The green shaded area depicts the angle measured from the centre of the Earth to the Prime Meridian to the longitude line west (in this example, measured at 080°W)
B is the centre of the Earth
C is the Prime Meridian
A is the 080°W line of longitude
The orange shaded area depicts the angle measured from the centre of the Earth to the Prime Meridian to the longitude line east (in this example, measured at 80° E)