Shadow lengths change dramatically throughout 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) is infinite, shadows can be extremely long. If the sun passes directly overhead, then θ = 90°, cot(θ)=0, and shadows disappear completely.
Above from Wikipedia's Shadow entry on "Variation with Time" and shadows have always interested me in photography. Here are a few from my collection over the years with captions below each.
Death Valley ~ Racetrack Playa
Sedona Arizona ~ Tuzigoot ~ Ancient Sinagua Ruins
Marin Headlands, CA ~ 7:30pm July 7, 2009
Marin Headlands, CA ~ riding Chester
Marin Headlands ~ June 2009
Marin Headlands, California ~ Sept 11, 2006 riding Red