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 […]
Do you like it?0
0 Read more
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 […]
Do you like it?0
0 Read more
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 […]
Do you like it?0
0 Read more
October 22, 2020

Parallax

The last correction to be applied is parallax.  So far the observer has been able to find the heavenly body’s centre above the horizontal plane.  All that is left is to allow for the earth’s radius.  In the drawing parallax is the angle OXC.  As you can see the true […]
Do you like it?0
0 Read more
Prev page
12345678910
Next page