<P> where D is in kilometres and h and h are in metres . </P> <P> As another example, suppose an observer, whose eyes are two metres above the level ground, uses binoculars to look at a distant building which he knows to consist of thirty storeys, each 3.5 metres high . He counts the storeys he can see, and finds there are only ten . So twenty storeys or 70 metres of the building are hidden from him by the curvature of the Earth . From this, he can calculate his distance from the building: </P> <Dl> <Dd> D ≈ 3.57 (2 + 70) (\ displaystyle D \ approx 3.57 ((\ sqrt (2)) + (\ sqrt (70)))) </Dd> </Dl> <Dd> D ≈ 3.57 (2 + 70) (\ displaystyle D \ approx 3.57 ((\ sqrt (2)) + (\ sqrt (70)))) </Dd>

How far is the sky at the horizon