Jacob's staff
CALIBRATED WOODEN ROD WITH SLIDING CROSSPIECE USED FOR MEASURING THE ALTITUDE OF CELESTIAL BODIES OR THE HEIGHT OF DISTANT OBJECTS
Jacob Staff; Jacob's Staff; Cross-staff; Cross staff; Cross head (surveying); Ballastella; Radius astronomicus
The term Jacob's staff is used to refer to several things, also known as cross-staff, a ballastella, a fore-staff, a ballestilla, or a balestilha. In its most basic form, a Jacob's staff is a stick or pole with length markings; most staffs are much more complicated than that, and usually contain a number of measurement and stabilization features.