It is a valuable and basic navigational instrument that is fully combined with the vessel’s safety.
It consists of the magnetic needle that is freely hanged on a horizontal plane. When at land, it is affected by the earth’s magnetism and is oriented towards the direction of the magnetic North.
In iron vessels, the magnetic needle of the compass shows the direction of the compass North, because of the combined action of the earth’s and the vessel’s magnetism.
Therefore, the compasses’ indications, in order to be true, must be taken under consideration in proportion with the variation, namely the magnetic declination, and the deviation.
Nowadays, except from compasses, the majority of the vessels is also equipped with gyrocompasses. The principle of the gyrocompass’s function is based on the gyroscope’s valuable property to retain its axis towards the direction of the true north of a place. Therefore, there is no deviation or variation in this type of compass and the courses and bearings indicated by it are true. It operates with the vessel’s electricity and, from the moment it starts running, it needs two to four hours to equilibrate towards the true north. In vessels there is only one gyrocompass. For the proper steering of the ship and the appropriate reception of the bearings there are electronic repeaters in applicable places of the ship that electromagnetically monitor the Master Gyrocompass and show the direction of the True North.
The measurement bearings and the azimuth, requires the azimuth circles that are an indispensable supplement of gyrocompasses and magnetic compasses. There are regular, azimuth or automated azimuth circles.
It is a kind of goniometric instrument, with which, when at sea, we can measure from the horizon the heights of the celestial bodies, as well as the vertical and horizontal angles of the earth’s objects.
The calculation of the Greenwich Hour Angle of the celestial bodies in comparison to the Prime Meridian requires the knowledge of the GMT time which is shown by the accurate timekeeping devices called chronometers.
A watch with a seconds hand that measures intervals of time.
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