Speed of light
Modern science is based on the scientific method and experimental design. So when scientific concepts are beyond our powers of observation, it becomes much harder to find out how they work. For this reason, Galileo and many other scientists like him believed that light traveled instantaneously.
It has been just within the last hundred years that scientists had the technology necessary to discover the exact speed at which light travels. They found out that the speed of light is constant and does not change. For instance, when a train is traveling, the light that is emitted from the train does not travel at the speed of light plus the speed of the train. It just always travels at the speed of light, which is 299,792,458 meters per second. This scientific fact is contrary to common sense, but is true nevertheless and has been proven time and time again in many different experiements.
Einstein's theory of the universe dictates that nothing can exceed the velocity of light. Light then becomes a sort of universal speed limit that nothing in the whole universe can travel faster than.
But even though light travels at an incredible rate of speed, that rate of speed is still measurable and finite. It still takes time for light to travel great distances. The distance between the Earth and many celestial bodies is so great that light cannot keep up. The light we see from certain galaxies and stars was actually emitted as long as hundreds of years before. So when you look into a telescope, you are actually looking into the past!