The most important astronomer of all time turns out to be the Italian spearhead of the Scientific Revolution, Galileo. Galileo was, in a sense, a lucky astronomer. To put it simply, he was fortunate to be alive when the telescope was invented (around 1607 AD). He caught wind of this amazing new device, and quickly made his own refracting telescope. This gave him absolutely unprecedented access to information on the heavens– and he was the first to capitalize on it. While military leaders across Europe were using the spyglasses to watch their enemies at sea, Galileo turned his telescope to the sky and discovered secrets that had lay waiting for millennia.
Because Galileo lived and worked at such an opportune time, he is considered by most to be the father of modern observational astronomy (not to mention the father of modern physics). Many aspects of his life lend themselves to this title. He was the first to lay eyes on the Rings of Saturn (though they looked more like handles from his perspective), and he also discovered and named various moons of Jupiter. He was also the first to observe sunspots, which was rather significant, because it was then believed by the church that the sun was perfect and without blemishes of any kind.
Probably what Galileo is most well known for is his staunch defense of the idea of a heliocentric solar system, regardless of the religious persecution he was subjected to. Unlike Copernicus however, he was able to provide hard evidence for the fact. One of his famous experiments included observing the planet Venus over a certain period, and recording the various phases it would go through (with a telescope, one can see that Venus goes through phases, not unlike our moon). Galileo used his data and made certain calculations, and was able to show that due to Venus’s phases, it had to orbit the sun. Despite this and other pieces of amazing science however, the church still argued that the sun orbited the Earth. Galileo was eventually placed under house arrest for his heretical views, and lived out the last eight years of his life in his villa near Florence.