When a ray of light passes at an angle from a less dense medium to a higher density medium, it is bent towards the normal (an imaginary line running at right angles to the surface between the mediums). This is referred to as refraction. The visible light from a heavenly…
This is the angular difference between the altitude measured between the apparent horizon and the body as seen through the observer’s eye and the altitude which would be measured between the body and the horizon as seen from the surface of the sea. The observer will always be a certain…
Sextant altitude, observed altitude and index error. The altitude is measured by a sextant and although they are precision instruments, there is always a slight difference between the sextant altitude and the observed altitude caused by the design and manufacture of the instrument. This difference is referred to as the…
When we take a sextant altitude of a heavenly body, we measure the angle between the body and the visible sea horizon as witnessed from the observer’s position on the bridge of a vessel. (angle DOX in the diagram). This is referred to as the observed altitude. In converting it…
Individual corrections. In order to be able to use the altitude obtained by sextant of a heavenly body, we need to convert it from a sextant altitude to a true altitude. To do this we need to apply five individual corrections to the altitude obtained from the sextant, namely corrections…
Adjustment screws. Every sextant is equipped with error adjustment screws. The design of these screws varies from sextant to sextant. Some have ordinary screw driver slots while others have round heads with a hole drilled through it. The first requires the use of a suitable screw driver whilst the others…
