Astronomical Tower was built in 1722 to a height of 68 meters. At the top stands lead statue of Atlas carrying the celestial sphere by Matthias Bernard Braun. Construction of the tower was related to  the development of astronomical studies in the country. At the time of the construction of Klementinum, such astronomers as Tycho Brahe, Johannes Kepler, or Thadeus Hájek worked in Prague. Astronomy was part of the university curriculum since its inception, but the observatory was founded in Klementinum at the instigation of Joseph Stepling, its first director, in the years 1751-1752. Astronomical instruments were installed in the tower and it became the main spot for astronomical measurements.

The observatory was from the foundation under state control, after the First World War it became part of the newly founded State observatory of Czechoslovakia and the Klementinum tower was until 1928 its only observatory in Bohemia. The observatory in Klementinum worked until World War II.

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With Josef Stepling are also associated meteorological measurements. They began to perform them in Klementinum in 1752, but those were not regular. Daily temperature measurements were recorded continuously from 1775, rainfall measurements were conducted since 1804. All mereorological measurements are performed in Klementinum today and has become one of the oldest ones in the world. 

Today, visitors of Klementinum can climb the tower to a height of 52 meters, where they have a unique view of the city and its finest monuments. 

Before reaching the top of the tower there is one more stop - the original workroom of the observatory. There used to be smaller astronomical, geophysical and meteorological instruments from the 19th century in here, coming mainly from the workshops of Jesuit scholars and mechanics. Most of them are still displayed here today. In addition to astronomical and meteorological observations, experiments in physics were also conducted in this room.