The attitude indicator (AI), formerly known as the gyro horizon or artificial horizon, is a flight instrument that informs the pilot of the aircraft orientation relative to Earth's horizon, and gives an immediate indication of the smallest orientation change. The miniature aircraft and horizon bar mimic the relationship of the aircraft relative to the actual horizon. It is a primary instrument for flight in instrument meteorological conditions. Attitude is always presented to users in the unit degrees (°). However, inner workings such as sensors, data and calculations may use a mix of degrees and radians, as scientists and engineers may prefer to work with radians. History Before the advent of aviation, artificial horizons were used in celestial navigation. Proposals of such devices based on gyroscopes, or spinning tops, date back to the 1740s. Later implementations, also known as bubble horizons, were based on bubble levels and attached to a sextant. In the 2010s, remnants of an artificial horizon using liquid mercury were recovered from the wreck of HMS Erebus. Use The essential components of the AI include a symbolic miniature aircraft mounted so that it appears to be flying relative to the horizon. An adjustment knob, to account for the pilot's line of vision, moves the aircraft up and down to align it against the horizon bar. The top half of the instrument is blue to represent the sky, while the bottom half is brown to represent the ground. The bank index at the top shows the aircraft angle of bank. Reference lines in the middle indicate the degree of pitch, up or down, relative to the horizon. Most Russian-built aircraft have a somewhat different design. The background display is colored as in a Western instrument, but moves up and down only to indicate pitch. A symbol representing the aircraft (which is fixed in a Western instrument) rolls left or right to indicate bank angle. A proposed hybrid version of the Western and Russian systems would be more intuitive, but has never caught on. Operation The heart of the AI is a gyroscope (gyro) that spins at high speed, from either an electric motor, or through the action of a stream of air pushing on rotor vanes placed along its periphery. The stream of air is provided by a vacuum system, driven by a vacuum pump, or a venturi. Air passing through the narrowest portion of a venturi has lower