Brief description

Optical navigation instrument used to measure the angle between two visible objects. Its primary use is measuring the angle between a celestial body and the horizon to determine latitude.

Use / Function

• Navigation: Determine latitude by measuring the height of the sun or a star above the horizon.
• Surveying: Measure horizontal angles between landmarks.

Operating principle

Uses the principle of double reflection.

1. A movable mirror (index) reflects the image of the celestial object to a fixed mirror (horizon).
2. The horizon mirror is semi-transparent: it allows viewing the horizon directly through it and seeing the reflection of the star superimposed.
3. By aligning both images, the angle of the index mirror indicates the height of the star.

How to create it

1. Frame: A circular sector of 60 degrees (a sixth of a circle, hence the name).
2. Index Arm: A movable arm with a mirror at the pivot.
3. Mirrors: Two small flat mirrors.
4. Scale: A precisely graduated arc.
5. Sight: A small tube or hole to align the view.

Materials needed

• Essential: Brass or bronze (stable and resistant to marine corrosion), glass for mirrors.
• Tools: Precision engraving tools for the scale.

Variants and improvements

• Octant: Earlier version with a 45-degree arc (eighth of a circle).
• Vernier: Auxiliary scale to increase reading precision.

Limits and risks

• Visibility: Requires seeing both the star and the horizon (difficult on cloudy days or moonless nights).
• Instrument Error: Must be adjusted and verified regularly.

Related Inventions

• Chronometer
• Compass
• Lens
• Mirror