A planar reflecting surface that can be adjusted to coincide with the astronomical horizon, that is, can be made perpendicular to the zenith. This instrument is used, usually in conjunction with others, in observing celestial bodies. See horizon.

An instrument which enables a pilot to determine the attitude of the aircraft in relation to the horizon, i.e. whether the aircraft is nose-up, nose-down, or banking left or right.

An instrument, operated by a gyroscope, devised to keep an indicator permanently parallel with the true horizon and thus show the pilot the attitude of his aeroplane when flying in cloud or bad visibility.