Scholars and navigators alike understood that if the positions of celestial bodies could be accurately measured at sea, sailors could determine their latitude with reasonable certainty. Achieving this required precision instruments.
Ptolemy described the astrolabe (also known as a star-measuring instrument); larger versions were used in observatories, while smaller ones were employed on ships. Operating an astrolabe required a team of three: one person held the instrument by its thumb ring, another took the sighting, and a third read the result from the dial. Naturally, when the ship rocked violently, the resulting measurements were not very accurate. Whenever possible, sailors would go ashore to take their readings.
Ancient astronomers used the cross-staff to measure the altitude of stars, a technique later adopted by sailors for navigation. The instrument consisted of a main staff and a cross-piece, with the lower end of the cross-piece held horizontally. While sighting a celestial body along the main staff, the user would slide the cross-piece until it appeared to touch the object (such as the sun or a star) within their field of view, then read the degree measurement from the staff. This instrument could be operated by a single person.
Both the astrolabe and the cross-staff required the observer to look directly at the sun. On clear days, the intense light made observation impossible. To solve this problem, the English captain and navigator John Davis invented the backstaff. It consisted of a main staff and a sliding cross-vane. The observer stood with their back to the sun and slid the cross-vane until it cast a shadow onto a small vane ahead; this method allowed the observer to sight the horizon.
With the assistance of Edward Wright, a mathematician from Cambridge, John Davis also invented the quadrant. This instrument featured an eyepiece on its cross-vane, allowing the observer to view both the horizon and the reflected image of the sun.
Pierre Bouguer, a hydrographer from Croisic, further improved the quadrant; the modified version enabled the observer to see the sun resting on the horizon through the eyepiece.
John Hadley of England invented the octant, which was first tested in 1732. It consists of a reflecting telescope and a spirit level. This instrument is far more precise than any other instrument previously used at sea.