Accession No


Brief Description

backstaff with Flamsteed arc, made for Thomas Halcott, by E. Blow, English, 1736


England; London; Wapping; Plow Alley


Blow, E.



Earliest Date

Jan. 1, 1736

Latest Date

Dec. 31, 1736

Inscription Date



wood (rosewood, boxwood); glass; metal (brass); ivory


Special Collection


Purchased from Sotherby's, London, England; lot 99, 29/4/1977. Purchased with assistance of Preservation Fund, administered by Science Museum.


in Plow Alley at Vnion Staires
Wapping. May ye 10. 1736’ (brace)
‘for Mr Thomas Halcott. 1737’ (ivory plate)

Description Notes

Rosewood limbs, boxwood arcs and braces, boxwood vanes. Horizon vane. 2 shadow vanes, one with glass (cracked), sight vane with pinhole sight backed by inset brass plate. 60˚ arc divided 0 - 60˚ by 10˚, subdivided to 1˚. 61 1/2˚ on arc, 3 1/2˚ on limb. Lesser arc divided 0 - 25˚ by 1˚ subdivided to 5´, and by transversals to 1´; also numbered 65˚ - 90˚ by 5˚ for zenith distance. Solar declination table on reverse of lesser arc.

Condition good (Flamsteed glass cracked); complete.


Joshua Nall; ‘Astronomy at Sea’; Explore Whipple Collections online article; Whipple Museum of the History of Science; University of Cambridge; 2020:


The backstaff is a navigational instrument used to find latitude. It was introduced during the 17th Century as a replacement for the crosstaff. With a crosstaff, the ship’s navigator would have to look directly into the sun which was often to bright. To use the backstaff the navigator was able to stand with their back to the sun and use the shadows cast on the instrument to take readings.

To use the backstaff the backsight and the horizon slit are lined up. The shadow vane on the small arc is moved until its shadow falls onto the horizon slit. The combined angle of the backsight and the shadow vane gives the angle of the sun and therefore the latitude.

The backstaff has a standard pattern of 2 main “limbs”, in ebony rosewood, pear wood or mahogany with the arcs and occasionally the braces in boxwood.

The backstaff was replaced by the octant which began to be made commercially from the mid-18th Century. Unlike the backstaff, the octant was not affected by the movement of the ship when the navigator attempted to take readings.

Both the octant and backstaff continued to be produced and used into the 19th Century despite competition from the more accurate and versatile sextant. This may be because these instruments were cheaper and more affordable than the sextant.


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