What is the Approximate Distance to the Horizon? If Earth was flat and the air was perfectly clear, you could see for an unlimited distance. Therefore, the fact that there is a horizon--that is, an apparent line that separates Earth from the sky--is evidence that Earth is round. So approximately how far is it to the horizon? If we let r represent the radius of Earth and let h represent the height of the observer's viewpoint, then the relationship to the desired distance L will be as follows, according to the Pythagorean theorem: ( r + h ) Therefore, 2 = r 2 + L 2 L = ( r + h ) 2 - r 2 = 2 rh + h 2 Since the radius of Earth (r) is approximately 6,378 km (which is 3,963 miles), if we assume that the height of a normal person's eye (h) is 0.0015 km (equal to 5 feet), then we can calculate L as approximately 4.4 km (2.75 miles). In other words, if we gaze at the ocean from the beach, we will only be able to see the ocean's surface out to a little more than 4.4 km (2.75 miles).