Mediaeval astronomical measurements assumed the model of a two sphere Universe. This model postulated the segregation of Earth and sky: the laws of motion and the constitution of the heavens were considered quite separately from the physical relationships governing the behaviour of matter on the Earth. Philosophers considered the heavens to be perfect and unchangeable: all celestial bodies moved with the most perfect motion, uniform circular motion, about the Earth. During the sixteenth century, however, Aristotelean physics and the concept of a geocentric Universe came under increasing attack. By the end of the seventeenth century, the postulated duality of Earth and Sky had been over¬thrown by the concept of a unified Universe in which Earth and Sky alike were governed by the law of universal gravitation.
The change in cosmological outlook was characterized by a new interest in the stellar sphere itself. Events such as comets, which in earlier times would have been attributed to atmospheric effects, were now watched enthusiastically in an attempt to explain the new evolving cosmos. This new interest in the heavens was accentuated by the demands of navigation. With trade and expansion dependent on accurate means of navigation, countries sought to solve the problem of determining longitude at sea. Many felt that the elm- lay in more precise knowledge of celestial positions, which could t hen be observed by navigators in order to deduce their own positions. So, around the middle of the seventeenth century, State observatories were set up in France, England, and later other countries, to prepare extensive catalogues of stellar and lunar positions. At the other end of the scale further attention was paid to the shape and size of the Earth to see if it satisfied the predictions of Newtonian gravitation.
The overall revolution in astronomical thinking began with the work of Nikolaus Copernicus. Although Copernicus began the so-called revolution, very few of the changes mentioned above were anticipated by him. Observed planetary positions at this time were found to differ by as much as several degrees of arc from their predicted positions. Copernicus reanalysed planetary motions and concluded that a new model of the Solar System would fit the observations more closely. ‘De Revolutionibus Orbium Coelestium’ adhered to the mediaeval concept of a compact Universe bounded by the sphere of the stars, but it positioned the Sun, rather than the Earth, at the centre. Unlike the ‘Almagest’, it provided a uniform planetary system, the mathematical application of which would reveal the relative sizes of the planetary orbits in space. In other words, from the Copernican system one could obtain the actual scale of the Solar System. The displacement of the Earth from the centre had another dramatic effect – it meant that the diameter of the hypothesized celestial sphere would have to be at least 2000 times greater. With the stars any closer than this, one would expect to be able to detect stellar parallax (cyclic changes in the observed positions of the stars due to the motion of the Earth round the Sun), which had not at the time been observed. Despite this massive increase in stellar distances, the Copernican world picture was much like the mediaeval one.
One of the early converts, Thomas Digges , added a new outlook to the Copernican picture. In a popular account of the Copernican theory published in 1573. he depicted the stars (stretching out wards beyond the orbit of Saturn and he maintained that they extended to infinity. It was Giordano Bruno, however, who became the leading proponent of the idea that the universe extended infinitely and had no particular centre and his doctrines spread throughout Europe like wildfire. Bruno’s theory implied a plurality of inhabited worlds and a rejection of traditional religious doctrine; this led to his execution in 1600, and also provoked suspicion of the Copernican theory among theologians.