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 body will be refracted when…
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 distance above the sea surface…
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 index error and must be…
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 to a true altitude we…
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 for: Index error. Dip. Refraction.…
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 require the use of a…
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 of the micrometer graduations. The…
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 the holes specially made for…
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 a lock or key and…
The sextant is made up of the following parts: The frame. The handle. The telescope or monocle. The rising piece. The arc. The index arm. The clamp. The worm and rack. The micrometer drum. The micrometervernier. Electric light. The index mirror. The index mirror clips. The index mirror (first) adjustment screw. The index mirror shades.…
