A Little History Of Science: Where is the Centre of the Universe?
Every morning, the sun rises in the east, and every evening, it sets in the west. We can see it slowly move throughout the day, with our shadows long or short, in front or behind, depending on where the sun is. Try the experiment at midday, and see your shadow tuck up under you. Nothing could be so obvious, and since it happens every day, if you miss it today you can catch the show tomorrow.
The sun doesn’t go around the earth each day, of course. You can understand how difficult it would be to convince people that what seems so obvious is not really what is going on. Put it this way: the earth is the centre of our universe, because that is where we are when we look at the sun, moon and stars. It’s our centre, but not the centre.
All the stargazers in the ancient world had put the earth at the centre. Remember Aristotle? After him, the most influential Greek astronomer, Ptolemy, built on the careful noting of the position of the stars night after night, season after season, and year after year.
Looking at the stars on a clear night is a magical experience, and being able to identify the groups, or ‘constellations’, of stars is great fun. The Plough and Orion’s belt are easy to trace across the sky when there are no clouds. From the Plough you can find the North Star, and this helped sailors at night to continue to sail in the right direction.
There were problems with a model of the universe in which the earth is at the centre and the heavenly bodies move around it in perfect circles. Take the stars, for instance. They change their positions only gradually, as the nights pass. The spring equinox – when the sun is directly above the equator, making the day and night of equal length – has always been important for astronomers, and, in fact, for everyone. It occurs on either 20 or 21 March, and the 21st is the official first day of spring. The trouble is, the stars are in slightly different positions each first day of spring, which they shouldn’t be if they were moving in perfect circles around the earth. Astronomers called this ‘the precession of the equinoxes’, and they had to make complicated calculations to explain why this happens.
The movement of the planets was also a puzzle. When you simply look at the night sky with your naked eyes, the planets appear as bright stars. Ancient astronomers thought that there were seven planets: Mercury, Venus, Mars, Jupiter and Saturn, plus the sun and the moon, which they also called planets. They were obviously closer to the earth than what they called the ‘fixed stars’, which we call the Milky Way. Observing the planets created more problems than the fixed stars, since they do not move as if they are circling the earth. For one thing, their movement does not appear to be constant, and the planets sometimes seem to go back on themselves. To solve this problem, astronomers said that the point that the planets were spinning around was not actually at the centre of the earth. They called this point the ‘equant’, and this and other calculations helped stargazers explain what they could see in the sky at night without having to throw away the model entirely.
It meant that they could still assume that the earth was at the centre of things and that the other heavenly bodies revolved around it.
What would happen if instead of placing the earth at the centre of things, you put the sun there, and assumed that the planets (now including the earth as one of them) revolved around it? We are so accustomed to this view that it is hard to realise what a dramatic step it was. It went against what we see every day, it went against the teachings of Aristotle and (more importantly) of the Church, for in the Bible Joshua is said to have asked God to command the moving sun to stand still. But putting the sun at the centre of things was exactly what a Polish priest named Copernicus boldly did.
Nicholas Copernicus (1473–1543) was born and died in Poland, but he studied both law and medicine in Italy. His father died when Nicholas was ten years old, so his mother’s brother took charge of educating the clever young boy, at the University of Cracow, in Poland. When his uncle became Bishop of Frauenburg, also in Poland, Copernicus obtained a job at the cathedral. This gave him a secure income, enabled him to study in Italy, and when he returned, to continue his passion: studying the heavens. He built a roofless tower, where he could use his astronomical instruments.
Since there were not yet any telescopes, these instruments simply allowed him to measure the angles between various heavenly bodies and the horizon, and the phases of the moon. He was also very interested in eclipses, which occur when the sun, moon, or one of the planets gets in the way of another planet and becomes partly or wholly covered from our sight.
We don’t know exactly when Copernicus decided that his model of the heavens and the solar system (as we now call it) was better at explaining the observations people had been making for thou- sands of years. But in 1514 he wrote a short manuscript and showed it to a few trusted friends. He did not dare publish it. In it, he stated quite clearly that ‘the centre of the earth is not the centre of the universe’, and ‘we revolve around the sun like any other planet’. These were pretty definite conclusions, and during the next three decades, Copernicus quietly worked on his theory that the sun, not the earth, is at the centre of the universe. Although he spent many hours observing the heavens himself, he was at his best in thinking about what other astronomers had seen, and how their difficulties could be smoothed out by placing the sun at the centre and assuming that the planets rotated around it. Many puzzles, such as eclipses, or the strange forward and backward movement of the planets, fell into place. Besides, the sun has such an important role in human life, giving us warmth and light, that making it central was a way of recognising that without it, life on earth would be impossible.
