How is a Sight Taken?
A navigator will decide which stars or planets to 'shoot', depending on time of day, clarity of the horizon and the heavenly bodies (stars and planets) that he or she can see clearly.
The taking of the altitude involves 'bringing the celestial body down to the horizon' when seen through the sextant. This means that the reflection of the body, as seen through the sextant, is slowly lowered down to a point when it is just touching the horizon. This is done by first setting the sextant to zero and slowly and smoothly moving the arm of the sextant, all the while keeping the reflection of the body in sight within the instrument. Finer adjustments are made with the micrometer.
When the navigator is satisfied that the body is perfectly on the horizon (with planets and the sun, the sextant is rocked from side to side to ensure that the circumference of the body is tangential to the horizon), the angle is then read off the scale on the sextant. This is the uncorrected altitude.
A navigator would typically go out on the navigation bridge wing. He or she then takes an altitude and notes the exact time. Calculations are then made to determine the ship’s position. This is usually done when the ship is far from land, and lighthouses and other land based objects cannot be used to ascertain position.