The globe is divided into navigational lines that run both horizontally and vertically. The vertical lines, which present the "longitude," were difficult to calculate when traveling and posed a serious problem for early seafarers. By contrast, the horizontal lines, or "latitudes," were understood relatively early on in the history of navigation, and so a number of different instruments have been used to help people determine their latitude.
Cross-staff and Backstaff
One of the earliest instruments used to determine latitude was the cross-staff, or Jacob's staff. It consisted of a long stick with a movable crossbar. The navigator placed the stick against his cheek, moving the crossbar until it touched both the sun, or a reference star, and the horizon. By calculating the elevation of the celestial body at its highest point --- noon, for the sun --- the navigator could then find his latitude. Because this required staring directly at the sun, an early revision of the cross-staff, known as the "backstaff," allowed the user to look away from the sun, taking a reading from its shadow instead.
A more sophisticated instrument than the bulky, unwieldy cross-staff was the astrolabe, which was designed to make measuring a simpler process. An astrolabe is a circle, weighted at the bottom so it always remains level relative to the horizon, with angle markings placed at regular intervals around it and a bar that could be rotated around the diameter. By aligning the bar with the position of a celestial body, its elevation could be determined, with the subsequent latitude calculation being the same as when using a cross-staff.
A further evolution of the techniques used in the astrolabe and cross-staff, the sextant is one of the quintessential instruments of manual navigation. In its basic form, it is an arc consisting of one-sixth of a circle, hence the name "sextant." A system is mirrors is used to simultaneously project an image of the horizon and the celestial body being used as a reference point; when the mirrors are adjusted so the image of the horizon and the star appear to cross, the sextant displays an accurate angle of elevation. Because of its high precision and ability to measure angles between any two objects, it could be used to calculate longitude as well as latitude.
Today, electronic equipment can determine latitude without the need for sighting a remote object, which could be difficult in rough seas or bad weather. Radio direction finding, for example, uses fixed transmitters to broadcast a signal so ships and aircraft can determine their location relative to the transmitter whose latitude they already know. The Global Positioning System, or GPS, uses satellites to determine location even more accurately; a navigator with a GPS device can determine her latitude simply by glancing at its screen.
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