The Sydney Observatory is located in the heart of Sydney, Australia, atop a hill now known as Observatory Hill. The site, which began as a fortress built on Windmill Hill in the early 19th century, was transformed into an astronomical observatory during the 19th century. Today it serves as a museum where evening visitors have the opportunity to observe the stars and planets through a modern 40cm Schmidt-Cassegrain telescope and through a period 29cm refracting telescope. The latter dates back to 1874 and is the oldest telescope still in use in Australia.