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An azimuth compass (or azimuthal compass) is a nautical instrument used to measure the magnetic azimuth, the angle of the arc on the horizon between the direction of the sun or some other celestial object and the magnetic north. This can be compared to the true azimuth obtained by astronomical observation to determine the magnetic declination, the amount by which the reading of a ship's compass must be adjusted to obtain an accurate reading. Azimuth compasses were important in the period before development of the reliable chronometers needed to determine a vessel's exact position from astronomical observations. ==Theory== In navigation, the true azimuth of a heavenly body is the arc of the horizon between the point where a vertical plane containing the observer and the heavenly body intersects the horizon and the direction of true north. The magnetic azimuth is the arc between the point on the horizon below the heavenly body and the direction of magnetic north. When the latitude and date are known, the bearing of the sun at sunrise or sunset relative to true north can be readily determined. If the sun is observed at some time between sunrise and sunset, its altitude must also be recorded to calculate the true azimuth. The true azimuth may be compared to the magnetic azimuth to find the magnetic declination, the angle between the direction that the compass indicates as north and the true north direction. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Azimuth compass」の詳細全文を読む スポンサード リンク
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