Final answer:
The angular difference referred to is the dip angle, which measures the tilt of a compass needle in a vertical plane relative to the horizontal. It varies with the geographic location, showing a variation in the Earth's magnetic field.
Step-by-step explanation:
The angular difference between horizontal and a compass needle turned on its side so that it can rotate in a vertical plane is referred to as the dip angle or magnetic inclination. This angle measures how much a compass needle is tilted relative to the horizontal plane. If we ignore other factors and consider a scenario where the Earth's magnetic north pole coincides with the geographic north pole, a compass at the equator would have a dip angle of 0°, meaning it would point straight out horizontally. However, as one moves away from the equator towards one of the magnetic poles, this angle will increase due to the Earth's magnetic field lines being vertical at the poles and horizontal at the equator.