How to Use a Sextant

A sextant measures the angle between a celestial body, such as the sun, moon, a planet or a star, and the horizon. With that angle, the exact time and a nautical almanac, you can find your position anywhere on earth without a single battery. Few sailors need it day to day, but it's the ultimate backup to GPS, and a satisfying skill to learn.

Never look at the sun through a sextant without its shade filters in place. The telescope concentrates sunlight and can permanently damage your eyes in an instant. Start every sun sight with all the shades down, and remove them one at a time.

The Parts of a Sextant

• Frame and arc: the triangular frame, with a curved arc graduated in degrees along the bottom.
• Index arm: the arm that swings across the arc. Its position gives the angle.
• Micrometer drum: the knob on the index arm that fine-tunes the angle and reads the minutes.
• Index mirror: the mirror on top of the index arm, which moves with it.
• Horizon mirror: the fixed mirror you look through. On a split mirror, one half shows the horizon directly and the other shows the reflected body; a full-view mirror shows both across the whole glass.
• Telescope: magnifies the view so you can bring the body exactly onto the horizon.
• Shades (filters): coloured glasses in front of each mirror to cut the sun's glare.

Check the Index Error First

Every sextant reads slightly off zero. Measure it before each session:

1. Set the sextant to 0° 0'.
2. Look at a sharp, distant horizon. You'll see two horizon images, one direct and one reflected.
3. Turn the micrometer until the two line up as one straight line, and read the angle. That's your index error.
4. If the reading is above zero ("on the arc"), subtract it from your sights; if it's below zero ("off the arc"), add it. Sailors remember: "If it's on, take it off; if it's off, put it on."

Which Body to Use

• The sun is the most used. It's up all day, bright and easy to find. The classic routine is a morning sight, a noon sight for latitude and an afternoon sight; advancing the earlier lines to the same time gives your position over the day.
• Stars give the best fixes. Sight them in twilight, before sunrise or after sunset, when the brighter stars are out and the horizon is still sharp. Three or more stars spread around the sky, sighted within a few minutes, give three lines that cross at once: a fix from one session.
• Planets (Venus, Mars, Jupiter and Saturn) work the same way as stars. Venus and Jupiter are bright enough to find early, and Venus can sometimes be seen in daylight.
• The moon is often visible in daytime along with the sun, so the two together can give a daytime fix. It needs extra corrections because it's so close to earth, and you need a sharp horizon below it.
• Polaris, the North Star, gives your latitude in the Northern Hemisphere. It can't be seen from the Southern Hemisphere.

You don't need a special chart for each body. One book, the Nautical Almanac for the current year, gives the position of the sun, moon, the four navigational planets and 57 selected stars plus Polaris for every second of the year. Add sight reduction tables (Pub. 229 works for any body; Pub. 249 has a volume for selected stars and two for the sun, moon and planets) or a navigation calculator, a plotting sheet or your normal chart, and an accurate watch. A star finder such as the 2102-D helps you identify the stars and plan which ones to sight.

Take a Sun Sight

1. Put all the shades in place.
2. Bring the sun down to the horizon. With the sextant at zero, look at the sun through the shades, then swing the index arm forward while tilting the sextant down until the sun's image sits near the horizon.
3. Fine-tune with the micrometer.
4. Rock the sextant gently side to side, like a pendulum. The sun swings in an arc; adjust until the bottom edge of the sun (its lower limb) just touches the horizon at the lowest point of the swing. That makes sure you're measuring straight down to the horizon.
5. Note the exact time to the second, in Coordinated Universal Time (UTC). Four seconds of error can put your position off by up to a nautical mile.
6. Read the angle: degrees from the arc, minutes from the micrometer drum. Write down the time, the angle and the limb you used.

From Angle to Position

1. Correct the sextant angle. Apply your index error, then the dip correction for your height of eye above the water, then the altitude correction from the Nautical Almanac (for refraction and the sun's semi-diameter). The result is the observed altitude.
2. Look up the sun's position for the exact time of your sight in the Nautical Almanac: its Greenwich Hour Angle and declination.
3. Work the sight from an assumed position near where you think you are, using sight reduction tables (such as Pub. 229 or 249) or a navigation calculator. That gives a calculated altitude and the sun's bearing (azimuth).
4. Find the intercept: the difference between the observed and calculated altitudes, where each minute of arc is one nautical mile. If the observed altitude is greater, move toward the sun ("Ho Mo To": Ho more, toward).
5. Plot a line of position on the chart at right angles to the sun's bearing. You're somewhere on that line. Cross it with a second sight hours later (advanced for the distance you've run), or with sights of two or three stars at twilight, for a fix.

