What is the difference between an octant and a sextant?

An octant spans an arc of 45 degrees (one eighth of a circle), allowing it to measure angles up to 90 degrees. The sextant, developed around 1757, covers a 60-degree arc and measures up to 120 degrees, offering greater versatility. Octants are generally earlier and rarer than sextants.

How can I date an antique octant?

Key dating clues include the material of the graduated arc (ivory before circa 1850, silvered brass or aluminium later), the type of vernier, the shape of the frame, and the maker's signature. Catalogues and trade records for makers such as Dollond or Heath and Co help narrow down the period of manufacture.

Which octant makers are most sought after by collectors?

The most prized names include Heath and Co, Spencer Browning and Co, and Dollond in London, along with the French instrument makers Canivet and Lenoir in Paris. A signed example in good condition, complete with its original case, commands prices well above the market average.

What should I expect to pay for an antique octant?

A standard maker in good condition typically sells for £250 to £700. A signed example by a prestigious workshop, complete with its original case and accessories, can exceed £2,000 to £2,500. Rare 18th-century French pieces are particularly scarce and tend to fetch the highest prices.

How should I care for an antique octant without causing damage?

Avoid abrasive products on the brass and ebony. A gentle wipe with a soft, dry cloth is sufficient for routine cleaning. Never attempt to dismantle the mirrors yourself. Store the instrument in its case, away from humidity and temperature fluctuations, to preserve the stability of both the woodwork and the metalwork.

Antique Octants — Historic Marine Navigation Instruments

Authenticating an antique octant begins with a close look at the frame: the finest instruments are built from solid brass with a genuine ebony stock, occasionally mahogany. The graduated arc — ivory on pre-1850 examples, silvered brass or aluminium on later pieces — should show clean, evenly spaced divisions. The presence of a precision vernier and a set of coloured shade glasses is a reliable sign of a professional-grade instrument. Always look for the maker's signature engraved on the limb or the index arm plate: names such as Heath & Co, Spencer Browning & Co, and Dollond in London, or Canivet and Lenoir in Paris, are marks of quality that significantly enhance value.

Condition is paramount. Check the mirrors for any lifting of the silvering, confirm the index arm moves freely, verify that the graduations remain legible, and — crucially — establish whether the original case survives. An octant complete with its original mahogany case, fitted with its accessories (telescope, shades, adjusting key), can be worth two to three times more than an unboxed example. Be wary of heavy-handed restoration that masks natural wear: a consistent, coherent patina is always reassuring.

In terms of budget, expect to pay between £250 and £700 for a standard maker in good condition, and £900 to £2,500 or more for a signed instrument by a prestigious workshop, complete with case and original accessories. Rare 18th-century French examples regularly command higher prices at auction. To broaden your collection, explore antique sextants and the wider range of antique scientific instruments available on the platform.

Every octant listed on Antiquités en France is offered by a verified professional dealer who can provide detailed photographs, provenance information and a certificate where available. Contact the seller directly with any questions about condition, dimensions or the history of a piece before committing to a purchase.

Antique Marine Octants

Antique octants and astronomical navigation instruments from specialist dealers

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How to choose and authenticate an antique nautical octant

Frequently asked questions about Antique Marine Octants