In this project you will read how Christiaan Huygens used continued fractions to build a Planetarium. Continued fractions are rational approximation for (ir)rational numbers. Continued fractions resembles the Pythagoras theorem, as neither the discoverer nor their origin is known. Examples of continued fractions can be found throughout mathematics over the last 2000 years. Nevertheless, we know that Christiaan was the first to demonstrate a practical application for them.