compassThe sun is slipping behind the wave tops
North
Magnetic north still differs from geographic north.
East
Compass points divide the circle into usable steering commands.
South
A stable reading can still be affected by the ship’s motion.
West
Heading must be recorded together with speed and time.
Double reflection brings two separate directions into one view
A navigator needs the angle between a celestial body and the horizon, yet the eye cannot look accurately in both directions at once. In a sextant, light from the body reflects from the index mirror to the horizon glass and then toward the telescope, while part of the horizon remains visible directly.
Moving the index arm rotates the first mirror until the reflected body meets the horizon in the same field of view. The mirror turns through half the angle measured between those directions, so the engraved scale expands the reading through the geometry of double reflection.
Mirrors, shades and a moving deck make the observation a practiced act
The horizon glass must show both the direct horizon and the reflected object. Coloured shades reduce glare when observing the sun, and the telescope helps align the two images. These parts make the sight possible; they do not make it automatic.
On a rolling deck, the horizon rises and falls through the field. The observer rocks the image gently across it and judges the lowest point of the apparent arc. A blurred horizon, poor adjustment or a hurried reading can place error into the angle before any calculation begins.
The number on the arc is a beginning, not yet a position
The raw sextant angle requires corrections for the instrument and for the geometry of observing from above sea level. Refraction and the apparent size of the sun may also matter. Accurate time and astronomical tables connect the corrected sight to the celestial body’s expected position.
One observation may support latitude or contribute to a line of position. Longitude normally demands accurate time or another way to recover it. The sextant is powerful because it makes an angle portable and repeatable—not because it turns one glance into a complete fix.
Questions
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Why is it called a sextant?
Early examples commonly used an arc of about one-sixth of a circle. Double reflection allowed that arc to measure approximately twice its physical angle.
Can a sextant observe only the sun?
No. Navigators can observe stars, planets and the moon, and the instrument can also measure horizontal angles between terrestrial objects.
Can a sextant be used without a marine chronometer?
Some latitude observations can. Reliable longitude usually requires precise reference time or another demanding astronomical method.
Sources
Continue the research
- SextantRoyal Museums Greenwich
- All About Nautical Charts: Early chart-makingNOAA Ocean Service