The sun casts shadows which change dramatically through the day. The length of a shadow cast on the ground is proportional to the cotangent of the sun's elevation angle—its angle θ relative to the horizon. Near sunrise and sunset, when θ = 0° and cot(θ) = ∞, shadows can be extremely long. If the sun passes directly overhead, then θ = 90°, cot(θ) = 0, and shadows are cast directly underneath objects. Such variations have long aided travellers during their travels, especially in barren regions such as the Arabian Desert.
