azimuth compass

azimuth compass, a compass designed to observe the value of the local magnetic variation. Early navigators had no knowledge of the phenomenon, and it was not until the 15th century that it became clear that the compass needle did not point true north. It is sometimes claimed that it was Columbus on his first voyage who discovered the existence of variation. However, the first recorded description of an azimuth compass occurs in John of Lisbon's Livro de marinharia, published in 1514, and for the next two centuries various types of compass were constructed to observe the variation. Most were completely separate from the steering compass. The function of the azimuth compass was simply to get a bearing of the sun, moon, or a star that could be compared with the calculated bearing of the body, the difference between the two being the variation. A drawback of the azimuth compass was that it required two operators, one to take the bearing, the other to read the compass scale. Nowadays variation and its annual rate of change are recorded on every chart. Azimuth rings are still used when swinging a ship to find the deviation, or to take bearings for coastal navigation.