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Bettesworth Construction
DIY

How to Build a Simple Sextant for a Classroom Activity

Build a simple sextant-style angle measurer with a protractor, ruler, string, and weight—and learn why it is for education, not navigation.

By Bettesworth Construction Team 2 min read
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You can make a basic sextant-style angle measurer with a protractor, ruler, string, tape, and a small weight. It demonstrates how a sight line and gravity can help estimate an angle, but it is not a precision marine sextant and should not be used to navigate.

Materials for a simple sextant

This build follows the educational activity from Thames & Kosmos. Gather:

  • A clear protractor
  • A straight ruler
  • String
  • Tape
  • A small washer or other small weight
  • A star wheel to help locate Polaris

How to build it

  1. Tie or attach the string at the protractor’s midpoint so it can hang freely over the curved, numbered edge.
  2. Attach the washer or other small weight to the free end of the string.
  3. Hold the protractor with its flat edge level. Check that the string crosses the 90° mark; adjust the attachment if needed.
  4. Tape the protractor to the ruler, aligning the ruler with the protractor’s straight edge. The ruler will act as a sight.

How to use the homemade device

  1. Outside, use a star wheel to locate Polaris.
  2. Point the ruler’s edge toward the star and hold the instrument steady.
  3. Ask a helper to read the angle where the hanging string crosses the protractor scale.

The ruler gives you a repeatable line of sight, while the weighted string hangs vertically under gravity. Together they provide a basic estimate of the target’s angle relative to a level reference. The activity does not report the device’s accuracy or repeatability, nor does it provide a calibration procedure, so treat the result as an educational estimate.

Why this is not a navigation sextant

A traditional marine sextant uses a frame, graduated arc, and mirrors to measure the angle between a celestial body and the horizon. In the double-reflection method, the observer sights the horizon and moves the index arm until the reflected celestial body appears aligned with the horizon image, then reads the angle on the instrument’s scales. That angle, together with the observation time, can contribute to calculating a vessel’s position. See the U.S. Naval History and Heritage Command’s record of an 1871 sextant and Museo Galileo’s explanation of octants and sextants.

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The protractor build has no mirrors or graduated arc like the museum-described instrument. The DIY activity supplies no evidence that its readings are accurate enough for a navigational fix. The marine sextant developed in the late 1750s, and its angle readings helped navigators estimate latitude, according to Royal Museums Greenwich.

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Do not use this build to look at the Sun

The classroom activity does not specify a solar filter or a safe procedure for observing the Sun. Do not sight the Sun through this device. Museo Galileo notes that sextants were often fitted with moving colored filters in the light path to reduce brightness and protect the observer from dazzle; this simple build has no such specified protection.

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