Copernicus’s model had another very significant consequence: it meant that the stars were much further away from the earth than Aristotle and other earlier thinkers had assumed. Aristotle thought that time was infinite but space was fixed. The Church had taught that time was fixed (to a few thousand years before, when God created everything), and so was space, except perhaps for Heaven itself. Copernicus accepted the Church’s ideas of time and creation, but his measurements told him that the earth was much nearer to the sun than the sun was to the other stars. He also calculated the approximate distances from the sun to the planets, and of the moon from the earth. The universe was much larger than people had thought.
Copernicus knew his research would shock people, but as he got older, he decided that he should publish his ideas. In 1542, he finished his big book, De revolutionibus orbium coelestium (‘Revolutions of the heavenly bodies’). But by then Copernicus was a sick old man. So he entrusted its printing to his friend, another priest called Rheticus, who knew about his ideas. Rheticus began the job, but then he had to go to work at a university in Germany, and the task was entrusted to yet another priest, Andreas Osiander.
Osiander believed that Copernicus’s ideas were dangerous, so he added his own introduction to this great book, which was finally printed in 1543. Here he wrote that Copernicus’s ideas were not actually true, but were simply a possible way of solving some of the difficulties astronomers had long recognised with their earth- centred idea of the universe. Osiander was entitled to his own opinion, but he did a very dishonest thing: he wrote this preface as if it was the work of Copernicus himself. Since it was not signed by anybody, people assumed that this was what Copernicus meant to say about his ideas, and Copernicus was by then close to death and unable to do anything to correct the false impression that the preface gave. Consequently, for almost one hundred years, readers of this wonderful book assumed that Copernicus was merely playing around with ways to explain what you saw in the heavens each night, but not really saying that the earth went around the sun.
This preface made it easy for people to ignore the revolutionary message in Copernicus’s book. Many people read it, however, and its comments and calculations influenced astronomy in the decades after he died. Two especially important astronomers took his work even further. One of them, Tycho Brahe (1546–1601), was inspired by Copernicus’s insistence that the universe must be very large, so far away were the stars. Observing an eclipse of the sun in 1560 fired his imagination, and although his noble Danish family wanted him to study law, the only thing that really satisfied him was studying the heavens. In 1572, he noticed a new, very bright star in the night sky. He wrote about this nova stella (‘new star’) and argued that it showed that the heavens were not completely perfect and changeless. He built himself an elaborate observatory on an island off the coast of Denmark, and equipped it with the most advanced tools. (Alas, telescopes had still not been invented.) In 1577 he followed the path of a comet; these were generally seen as bad omens, but for Tycho, the comet’s path merely signified that the heavenly bodies were not fixed in their own spheres, since the comet cut across them.
Tycho made many important discoveries about the positions and the movements of the stars and planets, although he eventually had to close his observatory and move to Prague, where in 1597 he established a new astronomical observatory. Three years later, he made Johannes Kepler (1571–1630) his assistant. Although Tycho never accepted Copernicus’s model of the sun at the centre of things, Kepler had a different outlook on the universe, and Tycho left him all his notes and manuscripts when he died in 1601. Kepler was dutiful to Tycho’s memory and edited some of his work for publication, but he also took astronomy into an entirely new direction.
Kepler had a stormy, chaotic life. His wife and young daughter died, and his mother was put on trial for witchcraft. He himself was an intensely religious Protestant in the early days of the Reformation, when most authorities were Catholic, so he had to watch his step.
He believed that the order of the heavens confirmed his own mystical appreciation of God’s creation. For all that, his lasting contribution to astronomy was very hard-nosed and precise. In the midst of his writings, which are often difficult to understand, he elaborated three concepts that are still known as Kepler’s Laws.
They were extremely important.
His first two laws were closely related, and his discovery of them was helped by the careful observations of the movements of the planet Mars that Tycho had left him. Kepler studied these for a long time before he realised that planets do not always move at the same speed; rather, they move faster when they are closer to the sun, and slower when they are further away from it. He found that if you draw a straight line from the sun (at the centre of the universe) to the planet, it is the curve of the arc made as the planet moves that is constant, not the planet’s speed. This was his second law, and its consequence was his first law: that planets move not in perfect circles, but in ellipses, a kind of flattened circle.
Although gravity had not yet been thought about, Kepler knew that some kind of force was acting upon the planets’ movements. And he realised that the ellipse is the natural path of something revolving around a central point, as planets do around the sun. Kepler’s two laws showed that the ancient idea of perfect circular motion in the heavens was wrong.
His third law was more practical: he showed that there is a special relationship between the time a planet takes to revolve completely around the sun and its average distance from the sun. This allowed astronomers to calculate the distances of the planets from the sun, and to get a sense of how large our solar system is, but also how small when compared to the enormous distances between us and the stars. Luckily, at around the same time a scientific instrument was invented to help us look further into those distances. The man who turned the telescope into a tool of immense power was the most famous astronomer of all: Galileo Galilei.