In the 1600s, astronomers noticed that the stars seemed to jump in a way that made no sense. During certain months, stars seemed to leap out of place through no phenomenon that they could understand. Then along came James Bradley, who used wind and rain to explain what he called the aberration of starlight.
In the 1600s people were finally reconciled to the idea that the Earth orbits the sun instead of the other way around. It was time to start looking for new odd movements of the spheres. They started scanning the sky to see how the stars "moved" to get a better idea of how the Earth actually moved. Particularly, they looked at parallax.
Parallax is easiest to understand when closing one eye and then the other. You'll notice that some objects in the foreground suddenly hop back and forth in relation to the background. The distance between your eyes gives you a slightly different view of the three dimensional objects in front of you. In the same way, the distance between one side of the Earth's orbit and the other should give a slightly different view of one side of a three dimensional field of stars.
The problem is, when people started looking up with sufficiently powerful telescopes, they noticed that the stars jumped three months early. The effect was particularly noticeable for those stars that the Earth's orbit seemed to cross during those three months. What was going on?
It was James Bradley who finally explained the phenomenon. He looked up and saw the flag at the top of his boat trailing directly back on a windless day. When the wind kicked up, blowing a bit to the side with respect to the boat, the flag changed direction. He realized that the Earth was like the boat moving along. Although the light from the stars didn't come in on the wind, it did move. That movement, along with the movement of the Earth, changed the apparent direction of the starlight coming to a telescope.
It's a bit like the direction of raindrops. If you're at a standstill beneath rain pouring directly down, it will hit you square on and you'll see it as coming straight down on top of you. If you start walking, or get in a car and start driving, your view of the drops will change and they'll appear to be coming directly towards you from slightly in front of you. And just when the Earth is moving perpendicularly to starlight, the light will appear to come from a different direction, making the star "jump."
Top Image: Arches National Park