In the midst of these cosmological expansions, several events culminated in the abolition of the concept of solid, classical spheres required by mechanistic models of the universe. Copernicus Universe deprived the stellar sphere of its earlier function as the driving force behind celestial motions. A succession of uncommon celestial events showed that the spheres were a redundant feature of the models. These rarities included the sighting of a supernova in 1572 and the appearances of the great comet, visible in daylight ,in 1577.Tycho brahe observed both of these and was ingenious enough to take a few measurements of them ; from the lack of visible parallax of the nova and the very small parallax of the comet he deduced that they must both be far beyond the atmosphere of transient events cast serious doubt on the existence of immutable planetary spheres and re-opened the questions of the whole mechanism of the Solar System

At this point, although the concept of space and the size of the Universe was changing, actual distance measures had not advanced. Copernicus’ value for solar parallax was remarkably similar to Ptolemy’s, and only the estimated distances to the stars had in¬creased. The most meticulous observer of the day, Tycho Brahe, had attempted to determine the trigonometrical parallax of the star Polaris with his great quadrant but met with no success. Consequently he rejected the notion of a moving Earth and instituted instead an alternative planetary scheme. Another objection to a moving Earth was the immense stellar diameters (of the order of 2000 times that of the Sun) obtained from the new distances if the apparent angular sizes were converted to linear measurements. This objection was eliminated by Galileo’s observations of stars through a telescope, the first-known telescope to be used for astronomical purposes. To Galileo’s surprise, the stars continued to appear as mere points of light, despite magnification; this demonstrated that apparent sizes were, after all, misleading. The telescope also revealed an infinitude of stars, hitherto unseen, particularly in the Milky Way where Galileo’s telescope could resolve star images. Gradually a picture of layers of stars at varying distances replaced the old model of a sphere of fixed stars. To deter¬mine their distances, Galileo suggested a new method of parallax determination based upon differential measurements of distances between the components of optical pairs of double stars. His idea was that of two stars close together in the sky, the brighter could be assumed to be the nearer; if this were true, the apparent separation of two close stars should vary as the Earth orbits the Sun, on account of parallax.

Contemporary with Galileo’s telescopic observations, Johannes Kepler was providing the final blow to the classical picture of the Universe. After several unsuccessful attempts over many years Kepler finally discovered that the orbit of Mars could be accurately represented by an ellipse. He extended the model to the rest of the planets, and also revealed the first accurate period-distance relation now known as Kepler’s third law. Kepler attempted to find the absolute scale of the Solar System by obtaining the parallax of Mars, but he was unable to detect any such parallax. He concluded that the distances of Mars and the Sun must be greater than had been supposed: in his estimate, the distance to the Sun would have to be three times greater than the currently -accepted value (that is, a solar parallax of about 1 arc minute) and the distances to stars at least 400000 million kilometres.

Estimates of solar parallax were to become more exact in the seventeenth century. In 1671, Jean Richer undertook an expedition to Cayenne, in co-operation with Picard and Cassini. Together with other observers back in France they determined the parallax of Mars and thus, solar parallax. 1671 was a particularly good time to observe Mars as it was then in opposition, the point at which the Earth and Mars come nearest to each other in their orbits. Their combined efforts produced a solar parallax of around 9.5 arc seconds, or approximately 140000000km.

Extensive star catalogues, particularly that of John Flamsteed, the first Astronomer Royal, were also being published. Despite the much greater accuracy of the instruments, parallax stiU escaped detection. A new method, developed by Christiaan Huygens around 1650, initiated what could be called the photometric method of parallax determination. In this method, Huygens compared the observed brightness of the Sun and Sirius. On the assumption that both stars had the same intrinsic brightness, he estimated the distance of Sirius to be about 29000 times that of the Sun (that is, a parallax of about 3 arc seconds). This method was used by Newton, Gregory, and others after them, to arrive at possible stellar distances. The historical importance of this development lay in the assumption that the Sun and the distant stars are physically the same, since their apparent brightnesses could only give reliable distance estimates if this were the case.

Throughout the seventeenth century, estimates of solar and stellar distances increased as more precise data became available. The development of the micrometer, first used by William Gascoigne and later, independently, by Christiaan Huygens, contributed to this greater accuracy. Jean Picard was the first to introduce the systematic use of micrometer sights on the telescope in about 1667 after which they became a common tool of the observational astronomer. At first, however, there was the occasional objector: Johann Hevelius, for example, insisted on the superiority of naked-eye observations throughout his life. With the establishment of the Greenwich and Paris observatories during the second half of the seventeenth century, astronomers, assured of limited State support, began thorough investigations of planetary and stellar positions. In about 1720 Edmond Halley undertook at the age of 63 the task of obtaining precise lunar positions over a complete synodic period (18.6 years).

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