Two hundred years ago, astronomers couldn't measure the distance to a single star or know what they were made of. Today we've mapped the universe back nearly to its beginning, detected gravitational waves, photographed black holes, and found thousands of worlds orbiting other suns. Big Think's rundown of the 20 most significant cosmic discoveries since the early 1800s traces how each breakthrough reshaped what we know: the realization that stars are suns like ours, the discovery that galaxies exist beyond the Milky Way, the evidence that space itself is expanding, the detection of the cosmic microwave background radiation left over from the Big Bang, and more recent finds like exoplanets and the direct imaging of a black hole's shadow. Each discovery followed the same pattern: someone looked harder or built a better instrument, and the universe revealed something nobody expected. The speed of discovery has only accelerated, with more known about the cosmos now than in all of previous human history combined.