Instructions for a Pocket Sextant

Written by hannah ardeb | 13/05/2017
Instructions for a Pocket Sextant
Sextants were renowned for their accuracy before GPS. (sextant image by serge simo from

The sextant is one of the instruments used on a ship in order to navigate. When charting out a ship's course, the navigator first looks at the stars by using an almanac to predict their approximate positions. The cartographer provides latitudes and longitudes so that the navigator has physical reference points from earth. Lastly, the sextant, a device that measures angles, is used to compare the positions of the stars to a reference point on earth.

Move the arm of the pocket sextant so that the light from whatever source is available (the sun or the moon) bounces off one mirror to a second mirror. There are two mirrors on a sextant. One is at the top of the sextant's arm, and the second is diagonal to it on the lower left-hand side of the sextant.

Twiddle the micrometer knob on the sextant to correct the sextant. When you move the arm so that the light bounces off the upper mirror on the arm onto the lower mirror, the light source appears on the land's horizon on the mirror. If you need minor adjustments to achieve this, use the micrometer knob.

Mark the angle of the sextant. The arm moves across an arc at the bottom of 60 degrees (hence the name of the sextant), and angles should be engraved on the arc. Also, mark the seconds, hours, and minutes that you successfully superimposed the light source on the horizon.

Things you need

  • Sextant
  • Light source

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