Instrument for constructing and measuring plane angles. The simplest protractor is a semicircular disk marked in degrees from 0° to 180°. A more complex protractor, for plotting position on navigation charts, is called a three-arm protractor, or station pointer, and consists of a circular scale connected to three arms. The centre arm is fixed, while the outer two can be rotated to any desired angle relative to the centre one. A related instrument is the course protractor, which allows navigators to measure the angular distance between north and the course plotted on a navigation chart.

This entry comes from Encyclopædia Britannica Concise.
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