People got great pleasure from watching the spectacular display of the recent HaleBopp comet. Such celestial displays played an enormously important part in the history of science. Simple observations and measurements, coupled with brilliant reasoning, dismantled the classical model of the universe as proposed by Aristotle and Ptolemy, and paved the way for the modern scientific revolution.
In the Aristotelian model of the universe, as intricately elaborated in the second century AD by the Alexandrian astronomer Ptolemy, the Earth sat at the centre of a massive machinery of solid concentric spheres that some believed to be more transparent and harder than diamonds. Each of the spheres revolved around the Earth, carrying one of the moving heavenly bodies - the moon, the sun, or one of the known planets. The outermost sphere carried the stars.
The universe beyond the earthly sphere of air was thought to be immutable, thereby reflecting the perfection of the Godly heavens. Birth, death, decay and change were possible only in the earthly sphere of air, inside the sphere of the moon. Obviously therefore, if changes and imperfections could be demonstrated in the universe beyond the earthly sphere, this would seriously challenge the Aristotelian model. This is in fact the way it happened, beginning in 1572.
Copernicus proposed in 1543 that the sun, not the Earth, sits at the centre of our solar system. His proposal didn't create much of a stir because it was based on a technicality. The Ptolemaic model of the universe worked so well and accorded so fluently with common observations and the theology of the church that it was pub under little pressure by the proposal of Copernicus. It would take practical measurements based on careful observations to rattle the Ptolemaic model and this was made possible by a fortunate series of celestial phenomena that occurred over the period 1570-1640.
In November 1572, the Danish astronomer Tycho Brahe saw a brilliant new point of light in the constellation Cassiopeia. It remained fixed in the sky for over a year, though it dimmed steadily. Tycho made extensive observations and measurements of this "new star", and wrote a book, Del Nova Stella (On the New Star). That book helped pave the way for one of the greatest intellectual revolutions in history.
WORD of the new star quickly spread, but was greeted with widespread disbelief. The Aristotelian model held sway and the appearance of a new star was impossible to accept for most astronomers of the day. Change was allowed only in the terrestrial sphere of air, and, there fore, if something new appeared in the sky it must reside in that sphere. Most astronomers explained the new light as some eruption of fire or vapour from the Earth. Others claimed that it was a comet (comets were believed to move in the earthly sphere), even though it never moved.
Tycho Brahe had a problem with the proposal that the new object was close to the Earth. His measurements showed that the new light had no detectible parallax. What is parallax? Hold your finger a few inches before your eyes and look at it with just your left eye, and then with just your right eye. Your finger will change position against the background as you change your angle of view.
Repeat this procedure with your finger at arm's length. This time your finger will not shift so much. The size of the shift can be used to calculate the distance of the finger from the eye. Parallax can also be used to measure the distance of celestial objects from the Earth. The absence of parallax for the new star virtually proved that it was well beyond the terrestrial sphere - "in heaven itself", as Tycho wrote.
Five years later the solid spheres of Aristotle were dealt another blow with the arrival of the great comet of 1577 - the largest comet ever seen. Tycho observed and made measurements on the new comet with the same enthusiasm he had shown for the new star. He was intrigued at the possibility that comets, the very embodiment of change, thought to move within the sphere of air, in fact also roamed the perfect heavens. Tycho's observations convinced him that the comet showed no parallax. His calculations also showed the comet to be at least 230 Earth radii away from the Earth, well beyond the 52 Earth radii separating the moon from the Earth. The evidence that comets reside beyond the moon was more destructive to the traditional cosmology than were the observations on the new star, because sightings of comets were common.
In 1604 another brilliant star appeared in the sky. Johannes Kepler and others could measure no parallax for this star, reinforcing the conclusion that change was possible in the realm of the fixed stars. The influence of astrology, however, remained strong. Every event was invested with great meaning and many astronomers thought that new stars and comets foretold impending disasters.
The appearance of the new star in 1572 was thought to foretell some great historical event. However, by 1604 when a second new star appeared, and no fittingly momentous historical change had happened since the star of 1572, many scholars began to accept that such celestial events were simply natural occurrences and not signs from God.
The stars of 1572 and 1604 were probably supernovas, the massive explosions of dying stars. Indeed, 20th century astronomers have detected a radio source, like the emissions from the remains of a supernova, close to the position of the 1604 star as measured at the time by Kepler. Possibly the most important effect of the supernova of 1604 was that it helped to direct Galileo's attention to astronomy. He gave three public lectures about the star.
The 1604 star faded from view after a few years, and Galileo turned the newly invented telescope on the heavens for the first time, thereby ushering in a new age of astronomy. The phenomena visible through the telescope, from the pockmarked appearance of our moon, to the moons of Jupiter, to the moon like phases of Venus, were devastating to Aristotelian cosmology. Also, in 1618 three spectacular comets appeared in quick succession. The last of them was as large as the comet of 1577 and a number of astronomers calculated that this comet had no parallax. Galileo used these data, and others, to mount a strong attack on traditional cosmology in two books - The Discourse on Comets, published in 1619, and The Assayer, published in 1623.
Another fortunate celestial phenomenon also supported Galileo's argument. Sunspots are common, but the crude early telescopes would have difficulty seeing most spots. However, the first telescopes looked at the sun at a time when spots were unusually common. In 1634 a French astronomer reported 32 spots at one time, a record for the early modern era. Galileo believed that the sunspots were smoky emissions from the centre of the sun and, as early as 1612, he declared that these spots disproved the Aristotelian claim that the sun was perfect and unchangeable.
Galileo fully acknowledged the part played by the observations of these celestial phenomena, the supernovas, the comets and the sunspots when he wrote in his Second Letter on Sun Spots - "so when he (Aristotle) argued the immutability of the heavens from the fact that no alteration had been seen in them during all the ages, it may be believed that had his eyes shown him what is now evident to all of us, he would have adopted the very opinion to which we are led by these remarkable discoveries".