For observed sunspots, the differential rotation can be calculated as: Ω Ω = Ω Ω 0 − − Δ Δ Ω Ω sin 2 ⁡ ⁡ Ψ Ψ where Ω Ω 0 is the rotation rate at the equator, and Δ Δ Ω Ω = ( Ω Ω 0 − − Ω Ω p o l e ) is the difference in angular velocity between pole and equator, called the strength of the rotational shear. Ψ Ψ is the heliographic latitude, measured from the equator.