Grade 11 – Nautical Science

October 22, 2020

Sextant altitude

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 […]
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October 22, 2020

Dip and apparent altitude

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 […]
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October 22, 2020

Refraction

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 […]
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October 22, 2020

Correction for semi-diameter

Because of their relative sizes and nearness to the earth, the sun and moon are not merely seen as points of light but as spheres with sizeable diameters.  Therefore, in order to take a sextant altitude, the observer needs to measure the angle between the horizon and their upper or […]
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