What are the most important discoveries made through research in astronomy?
In 1543, Copernicus proposed the, at the time the heretical idea that the Earth and all the planets revolved around the Sun. The view at the time was that the Earth was the centre of the universe and everything, including the Sun, revolved around us. This also went against the Church dogma of the time and was a risky thing for him to propose. As time went on, this idea gained more and more evidence and has now become scientific fact. This Copernican Revolution radically changed not only astronomy, but human culture dramatically as it showed we were not the centre of the universe!
The Big Bang Theory was the other major revolution in astronomy that essentially defined how the universe got started some 13.7 billion years ago. This idea was proposed in 1927, but it took over 50 years or so to gain widespread scientific acceptance. This also rock our human cultural ideas in that the Big Bang did not postulate a God(s) to get the process going. This idea also annoyed the Church, but they found a way to give God a role in kick-starting the Big Bang, which annoys astronomers!