Around 1590, English seaman and explorer John Davis wrought a revolution in celestial navigation by inventing the backstaff.

This new tool was so simple and so practical that it remained in use for more than 200 years and sailors were so grateful that the Davis Strait, between Baffin Island and Greenland, was named in John Davis' honor. 

The backstaff itself was actually an improvement on the design of the cross-staff.  The backstaff improved on it's predecessor by eliminated the frequent parallax errors (mistakes in calculation resulting from taking sightings from objects in irregular orbital patterns). In addition, there was no glare to contend with because the user stood facing away from the sun (hence the name).

The backstaff consisted of three vanes (a sight vane, a shadow vane, and a horizon vane) and a pair of wooden arcs attached to a staff.  Davis’s invention was as significant an advance in his time as GPS has been in ours: the back-staff made it possible to read the altitude of the sun and moon in minutes rather than degrees so, for the first time, navigators could consistently find their latitude to within a few miles.

The backstaff was later fitted with mirrors so it could be used for star and planet sights as well.