Determine Object Heading Is there a sensor I could use which would let me know which direction an object is heading/facing? i.e. I have a servo that can rotate 0 to 180 degrees, and I have another sensor on a block. When I turn the block 45 degrees, I want the servo to also turn 45 degrees - I thought I could do this with a compass, but the varying magnetic fields make it impossible. Gyro/acc maybe? Except I don't know how to use that to solve this problem. 

So in other words we need to know the block was rotated 45 degrees using a sensor on the block, which tells the servo to rotate 45 degrees.
Use a gyro, it will give you the angle. Gyros are difficult to find spare, so you should buy an IMU like MPU6050 that contains also an accelerometer.