some historic figures are absorbed in work and discussion. At the back, we spot a Chaldean astronomer in a Phrygian cap making observations between rough-hewn log pillars. It is in the Babylonian and Chaldean civilizations that we find many of the earliest traces of Western mathematical astronomy. Their priest-scribes compiled catalogues, recognized many cyclic occurrences, and tried to make predictions arithmetically. They observed that the movements of the planets were not uniform. To describe and explain this behaviour became the central theoretical problems for astronomers ever since the Greek era. Kepler also knew well that the sexagesimal number system had its origins in ancient Mesopotamia, and sexagesimal coordinates were something that he made frequent use of in his Tables.The next two columns are portrayed as fragile piles of stone blocks, but proceeding to the brick pillars, we find our first two names. On the right, the name Meton sticks out. Active at the time of Socrates, he was a well-known Athenian geometer and astronomer. He tried to improve the calendar and gave name to the Metonic cycle, which is based on the observation that 19 years are (coincidentally) very close to 235 months counted as the period of the moons phases. Many civilizations used this cycle to form lunisolar calendars. On Metons column hangs a tribute to them: a disc divided into 19 sections, decorated with sun and moon. In Keplers days Gregorian calendar reform was fiercely debated. Although criticized by other Protestants, both Tycho and Kepler stood up for it.On the left side, it may at first seem odd that Kepler chose the Greek poet Ara- tus as one of the pillars of astronomy. Aratus is mainly known for a long didactic hexameter poem called Phaenomena (Appearances). This work, a blockbuster in antiquity, is actually a commissioned verse setting of treatises by Eudoxus, one of the finest Greek mathematicians and astronomers. None of his works have survived, however, and we have to rely on others to get glimpses of his contributions and brilliance. In Aratuss poem we find the first description of our familiar star constellations. Eudoxus combined a number of nested rotating spheres to represent a planets motion around the Earth. With this clever geometric model he could qualitatively account for apparently irregular motions seen in the sky, not only deviation from the ecliptic, but also retrograde motion (where he needed four spheres). The first mathematical model combining the Pythagorean and Platonic ideas of uniform circular motions was born, the starting point of a long tradition.Nailed to Aratuss column is an old astronomical instrument, the armillary sphere. The Earth in the middle is surrounded by the equator, the ecliptic, and other great circles of interest. It was widely used as a quick and simplified way to determine celestial positions in various frames of reference. Look at the flag of Portugal and you will see another example honouring a seafaring nation.While the last two brick columns were associated with the foundations of astronomy in the Greek-speaking world, the next two represent the culmination.The armillary sphere can be seen as a skeleton