The first telescopic observation of Mars was by Galileo Galilei in 1610. Within a century, astronomers discovered distinct albedo features on the planet, including the dark patch Syrtis Major Planum and polar ice caps. They were able to determine the planet's rotation period and axial tilt. These observations were primarily made during the time intervals when the planet was located in opposition to the Sun, at which points Mars made its closest approaches to the Earth.