bubble horizon

The Oxford Companion to Ships and the Sea | 2006 | 700+ words | Copyright

bubble horizon, an attachment that screws onto the frame of a marine sextant immediately in front of the horizon glass to provide an artificial horizon in conditions when the real horizon is indistinct or invisible. When taking an altitude of a celestial body, the navigator's line of sight through the sextant's telescope passes through the clear part of the horizon glass and enters the attachment where, by means of mirrors and a lens, the rays of light from a bubble inside the attachment are projected in a true horizontal direction. This provides an artificial true horizon, and when the reflected image of the body, seen in the silvered part of the horizon glass, is brought down to the artificial horizon, its altitude can be read off on the scale.

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