Want to check your working? Try the Sight Reduction Calculator below.

Two Simpler Sights

• Noon sight for latitude. At local noon the sun reaches its highest point. Follow it up and record the highest altitude. Subtract the corrected altitude from 90° to get the zenith distance, then add the sun's declination if it's on the same side of the equator as you, or subtract it if it's on the other side. That's your latitude, and no exact time is needed. (This simple rule works when the sun is between you and the equator, as it always is in Canadian waters.)
• Polaris for latitude. In the Northern Hemisphere, the corrected altitude of Polaris, the North Star, is very close to your latitude. The Nautical Almanac's Polaris tables give the small correction.

Sight Reduction Calculator

Taken a sight and looked up the body in the Nautical Almanac for the current year? This calculator does the rest. It applies the index error, dip and altitude corrections to your sextant angle, then does the job of the sight reduction tables: it finds the calculated altitude, the azimuth, the intercept and a point to plot your line of position through. It can also work a noon sight for latitude.

Use it to learn and to check sights you've worked by hand. It needs power, so it's no substitute for the printed Nautical Almanac and sight reduction tables.

Using a Sextant on Landmarks: Coastal Navigation

Near the coast, a sextant is one of the most accurate position-finding tools there is, and you don't need the sky at all. Because it measures the angle between two objects directly, compass variation and deviation don't matter, and a careful reading is good to a fraction of a degree.

Horizontal angles for a fix

1. Pick three charted objects you can identify, such as a lighthouse, a church spire, a radio tower or the edge of a headland, spread across your view.
2. Hold the sextant with its frame horizontal. Look directly at one object through the horizon mirror, then move the index arm until the reflected image of the next object lines up exactly with it. If the reflection won't come in from that side, turn the sextant over and start from the other object.
3. Read the angle, then measure the angle between the middle object and the third one the same way. You now have two angles that share the middle object.
4. Plot the fix with a station pointer, a three-armed protractor. Set its outer arms to your two angles, lay it on the chart, and slide it until each arm runs through its object. The centre is your position. Without one, draw the angles on tracing paper and fit them the same way.

Choose your objects well. Angles between about 30 and 120 degrees give the best fix. Avoid sets where you and the three objects lie on or near the same circle, for example a curving shoreline with you in the middle of the curve; the fix becomes indeterminate. A middle object closer to you than the outer two is a safe choice.

The danger angle

To pass a hidden hazard such as a reef between two charted landmarks, draw a circle on the chart that passes through both landmarks and keeps the hazard inside it, then measure the angle the landmarks make from any point on that circle. That's your danger angle. As you sail past, keep measuring: an angle smaller than the danger angle means you're outside the circle and clear of the hazard; a larger one means you're inside it, so turn away.

Vertical angles for distance off

Hold the sextant upright and measure the angle from the top of a lighthouse or other charted object down to the waterline below it. Then:

• Distance (nautical miles) ≈ 1.856 × height in metres ÷ angle in minutes of arc
• or, with heights in feet, 0.565 × height in feet ÷ angle in minutes.

Use the height printed on the chart; lighthouse heights are usually given above high water, so allow for the tide. A distance off a charted light, combined with a compass bearing of it, gives you a quick fix.

What You Need

• A sextant, kept in its case and handled by the frame, never by the arc or mirrors
• An accurate watch set to UTC, checked against a time signal
• The Nautical Almanac for the current year. The official edition is sold, not free, but there are free options for practice: the U.S. Naval Observatory's Air Almanac is a free download and accurate enough for small-boat navigation, and its celestial navigation calculator gives each body's position, computed altitude and bearing for any time and assumed position. The calculator needs power, so carry a printed almanac too.
• Sight reduction tables or a navigation calculator, plus spare batteries if it's electronic
• Plotting sheets or charts, a pencil, dividers and a plotter
• A notebook to record every sight

How to Learn

• Practise on land with an artificial horizon, such as a still dish of water. The angle you measure is twice the body's altitude, so halve it.
• Take a course. Boating education groups such as CPS-ECP in Canada and America's Boating Club in the U.S. teach celestial navigation.
• Practise at sea in good weather, comparing each fix with your GPS until you trust your results.

See also: Marine Navigation & Compass, Top 10 Sailing Safety Tips and How to Use a Compass.