An astronomical clock designed before the discoveries of Galileo and Copernicus

On May 24, 1543, Nicholas Copernicus lay in bed, dying of brain haemorrhage. It is said that a copy of his book De Revolutionibus Orbium Coelestium — the publication of which he delayed by nearly 30 years — was brought to his deathbed so that he could have a last glimpse of it. In this book, he had detailed a celestial system with the Sun at the centre and the planets going around it in fixed orbits. Copernicus had stopped the Sun and set the Earth in motion.

Today, we take for granted the idea of a solar system made of the Sun at the centre with eight planets and thousands of other smaller bodies orbiting it. Have you ever wondered how tortuous was the road to this knowledge? It is a tale of extraordinary blunders, irresponsibilities and damaging effects of religious suppression of science.

All ancient civilisations knew of five (and only five, not nine!) planets — Mercury, Venus, Mars, Jupiter and Saturn. However, their motion in the sky posed a puzzle. Of these five planets, the first two do not move too far away from the Sun (as for instance, one never sees Venus overhead at night). The other three are known to exhibit a strange behaviour called “retrograde motion”. These planets systematically move through the sky in one direction for some time, stop and backtrack for some time and again move forward. These features are difficult to understand if the Earth was at the centre of the universe and all the planets were moving around it.

Around 350 BC, there lived a Greek astronomer, Aristarchus, who wrote a short treatise proclaiming that the Sun — and not the Earth — was at the centre of our world and that all planets revolved around it. The book became a classic of antiquity and both Archimedes and Plutarch knew about this work. So we should have had a helio-centric theory (Sun at the centre) of the solar system all the way from 350 BC, right? No! Incredibly enough, Aristarchus was forgotten. Instead, the geocentric system (Earth at the centre) of Ptolemy, a much more complicated and aesthetically unappealing model, held sway even around the second and third centuries AD. The key architect of this model, Ptolemy (127-145 AD), drew extensively on the work of another Greek astronomer, Hipparchus (190-120 BC), and developed a complex scheme which had the Earth at the centre of the universe with the Moon, Mercury, Venus, Sun, Mars, Jupiter and Saturn (that still makes seven, not nine!) revolving around it. He had to use a system of circles, whose centres moved on another system of circles (usually called epicycles) and so on, to explain the observations.

After the decline of Greek civilisation, their heritage passed on to the Romans who did not bother too much about this legacy. It fell on the Arab civilisation — which was the dominant force around the Mediterranean with Cordoba, Baghdad, Damascus and Samarkhand emerging as centres of learning and culture. (The streets of Cordoba were paved and lit by lamps — amenities not available in London or Paris for another seven centuries!)

The Caliphs in Baghdad encouraged translations of every major scientific work into Arabic. The “western” science of the later centuries originated from the study of these Arabic texts, retranslated into Latin. Later, as Europe went through the Renaissance, one would have again hoped for the right ideas to emerge. But the strong religious dogmas and theological interpretations of Aristotle’s outdated ideas suppressed the truth for centuries. As A.N. Whitehead, an English historian of science, remarks: “In the year 1500, Europe knew less about nature than Archimedes, who died in 212 BC, knew.”

Of course, the system developed by Ptolemy soon started cracking up. The predicted positions of the planets were getting to be far away from the observed ones in spite of the several ad hoc corrections introduced by later astronomers. It occurred to Copernicus — born in 1473 in Torun in Eastern Poland, who studied medicine and canon law for 10 years before turning to astronomy — that the calculations could be considerably simplified if one adopted the heliocentric system. Copernicus’ genius was in putting this idea into practice and meticulously working out the details of the new model. Interestingly enough, he relied on the observations of others, mainly because he was not good in this. His instruments were less accurate than those used in Alexandria 2,000 years ago, and he made a mistake even in getting the terrestrial co-ordinates of his observatory right!

All these problems regarding the motion of planets disappear if we assume that the planets revolve around the Sun in the order of Mercury, Venus, Earth, Mars, Jupiter and Saturn. Because the orbits of Mercury and Venus are closer to the Sun than that of the Earth, they will never appear overhead at night. The backwards motion of the other three planets is also easy to understand: if the Earth revolves around the Sun at a faster rate than the outer planets, then the earth will “overtake” these planets every once in a while. Seen from the Earth, the outer planets will appear to go backwards. Also, since the distance between the Earth and Venus varies considerably, the appearance of this planet will be altered periodically — something that was known from ancient days.

Thus the Copernican idea could at one stroke resolve all the discrepancies faced by the earlier astronomers. The only place where Copernicus erred was in sticking to the circular motion. As a result, he also needed epicycles in his model (in fact, Copernicus used more epicycles than Ptolemy did!). And they remained, till Kepler later changed the paths of the planets to ellipses, which in turn played a crucial role in our understanding of gravity. But that is another story which we will explore later.

T. Padmanabhan is an astrophysicist with the Inter-University Centre for Astronomy and Astrophysics, Pune. This is the first of a new series of articles on the wonders of the cosmos.