Bubble Sextants
People
are often curious about the purpose of the bubble sextant and wonder how to
use it. The basic idea is that from an aircraft, the horizon no longer
serves as a reference for horizontal as it does at sea level. So a bubble
level was used instead. Some models also had an interesting clockwork device
to help the operator average many readings devised to help sailors locate
themselves on the featureless ocean. A bubble attachment is a special sextant telescope containing an internal artificial horizon in the form of a small spirit level whose image, replacing the visible horizon, is superimposed with the image of the body. Bubble attachments are expensive and not very accurate because they require the sextant to be held absolutely still during an observation, which is difficult to manage. A sextant equipped with a bubble attachment is referred to as a bubble sextant. Special bubble sextants were used for air navigation before electronic navigation systems became standard equipment.
A pan filled with water, or preferably a more viscous liquid, e. g., glycerol, can be utilized as an external artificial horizon. Due to the gravitational force, the surface of the liquid forms a perfectly horizontal mirror unless distorted by vibrations or wind. The vertical angular distance between a body and its mirror image, measured with a marine sextant, is twice the altitude. This very accurate method is the perfect choice for exercising celestial navigation in a backyard.
