by Doris Vickers (Vienna)
It is a Copernicus year. We celebrate his 550th anniversary of birth and 480th anniversary of death this year and this alone would be reason enough to write a blog post. However, it is not his theory that my thesis is concerned with, but the theory that suggested he was right. We need to step back in time to get a clearer picture.
Copernicus changes the world
Ever since Aristotle, people had thought that the Earth is the immovable centre of the Universe. Ptolemy came up with the mathematical model to accurately calculate the seemingly weird and wonderous paths of the planets within this system and for almost 1,500 years, people had no reason to think of the Universe as something other than an elaborate system of spheres that moved the planets and stars to their calculable positions in the night sky.Read more: When were Kepler’s laws accepted?
Then Nicolaus Copernicus (1473-1543) came along. He thought it would be much easier if the Earth was removed from the centre of the Universe and the Sun was placed in the centre of the Solar System (not the Universe), with the planets orbiting the Sun on perfect spheres.
His book De Revolutionibus Orbium Coelestium (“On the Revolutions of the Heavenly Spheres”) was published shortly before his death in 1543 but failed to make a huge impact on the scientific community – the initial 400 printed copies did not even sell out. Nevertheless, the idea that something was amiss with the mathematics of the Solar System was out there and other astronomers at the time started looking into empirical evidence for either Ptolemy’s or Copernicus’ world system.
Tycho finds a different solution
Empirical evidence, however, spoke against the Copernican system because there was no (at the time) satisfying answer (and proof) to the question why, if the Earth was moving and rotating, scientists could not detect this motion – the natural sciences would only develop the answer to this problem (gravity) in the 17th century.
This led Tycho Brahe (1546-1601) to invent a model of the Solar System that was halfway between Ptolemy and Copernicus: Brahe placed the Earth back in the centre of the system, with the Sun, the Moon and the fixed stars orbiting it, while the other planets (Mercury, Venus, Mars, Jupiter, Saturn) orbited the Sun.
Johannes Kepler as a data scientist
Johannes Kepler, who had been Tycho Brahe’s assistant in Prague since February 1600, was a brilliant mathematician but no observer because he was left with weak vision after contracting smallpox as a child. On top of that, he had no access to the observational data until Brahe’s death in 1601 – what followed was Kepler’s most productive phase. Kepler closely inspected Brahe’s data for clues about the pattern of the path of the planet Mars to improve Brahe’s Solar System model.
But what Kepler found was something else entirely. He realised that the path of Mars was not a perfect circle, but an ellipse and he replaced the planetary sphere by an orbit. Further research into the data went on to show that the paths of the remaining planets were in fact also elliptical – Kepler had discovered his first law of planetary motion!
Using the data, he also drew up a formula about the relationship of a planet’s motion to its distance from the Sun. After several attempts, he came up with a seemingly simple formula: planets sweet out equal areas in equal times – now known as the second law of planetary motion:
Kepler published these research results in the 1609 Astronomia Nova ΑΙΤΙΟΛΟΓΗΤΟΣ seu physica coelestis, tradita commentariis de motibus stellae Martis ex observationibus G.V. Tychonis Brahe (“New Astronomy, reasoned from Causes, or Celestial Physics, Treated by Means of Commentaries on the Motions of the Star Mars, from the Observations of the noble Tycho Brahe”). Kepler’s third law of planetary motion was published in his 1619 Harmonices mundi libri V (“Five books about the Harmony of the World”), after having what he described as an epiphany on March 8, 1618 (although Kepler does not explain how he arrived at the third law), that the square of the periodic times are to each other as the cubes of the mean
Between 1609 and 1619 Johannes Kepler had given the world the key to the riddles of the Solar System and the mathematical model to accurately calculate positions of the planets. And yet, the scientific world had a hard time accepting them.
Arguments for and against
Johannes Kepler had used Tycho Brahe’s observational data to calculate and compile the so-called Tabulae Rudolphinae (“Rudolphine Tables”), a star catalogue and planetary tables published in 1627 and named in honour of Holy Roman Emperor Rudolf II. These tables were so accurate that astronomers who used them drew the conclusion that the fact that Kepler had used his planetary laws to calculate them means that these laws must be true and represent the true nature of the Solar System. Kepler’s three laws were not equally easy to accept for astronomers and most had serious doubts specifically about the second one – Aristotle’s uniform, circular motion was not easily let go of.
