Find the sun's angle using the NOAA Solar Position Calculator located in the resources section. Plug in your location, date and time to determine the sun's angle in that particular spot and time. For example, assume the sun's angle is 40 degrees.
Find the tangent of the sun's angle using your calculator. In the example, the tangent of 40 degrees equals 0.839.
Divide the height of the object by the tangent of the sun's angle. In the example, if the height is 5 feet, then 5 divided by 0.839 equals a shadow length of 5.959 feet.
Compare your shadow length to other shadow lengths to determine the longest shadow.