Is the Speed of Light Really Constant?

Light - one of the most profound ideas in the history of science. It travels across the universe faster than anything else we know of. It is how we see what our world around us looks like and how we communicate with each other. Able to both give life and destroy it, light is one of the most diverse and powerful things in the universe.

Many of our current ideas on how the universe was created and how it all operates are based on light and the idea that it is constant and moves at a constant speed through the vacuum of space. Several important theories and formulas use this idea as a constant to define physical interactions on every level. Scientists can measure the speed of light in a vacuum set up here on Earth and every time they will arrive at the same exact outcome. They call that number c and use it to define the length of a meter. But is the speed of light indeed constant?

According to Albert Einstein's theory of relativity, any measurement made can only be taken as accurate according to its relative position to the observer. With that in mind, how can we say that the speed of light is indeed constant to anyone other than the observer unless they are also looking at the system used to measure it from the same vantage point of the observer?

Let's look at an example. We are going to play a game of catch. Person A stands facing due east. Person B stands facing Person A so that the line from A to B is parallel to the Equator. Now, at noon, we throw a ball from A to B at 75 km/h. To anyone on Earth, the ball is traveling due east at 75 km/h. We do the same thing at midnight and it will still measure 75 km/h. It's constant, but only if you are on Earth. If you were standing on the sun, other than being burned up, of course, you would observe the Earth moving around the sun at 107,218 km/h on average. If you look at the ball thrown at noon, its velocity, relative to the sun, is 107,143 km/h, but the one at midnight is traveling 107,293 km/h. The velocity of the ball is not actually constant, even though our own observations showed that it was.

If something as simple as a game of catch can show how our idea of a constant is not really accurate or absolute, how can we assume that we fully understand something as complex and profound as the speed of light and the nature of how it travels through our universe?