You are approaching the island of Hawaii in a small boat. The highest point on Hawaii is Mauna Loa at 13,677 feet. You see it just barely above the horizon. The radius of Earth is 3,963 miles. Ignoring atmospheric effects, you figure that you are miles in a straight line from the top of Mauna Loa.
Hint: Draw the triangle whose vertices are the top of Mauna Loa, your boat, and the center of the earth. Apply the Pythagorean Theorem. A mile has 5,280 feet.