has an opening in it to observe the horizon, a shadow vane (B) to cast a shadow on the horizon vane and a sighting vane (C) that the navigator uses to view the horizon and shadow at the horizon vane. This serves to ensure the instrument is level while simultaneously measuring the altitude of the sun. The altitude is the difference in the angular positions of the shadow and sighting vanes. With some versions of this instrument, the sun's declination for each day of the year was marked on the arc. This permitted the navigator to set the shadow vane to the date and the instrument would read the altitude directly. References Ephraim Chambers, Cyclopædia, The First Volume, 1728 explaining the use of a backstaff Maurice Daumas, Scientific Instruments of the Seventeenth and Eighteenth Centuries and Their Makers, Portman Books, London 1989 Gerard L'Estrange Turner, Antique Scientific Instruments, Blandford Press Ltd. 1980 Notes External links "Backstaff" at answers.com – Good diagram of how a backstaff is held in use. Attribution Navigational equipment Measuring instruments Astronomical instruments Celestial navigation Historical scientific instruments