even initially horizontal rays normally curve down towards the earth, so that objects at the observers own level, as well as those above and below it, appear elevated. to understand this phenomenon, consider a wave front normal to the surface of the earth and, consequently, moving horizontally. if now, the density of the air at the place in question decreases with elevation, as it nearly always does, the upper portion of the wave front will travel faster than the lower, and the path will be bent towards the earth along a curve whose radius depends upon the rate of this density decrease. to make a long story short, you can see further laying on your stomach than standing because the light curves somewhat with earths curve. if earth were 6 times bigger with the same atmosphere, you could theoretically see all the way around to the back of your own head. i was in military... the earth is round.