Copernicus: when the Earth moved from the centre

In the morning the Sun rises in the east, travels across the sky through the day and sets in the west. At night the stars appear to turn in the same way. The eye says the Earth is still and the sky circles around it. For centuries most astronomers in the world made their calculations on this basis. Then, in the 16th century, an astronomer from Poland said that if the Earth is taken to be turning and moving around the Sun, many puzzles of the sky resolve themselves. His name was Nicolaus Copernicus.

From Toruń to Frombork

According to the MacTutor archive of the University of St Andrews, Copernicus was born in the Polish city of Toruń on 19 February 1473 and died in the Baltic coastal town of Frombork on 24 May 1543. He studied first in Kraków and then in the Italian cities of Bologna, Padua and Ferrara. After returning from Italy, he did his astronomical work in Poland.

The old map and its puzzles

The astronomical system then in use was that of Ptolemy, the second-century astronomer of Alexandria. According to MacTutor, in it the Earth stood still at the centre. This view prevailed for about 1,400 years. One major puzzle of the system was the “backward motion” of the planets. A planet such as Mars moves forward in the sky for a while, then appears to move backwards for some weeks, and then moves forward again. To calculate the positions of the planets, Ptolemy used small circles on top of circles, known as epicycles, and some other geometrical devices. This made the system quite complicated.

A short tract

According to MacTutor, around 1514 Copernicus wrote his ideas in a short handwritten tract and shared it with a few people. It was later called the “Commentariolus”, meaning “short commentary”. It contained seven basic points. Three of them were the most important:

• The daily turning of the stars from east to west is in fact the result of the Earth rotating on its axis.
• The Sun’s journey through the year is in fact seen because the Earth goes around the Sun.
• The backward motion of the planets appears because we are watching them from a moving Earth.

The last point was the most elegant. When a fast car overtakes a slower one, the slower car seems to go backwards. In the same way, when the Earth passes an outer planet, that planet appears to move backwards for a while. The puzzle for which complicated circles had been drawn now had a simple explanation.

The book of 1543

Copernicus worked on his major book for years but was in no hurry to have it printed. According to MacTutor, in 1539 a young German mathematician, Rheticus, came to him. He helped prepare an early summary of the system, the “Narratio prima”, and assisted with the publication of the book. Finally “De revolutionibus”, meaning “On the Revolutions of the Heavenly Spheres”, was printed in Nuremberg, Germany, in 1543, the year Copernicus died.

MacTutor also makes clear that the system of Copernicus was not the same as today’s solar system. In his system the Sun was not exactly at the centre but motionless close to it. Since he held all motion to be circular, he too had to use some small circles, or epicycles.

Those before and after

Copernicus was not the first to think of placing the Sun at the centre. According to MacTutor, the Greek astronomer Aristarchus, who lived roughly between 310 and 230 BC, also put forward the idea. It is mentioned in a book by Archimedes, but the Greeks did not accept it. The models of Copernicus also contain a device of two circles that Nasir al-Din al-Tusi had used at the Maragha observatory in Iran about 250 years earlier. According to MacTutor, many historians think it came from al-Tusi, while some disagree.

The work moved forward after Copernicus. According to MacTutor, the German astronomer Johannes Kepler accepted the Copernican system. Using years of observations by Denmark’s Tycho Brahe, he showed in 1609 that Mars moves not in a circle but in an elliptical orbit. The new observations made by Galileo’s telescope are discussed in instalment 49 of this series.

The lesson today

If you watch Mars closely for a few weeks on a clear night in Quetta today, you may see its backward motion yourself. The reason is the one Copernicus gave 500 years ago: we are sitting on a moving Earth. His real achievement was not a new instrument but a new point of view. When a puzzle becomes too complicated, perhaps the question needs to be seen from a different place.