The backstaff is a navigational instrument that was used to measure the altitude of a celestial body, in particular the Sun or Moon. When observing the Sun, users kept the Sun to their back and observed the shadow cast by the upper vane on a horizon vane.

What does back staff mean?

: an instrument similar to a cross-staff but fitted with a reflector and formerly used for taking the altitudes of heavenly bodies.

Is the backstaff still used today?

The backstaff continued to be used by mariners for over 150 years. By the mid eighteenth century, however, it became replaced by more reliable instruments such as the octant and sextant. Backstaffs find altitude by measuring the angle between the horizon and the sun.