Light travels at a finite speed, with the speed depending upon the medium. With mirages, the medium is air. In air, the speed of light is only slightly less than it is in a vacuum, and the speed of light in air depends upon the temperature of the air. Simply put, the speed of light is slightly greater in warmer air than it is in cooler air. In physics, we usually express this behavior reciprocally as the index of refraction, n:

It is in evidence that, if a projectile be fired from a rapidly moving body in an opposite direction to that in which the body is going, it will fall short of the distance at which it would reach the ground if fired in the direction of motion. Now, since the Earth is said to move at the rate of nineteen miles in, a second of time, "from west to east," it would make all the difference imaginable if the gun were fired in an opposite direction. But, as, in practice, there is not the slightest difference, whichever way the thing may be done, we have a forcible overthrow of all fancies relative to the motion of the Earth, and a striking proof that the Earth is not a globe.

54.) The aeronaut is able to start in his balloon and remain for hours in the air, at an elevation of several miles, and come down again in the same county or parish from which he ascended. Now, unless the Earth drag the balloon along with it in its nineteen-miles-a-second motion, it must be left far behind, in space: but, since balloons have never been known thus to be left it is a proof that the Earth does not move, and, therefore, a proof that the Earth is not a globe.
121) When you observe the Sun and Moon you see two equally-sized equidistant circles tracing similar paths at similar speeds around a flat, stationary Earth. The “experts” at NASA, however, claim your common sense every day experience is false on all counts! To begin with, they say the Earth is not flat but a big ball; not stationary but spinning around 19 miles per second; they say the Sun does not revolve around the Earth as it appears, but Earth revolves around the Sun; the Moon, on the other hand, does revolve around the Earth, though not East to West as it appears, rather West to East; and the Sun is actually 400 times larger than the Moon and 400 times farther away! You can clearly see they are the same size and distance, you can see the Earth is flat, you can feel the Earth is stationary, but according to the gospel of modern astronomy, you are wrong and a simpleton worthy of endless ridicule if you dare to trust your own eyes and experience.