Cosmos: A Historical Perspective

Everyone knows that the universe is extremely old and extremely large. But how did scientists determine just how old and how large? How do astronomers know that there are upwards of 100 billion galaxies in the universe if the nearest one is over 40,000 light—years away? How do we know what the stars are made of? The answer is that our current knowledge of the universe has arisen from the work and ideas of scientists and philosophers over hundreds of years. While it's only been during the la...