In response to some questions we've been getting in relation to the Apollo panel '8 balls', here's a brief summary...
The IMU (Inertial Measuring Unit) is fixed onto the cockpit and moves with the spacecraft. There are three gimbal axes measuring movement of the spacecraft in the X, Y and Z axes. The output from the IMU axes then turns the 8 ball to give a visual indication of the position of the non-moving stable platform (the 8 ball) in relation to the spacecraft.
Although bearing some resemblance to FDs (Flight Directors) used in general aviation, the FDAIs are not artificial horizon indicators. Rather, they are attitude indicators
showing the yaw, pitch and roll of the spacecraft with reference to an initial inertial state, which changes depending on the mission.
The horizontal lines are the pitch lines.
The vertical lines are the yaw lines.
The center semicircle indicates the current pitch and yaw.
The triangle points to the roll.
The red area is the dreaded gimbal lock region.