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. Compass designs An azimuth compass is a magnetic compass where the circumference of the card is divided into 360 degrees. It has two sights, diametrically opposite each other, through which the sun, planet or star is viewed. The sun is typically observed when the sun's center is about one solar diameter above the horizon. Typically the compass will have a stop in the side of the box, which the observer pushes when the sun is lined up in the sights. This fixes the compass card, from which the magnetic azimuth or amplitude can be read. The sights may consist of two vanes, one with a narrow slit and the other with a wider slit bisected by a thread. The observer looks towards the sun, seen through a dark glass, lining up the thread and the narrow slit on the center of the sun. Another arrangement has a vertical bar on one vane and a slit bisected by a thread on the other. The instrument