n.an instrument used to determine the altitude of a heavenly body (sun, moon or star) for the purpose of navigation. An octant is an instrument so designed that one can look through an eyepiece and (by the action of mirrors) see the horizon while at the same time viewing a heavenly body. This enables one to measure the heavenly body's height in the sky (and thus get information to determine one's position in relation to it). When the horizon is not visible, as at night or when weather conditions are unfavorable, one can use a bubble octant to make a similar measurement. This device contains a chamber of liquid with a bubble of air in it. The bubble octant is held so that the bubble lies in a certain position that shows the instrument is being held level to the horizon. The heavenly body is then viewed and measured similarly to how it is done with a regular octant.