Historically, astronomy had always been a mathematical discipline: the theory of predicting the movements of the heavenly bodies. However, in the seventeenth century, thanks to the invention of the telescope (which made the discoveries of the Italian scholar Galileo Galilei and other astronomers possible) and the theories of the French philosopher René Descartes, the structure and the nature of space itself became central to the science of astronomy.
Huygens was thoroughly familiar with the mathematical aspects of astronomy, but he was primarily interested in the question of how space is structured. As an astronomer, his primary function was that of an observer. He explored the heavens with his home-made telescopes, and achieved particular fame in this science thanks to his discoveries about the planet Saturn.
His work in astronomy includes a planetarium, built by order of the King of France, and the book Cosmotheoros, which he wrote towards the end of his life. In this book he discussed the structure of the heavens and the dimensions of heavenly bodies, as well as the question of whether life could occur on other planets.
See also the website of Museum Boerhaave for Huygens and Astronomy. (Text in Dutch)