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Building a Mariner's Sextant — The Instrument That Conquered the Oceans
The sextant is the instrument that made global navigation possible. Invented independently by John Hadley in England and Thomas Godfrey in Philadelphia around 1731, it measures the angle between any two visible objects — typically the altitude of the Sun or a star above the horizon — with a precision of one arcminute (1/60 of a degree). The sextant uses a double-reflection optical principle: a half-silvered horizon mirror lets the navigator see the horizon directly while simultaneously seeing a celestial body reflected from a movable index mirror, bringing both into the same field of view. When the two images are aligned, the angle is read from a graduated arc. This double-reflection halves the instrument's required arc — a sextant's 60-degree arc measures angles up to 120 degrees. Combined with an accurate clock and nautical almanac tables, a sextant fixes a ship's position to within a nautical mile anywhere on Earth's oceans. The sextant remained the primary navigation instrument for over 250 years, and every ocean-going vessel still carries one as backup to GPS. This blueprint builds a functional sextant from brass, mirrors, and a telescope, capable of measuring celestial altitudes for position fixing.
Advanced
20-30 hours
Instructions
1
1
Understand the double-reflection principle
Understand the double-reflection principle
The sextant uses two mirrors to bring a celestial body and the horizon into the same field of view. The index mirror is mounted on a movable arm (the index arm) and rotates with it. Light from a star hits the index mirror, reflects to the horizon mirror (which is half-silvered or split), and enters the telescope. The navigator sees the horizon through the clear half and the star reflected in the silvered half. When the star image is brought down to sit exactly on the horizon, the angle on the graduated arc equals the star's altitude. By the laws of reflection, the measured angle is exactly twice the angle turned by the index mirror.
2
2
Cut the frame (arc sector)
Cut the frame (arc sector)
Cut the sextant frame from a piece of thick brass sheet (2-3 mm). The frame is a sector of a circle — traditionally one-sixth of a full circle (60 degrees), though the double-reflection means it measures angles up to 120 degrees. Make the radius about 15-20 cm. The frame needs a curved arc at the bottom edge, two radial arms meeting at the pivot point (where the index arm pivots), and enough rigidity not to flex when handled. Use a jeweller's saw to cut the shape and file all edges smooth.
Materials for this step:
Brass Sheet1 pieceTools needed:
Jeweler's Saw
Metal File3
3
Graduate the arc
Graduate the arc
The curved arc at the bottom of the frame carries the degree scale. Because double-reflection halves the angle, each degree of arc on the sextant corresponds to two degrees of measured angle — so a 60-degree arc reads from 0 to 120 degrees. Using dividers and a diamond scriber, mark degree divisions along the arc. Subdivide each degree into thirds (20 arcminutes each) if space permits. Engrave numbers every 10 degrees. The accuracy of your sextant depends entirely on the precision of this graduation — take extreme care.
Tools needed:
Dividers
Diamond Scriber4
4
Make the index arm
Make the index arm
Cut a brass strip about 1.5 cm wide and as long as the radius of the arc. This is the index arm — it pivots at the centre of the arc and swings across the graduated scale. At one end, drill a pivot hole that aligns exactly with the centre of the arc. At the other end, cut a pointer or vernier scale that reads against the arc graduation. The arm must be perfectly straight and pivot freely without wobble. A small friction clamp or set screw allows the arm to be locked at any angle.
Materials for this step:
Brass Strip1 meterTools needed:
Jeweler's SawMetal File5
5
Mount the index mirror
Mount the index mirror
Mount a small flat mirror (about 3-4 cm across) at the pivot end of the index arm, perpendicular to the plane of the frame. This is the index mirror — it rotates with the index arm. The mirror must be a first-surface mirror (reflective coating on the front face, not behind glass, which would cause double reflections). Mount it with small brass clips so it stands exactly perpendicular to the sextant frame. Any tilt produces systematic error in all measurements.
Materials for this step:
Flat Mirror1 piece6
6
Mount the horizon mirror
Mount the horizon mirror
Mount a second flat mirror on the frame, about halfway along the left radial arm, perpendicular to the frame and facing the index mirror. The horizon mirror is split or half-silvered: the half nearest the frame is silvered (reflects light from the index mirror) and the other half is clear glass (the navigator looks through it to see the horizon directly). To make a split horizon mirror, carefully scrape the silvering off one half of a small mirror, or mount a half-mirror beside a clear glass piece.
Materials for this step:
Flat Mirror1 piece7
7
Add the sighting telescope
Add the sighting telescope
Mount a small telescope or sighting tube on the frame, pointing at the horizon mirror. The telescope magnifies the view, making it easier to bring the star image precisely to the horizon. A simple tube with a convex lens at the front and an eyepiece lens at the back, giving about 3-4x magnification, is sufficient. Align the telescope so that when looking through it, the horizon is visible through the clear half of the horizon mirror and reflected images from the index mirror are visible in the silvered half.
Materials for this step:
Convex Lens1 piece8
8
Add shade filters
Add shade filters
For solar observations, the sextant needs dark shade filters to protect the eyes. Mount two sets of swing-out filters: one set between the index mirror and the horizon mirror (shades the reflected Sun image) and one set between the horizon mirror and the telescope (shades the direct horizon glare). Use pieces of dark-tinted glass or multiple layers of exposed photographic film in small brass frames that swing into and out of the light path. Never observe the Sun without filters — the sextant telescope concentrates enough light to cause eye damage.
9
9
Collimate and adjust the mirrors
Collimate and adjust the mirrors
Set the index arm to exactly 0 degrees on the arc. Look through the telescope at a distant object (a star or a sharply defined landmark). You should see two images: one direct (through the clear half of the horizon mirror) and one reflected (via the index mirror to the silvered half). If the two images are not perfectly aligned when the arm is at zero, the mirrors are misaligned. Adjust the horizon mirror's tilt screws until the direct and reflected images overlap exactly at zero. This is the index error adjustment — without it, every reading will be off by a constant amount.
10
10
Measure the altitude of a star
Measure the altitude of a star
Hold the sextant vertically in your right hand with the arc curving downward. Look through the telescope at the horizon. Swing the index arm until the reflected image of a bright star appears in the silvered half of the horizon mirror. Slowly lower the star image until it sits exactly on the horizon line. Lock the index arm and read the angle from the graduated arc — this is the star's altitude above the horizon. Rock the sextant slightly side to side during the observation to ensure the star is at its lowest point in the arc (confirming the sextant is truly vertical).
11
11
Take a noon Sun sight for latitude
Take a noon Sun sight for latitude
The simplest celestial navigation fix is the noon sight. Near local noon, take repeated altitude measurements of the Sun (through the shade filters) as it climbs to its highest point. The maximum altitude occurs exactly at local apparent noon. Your latitude equals 90 degrees minus the noon altitude plus the Sun's declination for the date (found in a nautical almanac). This single observation gives your latitude to within a few nautical miles — the same technique used by every navigator from the 18th century to the GPS era.
12
12
Understand the complete celestial fix
Understand the complete celestial fix
To find both latitude and longitude, a navigator takes altitude sights of two or more celestial bodies at known times (from an accurate chronometer). Each sight produces a circle of position on a chart — the set of all points on Earth where that body would appear at that altitude at that time. Two circles from two different bodies intersect at the navigator's position. This is the method of celestial navigation perfected in the 18th century and used by Cook, Nelson, Shackleton, and every ocean-going vessel for over 250 years. Your sextant is the same fundamental instrument they carried.
Materials
4- 1 piecePlaceholder
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