Navigational instruments are instruments used by nautical navigators and pilots as tools of their trade. The purpose of navigation is to ascertain the present position and to determine the speed, direction, etc. to arrive at the port or point of destination. Charts and drafting instruments Charts are maps of the areas to be navigated with details specific to the marine environment. Computing aids: used in the necessary mathematical calculations. Today electronic computers or calculators are used. Other traditional aids used included tables (trigonometric, logarithms, etc.) and slide rules. Dividers used for measuring lengths of lines and approximate lengths of non-linear paths on a chart. Nautical almanac used to determine the position in the sky of a celestial body after a sight has been taken. Parallel rules used for transferring a line to a parallel position. Also used to compare the orientation of a line to a magnetic or geographic orientation on a compass rose. Direct measuring Chip log and sand glass serve to measure the ship's speed through the water. Sounding line used to measure the depth of the water and to pick up samples from the bottom. Drift meter optically measures the effects of wind on an aircraft in flight. Position finding instruments Celestial navigation instruments These instruments are used primarily to measure the elevation or altitude of a celestial object: Back staff, the best known of which is the Davis' quadrant. It could measure the altitude of the sun without having the navigator directly observe the sun. Cross staff, an older instrument long out of use. Kamal Very simple instrument used primarily by Arabian navigators. It consists of a small board with a knotted piece of twine through the center. The observer holds one of the knots in his mouth and extends the board away so that the edges make a constant angle with his eyes. Mariner's astrolabe Derived from the astrolabe, it was developed in late 15th century and found use in the 16th to 17th centuries. It was replaced by the back staff and later by the octant and sextant. Quadrant A very simple instrument which used a plumb bob. These instruments are also used to measure the angular distance between objects: Octant, invented in 1731. The first widely accepted instrument that could measure an angle without being strongly affected by movement. Sextant, derived from the octant in 1757, eventually made all previous instruments used for