crossstaff

— Yet even with these corrections, Harriot was not satisfied with the crossstaff as currently made. … To take a precise navigational sighting using a crossstaff it is necessary to look two ways at once—at the sun, for instance, and at the horizon. … The National Maritime Museum at Greenwich has 1 crossstaff, 15 backstaffs, and 98 octants; the Peabody Museum at Salem, Massachusetts has 2 crossstaffs, 11 backstaffs, and 82 octants.