position
Sight reduction uses an assumed position to calculate altitude, Hc, and azimuth. Comparing corrected observed altitude, Ho, with Hc gives an intercept. Plot the intercept toward or away from the body along its azimuth, then draw the line of position perpendicular to that direction.
Illustrative intercept example: Suppose Ho is 35°40′ and Hc is 35°30′. The difference is +10′, corresponding to a 10-nautical-mile intercept toward the body’s geographical position. A negative difference would place the intercept away. The numbers illustrate this calculation only; they are not a complete navigational sight.
One sight normally establishes a line of position rather than a unique latitude and longitude. Additional suitably separated position lines help establish a fix. When sights occur at different times, account for vessel movement before combining them.
Common Sextant Mistakes
Confusing angular units. Keep degrees, minutes of arc and decimal degrees clearly labeled. Record time in a separate field.
Reversing index correction. Write down whether the error is on or off the arc before applying its correction.
Using the wrong time. Record UTC and any known clock error. A precise-looking time is useful only if the clock is accurate.
Using the wrong limb. A lower-limb Sun sight uses the bottom edge. Record the limb and select the matching corrections.
Reading before checking verticality. Judge the contact at the lowest point of the apparent arc when rocking the sextant.
Treating a single sight as a fix. Complete sight reduction and combine appropriate position information before accepting a position.
Other Uses of a Sextant
A sextant can also measure horizontal angles between charted landmarks. With suitable chartwork and additional observations, those angles can help establish position near land.
Vertical angles to objects of known height can help estimate distance off when the observation geometry and relevant height or tidal information are known. Each application needs its own calculation method; a measured angle alone does not display distance.
Practicing Celestial Navigation
Begin from stable ground with a clear sea horizon, then compare repeated observations and completed calculations against a known position. Move to observations at sea as your technique improves. Horizon definition, vessel motion, timing and instrument condition all affect results.
If a sea horizon is unavailable, an artificial horizon can support practice. Follow its instructions: a reflecting artificial horizon measures the angle between direct and reflected images, so its reading and correction sequence differs from a normal sea-horizon sight. Do not apply the sea-horizon procedure unchanged.
Celestial navigation provides an independent skill when electronic positioning is unavailable. Its usefulness depends on preparation and practice, and observations may be limited by cloud, haze or an indistinct horizon.
American Nautical Services offers marine sextants, navigation publications and nautical instruments to help you assemble a celestial navigation setup. Choose an instrument and references suited to your training and intended use.
Frequently Asked Questions
What does sextant mean
A sextant is an instrument for measuring angular distance between visible objects. Its name refers to an arc spanning one-sixth of a circle. In marine navigation, it commonly measures a celestial body’s angle above the horizon.
What does a sextant measure
A sextant measures an angle, usually expressed in degrees and minutes of arc. A celestial body’s angle above the horizon is called altitude. The instrument does not directly measure latitude, longitude or distance.
Can a sextant determine longitude
Sextant observations can contribute to determining longitude when combined with accurate time, astronomical data and celestial navigation calculations. The measured angle alone is insufficient to establish longitude.
Can you use a sextant at night
Yes, provided an appropriate celestial body and usable horizon are visible. Twilight is often useful because navigational stars or planets and the sea horizon may be visible together.
Can you practice with a sextant on land
Yes. A shore location with a clear sea horizon allows practice, provided you use the correct height of eye. A purpose-built artificial horizon offers another option, using its own observation and correction procedure.
Does a sextant work without GPS
Yes. A marine sextant measures angles without a GPS signal or electrical power. Calculating position still requires reliable time, astronomical reference data, suitable observations and a sight reduction method.
