Sally is the surveyor in badlands in South Dakota. At point A on south side of the deep canyon, she sights across to the fire tower on north side of canyon at point B. Then she moves measured 500 feet along south rim of canyon to the point C on line perpendicular to original sight line AB. Setting up transit again, she discovers that her line of sight, CB, to tower is 87 degrees. How far is point C from tower?