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Serial Number



Radius 220mm, Arc 310mm


Nautical sextant


This nautical sextant is specially designed for navigational and astronomical purposes and allows horizontal and vertical angles to be measured. It is used in marine navigation for measuring vertical angles to stars, occasionally, also for measuring angles between terrestrial targets. The instrument is constructed in a brass pillar frame and is a fairly large instrument with a graduated silver arc of about 220 mm radius. The instrument includes two plane mirrors, both of which are fastened on the frame normal to the graduation surface, and the angle between these mirrors can be varied. The vernier enables angles to be read to 10 seconds and the range of the scale is such as to enable angles up to 120 to be measured. The instrument is also fitted with filters, or dark-coloured glasses, which can be swung into position when needed, and are generally used for solar observations. A clamp attached to the index arm enables the vernier to be clamped in any position to the graduated arc, and a tangential slow-motion screw provides the necessary fine-motion adjustment to bring the images into contact.

The structure of the sextant is based on the theorem that a ray reflected by two mirrors is deflected by twice the angle enclosed by the mirrors.  

History & comments

The sextant is a free hand instrument and so it's accuracy may be limited. It must be noted that the sextant measures angles, and not directions or bearings, and is a more accurate instrument than the compass, but not as accurate as the theodolite. The sextant however, in general, is lighter, more portable, and quicker to use than the theodolite. It can also be used from a moving platform, such as a ship or a small boat, where it is impossible to use a theodolite.


In a case


Good, mild rust on brass surface


  • Signed by Rogerson, Liverpool, England
  • Another Nautical Sextant is available in the collection (see also 0132)
  • Catalogued by T. Ko
  • Updated by F. Pall


Manufactured in 1857 (approx). Catalogued in 1997

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