In October 1604 a completely new star suddenly appeared in the constellation of the Serpent-Bearer. The star was seen across the whole of Europe, and aroused a great deal of interest. At that time the public was practically obsessed with interpreting signs in the sky, and other places. Naturally, the star was viewed as a bad omen on the whole, because people were used to war, famine and disease.
New stars appearing in the sky was not a completely unknown phenomenon. They were labelled stella nova ("new star") or simply nova. The nova of 1604 was in fact what we now call a supernova, a very rare stellar catastrophe which for a short period increases the light output of the exploding star a billion times or more. It was the German astronomer Johann Kepler in Prague who first noticed the phenomenon - and so the 1604 nova is known as "Kepler's nova" in consequence, and is the most recent supernova registered in the Milky Way.
Neither Kepler, Galileo nor anyone else had the slightest explanation for how the nova had come about. What they were able to do, though, was to say something about how far away it was. And this was a question of the greatest astronomical, philosophical - and therefore also religious - interest.
Kepler, mathematician to the Imperial court, wrote a book - About the New Star - which was largely concerned with astrological speculations. The more rationalistic Professor Galilei gave three lectures on the subject. But they both shared the same opinion about its remoteness.
The key word was parallax, or the angle that can be measured when one observes an object from two different points. Naturally, the greater the distance between the observations, the greater the angle. But also, the closer the object is to the observer, the greater the angle if one moves and observes it from another place. (If something is close enough, we can clearly register parallax simply by looking at it with one eye and then the other.) Or the reverse: if one looks at a star from two different places, and cannot measure any change in the angle of vision, it must be extremely far away, at a distance that is of quite another order of magnitude to the distance between the observation points.
Galileo did not travel about observing the nova, but both he and Kepler could easily compare data from observations all over Europe. And on one point they agreed: there was no measurable parallax. In other words, the nova was very far away - considerably further away than the moon.
The reason must once more be sought in the Aristotelian-Ptolemaic system and the theological interpretations of it. In Aristotle there is a clear distinction between what is found under the Moon (more accurately: what is found within the sphere the Moon is attached to and which revolves around the Earth), and what is further away: stars, planets and the heavenly spheres pertaining to them.
Under the Moon - in the sublunar zone - all was composed of the four elements: earth, air, water and fire. Here mutability and transition reigned: seasons shifted, plants grew, bloomed and withered, people were born and died, balls fell heavily to the ground if they were dropped from towers. Beyond the Moon, however, quite different natural laws applied. Everything there was made up of one single element - ether or quintessence. This had no weight (otherwise everything would have fallen down on to the immobile Earth, the centre of the universe), and the only change or movement that took place there was the "natural motion" of the spheres, in perfect circles around the Earth. By contrast, all natural movement below the Moon is straight, as a ball falls, or raindrops fall from the clouds.
It is clear that this notion had profound theological implications. Eternal perfection reigned in the heavens; earthly existence was, on the other hand, characterised by temporal frailty and change.
So by definition a "new star" could not be a star at all - as in that case a change must have occurred in the heavenly sphere where the fixed stars belonged. The nova must be some kind of natural phenomenon in the space between the Earth's surface and the Moon - in the same category as the northern lights, or the clouds for that matter.
If Galileo and Kepler were to be believed, Aristotle had made an elementary mistake on this point as well: heaven was not perfect and immutable. If the new star was not a direct argument in favour of Copernicus, it certainly put another question mark against accepted wisdom.
For Galileo, perhaps the most important result of the nova was that he had to apply himself seriously to astronomy, a corner of the "curriculum" he had not studied in depth up to that point. But he was certainly aware that the parallax question could also be turned into a serious argument against the Copernican theory of the Sun in the centre and the Earth in orbit, presumably the best scientific argument the Church and the defenders of tradition had.
If the Earth really orbits the Sun, said the sceptics, it must move an enormous distance in the course of the year. So, if we observe a star in the spring, and make the same observation in the autumn, the Earth will, in the meantime, have moved through space to a point diametrically opposite on its orbit, a distance many, many times greater than any we can measure on the Earth's surface. So why can we still not measure any parallax for that star? ("The greater the distance between the observations, the greater the angle.")
Copernicus had himself answered this objection. The parallax is there, but because the stars are so very far away, even in comparison to the Earth's orbit round the Sun, it is almost unmeasurably small. But this undeniably had the incontrovertible feel of the ad hoc argument. Anything can be proved if one can postulate data at random. (The argument was in fact right, but stellar parallax was first measured two centuries later, in 1838.)
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