The term "cross-staff" refers to a navigational instrument for measuring angles in the sky, also known as a Jacob's staff or simply a staff. Instead of the term "cross-staff", the instrument may be referred to as a quadrant, as it measures angles up to 90 degrees. Alternatively, it may be called a sextant, which is a more refined instrument that measures angles up to 120 degrees and is often used in marine navigation. Another synonym for the cross-staff is the backstaff or Davis quadrant, which was developed in the 18th century and allowed sailors to measure the altitude of the sun without looking directly at it.