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…
Reading the sextant is very simple: Degrees are read directly from the graduated arc opposite the index mark on the index arm. Minutes are read from the micrometer drum opposite the vernier index mark. Seconds are read from the vernier where one of the vernier graduations lines up with one…
Before housing the sextant in its case, set the index arm to the middle of the graduated arc and turn all the shades inwards (ie towards the centre of the instrument). Place the sextant carefully in the box ensuring that the three legs at the rear of the instrument enter…
Depending on type and manufacturer, the sextant case will either be made of a hardwood or plastic. The wooden cases are fitted with a carrying handle, patent fasteners, lock and key. Inside the case are wooden chocks built to prevent the sextant being dislodged. The plastic case does not have…
