20 feet Any height. When the Sun is low, in the morning or evening close to the horizon, even a short object will cast a long shadow. When the Sun is directly overhead (at noon between the Tropic of Capricorn and the Tropic of Cancer) objects cast no shadow. If you tell us your location and the time you are measuring then there would be a possibility of calculating an answer. At noon on one of the equinoxes in Montpelier VT (or anywhere at the latitude of 45 degrees north or 45 degrees south) a 50 foot tall tree will cast a 50 foot shadow.