But the true stumbling point for Kepler’s laws was the same argument that astronomers brought up when Copernicus formulated his model of the Solar System – if the Earth moved, then we should feel it, or at least be able to measure it. One direct evidence for the Earth’s movement is something called a “parallax” – a star shifting against the stellar background when observed from two points on opposite sides of the Earth’s orbit.
The usual answer to the lack of detection of a parallax was that the stars were simply too far away for this slight shift to be detected.
Along came James Bradley
Since the stellar parallax was generally regarded as the easiest and most direct evidence for the motion of the Earth, several astronomers specialising in observation tried their luck. One of these astronomers was Englishman James Bradley (1692-1762), who observed Gamma Draconis, a star in the constellation Draco, together with Samuel Molyneux (1689-1728) to determine its parallax.
Bradley had already calculated the expected parallax, but the star failed to show the predicted motion. They improved their equipment and continued to observe Gamma Draconis and other stars and found an entirely different cyclical motion instead. Molyneux died before Bradley came up with a solution to the problem: the direction of light coming from the stars is influenced by the Earth orbiting the Sun – a phenomenon now called the “aberration of light”.
The stellar parallax, which James Bradley and Samuel Molyneux were after, was not detected until 1838, when the telescopes used by Friedrich Bessel (1784-1846) were finally developed enough to measure it.
Foucault and the demonstration of Earth’s rotation
The first direct evidence of the Earth’s rotation was given by a simple yet ingenious device conceived by French physicist Léon Foucault in 1851. He had a heavy pendulum suspended from the roof of the Panthéon in Paris that showed over an extended period that the plane of oscillation rotated.
If therefore these oscillations are perpetuated for a certain time, the movement of the earth, which does not cease to turn from west to east, will become sensitive in contrast to the immobility of the oscillation plane whose trace on the ground will seem animated by a movement consistent with the apparent movement of the celestial sphere; and if the oscillations could be perpetuated for twenty-four hours, the trace of their plane would then execute an entire revolution around the vertical projection of the point of suspension.Léon Foucault, “Démonstration physique du movement de rotation de la terre au moyen du pendule.”, p. 364.
Nowadays, Foucault’s pendulum is one of the most famous scientific experiments in the history of mankind, with pendula suspended from many a museum’s ceiling.
The take-away message is: it has taken over 300 years to scientifically prove the correctness of Copernicus’ system. The proverb “Good things come to those who wait” turned out to be true, although that did not help Copernicus, who died without getting the recognition he deserved during his lifetime.
- Bradley, James (1728). “A Letter from the Reverend Mr. James Bradley Savilian Professor of Astronomy at Oxford, and F.R.S. to Dr. Edmond Halley Astronom. Reg. &c. Giving an Account of a New Discovered Motion of the Fix’d Stars”. Philosophical Transactions of the Royal Society of London. 35: 637–661.
- Foucault, Léon (1851). “Démonstration physique du movement de rotation de la terre au moyen du pendule.” In: Comptes Rendus Hebdomadaires des Séances de l’Académie des Sciences. Volume 32, Number 5. Paris: Bachelier, 3 February 1851.
- Understanding, disseminating, and interpreting Kepler : the role of Giovanni Battista Riccioli and the law of orbits / Marcacci, Flavia, 1976- in “Società italiana degli storici della fisica e dell’astronomia : Atti del XLI Convegno annuale = Proceedings of the 41st Annual Conference : Arezzo, 6-9 settembre 2021. – ( Atti / Società italiana degli storici della fisica e dell’astronomia) – Pisa : Pisa University Press, 2022- Casalini id: 5327302”, p. 307-316. DOI: 10.12871/978883339694137
About the author
Doris Vickers is a Classical Philologist and currently pursuing a PhD about Wilhelm Schickard’s Astroscopium, a text dating to 1623 that deals with contemporary issues of astronomy, star names and star maps. During her research, she often comes across interesting factoids that she would like to share with you here. She is also an associate member of the International Astronomical Union (IAU) and the Etymology Task Group Chairperson of the Working Group on Star Names (WGSN).