For centuries it was commonly believed that heavenly bodies must (i.e., can only) move in circular paths.
This is an example of a widely accepted historical scientific conception that impeded the progress of science.
Read about historical scientific conceptions
Read historical examples of alternative conceptions
We now understand that bodies moving through space can take quite complicated paths. Newton's first law suggests they should continue to move in a straight line unless acted upon by some force. However, Newtonian theory also suggests that every body in the universe is subject to gravitational forces from its interactions with all the others, so the default situation is that a moving body will be accelerated by a net force acting on it.
Newtonian gravitation was seen (even by Isaac Newton himself to some degree) as something of an occult influence as it appeared to act as a distance as if by magic. Current scientific theories have replaced the idea of a gravitational force with theories about the complex topology of spacetime – but for most purposes Newton's theory works well as a description of the universe (as well as being much easier to understand).
So, if we take the solar system:
To a first approximation, the earth is subject to a gravitational attraction with the sun*, which causes it to curve around rather that follow a straight path – and so follow an elliptical path. (Learners commonly find difficulty in understanding how orbital motion of this kind is accelerated, due to an unbalanced centripetal force.)
Read about conceptions of planetary orbits
[* I deliberately wrote 'the earth is subject to a gravitational attraction with the sun' rather than 'from' as, as is required by Newton's third law, the force is a mutual interaction and acts equally on both bodies.]
However, the earth is also attracted to the moon, so that causes a 'wobble' in the elliptical path as it moves around the sun. The earth is also attracted by the other planets in the solar system, and although the resulting forces tend to be VERY much smaller than that due to the interaction with sun, they still have small effects. Indeed, technically, the earth's path will be modified, if ever so slightly, by a passing comet.
However, for most practical purposes we can describe the orbits of the planets about the sun, and of moons around planets, as, near enough, elliptical. This will also apply to any other similar solar systems elsewhere in the universe (and scientists are regularly adding to the list of stars that observations suggest also have orbiting planets.) The conditional 'similar' is important: scientists know that a good proportion of stars are in dual systems where two stars orbit each other (or, to be more pedantic, orbit their shared centre of gravity) and, if planets can exist in stable orbits in such systems, their orbits are likely to be more complex.
A circle is an ellipse
Now, strictly, a circle is an ellipse in the same way that a square is a rectangle. A rectangle has four right angles, and the square is a the special case when adjacent sides have the same length. The circle is the special case of the ellipse with zero eccentricity (as the more eccentric an ellipse, the greater the difference between the lengths of its two principle axes – that is the more 'squashed' it appears compared with a circle). So planets could have circular orbits, and most of the planets in our solar system only have moderate eccentricity, so their elliptical orbits are 'not far off' from being circular.
So, why is it so important to emphasise so much that the orbitals are (approximately) elliptical if in fact they are mostly (approximately) circular anyway? This seems something of a detail, but it reflects a major conceptual block that had to be overcome in understanding the solar system: a block that impacted on the great scientific revolution in astronomy.
Copernicus as a limited revolutionary
Nicolaus Copernicus is often considered one of the great revolutionaries in science. He published a great work that suggested it was easier to understand the solar system if the earth was not seen as fixed at the centre of the universe, but rather if the earth moved – both by rotating and circling [sic – see below] around the sun. This seems obvious now, but at the time was not just against commonly accepted religious dogma (that is the official and enforced interpretation of Christian scriptures) but considered by many as a silly idea as if the earth was moving through space at great speed we would expect to fly off! (Conversely, if the earth was indeed still, it meant the distant stars must be moving around the earth at incredible speeds; but as the nature of those stars was not well understood many people preferred to believe that.)
So, Copernicus moved the earth (so to speak) from the centre of the world, which really was revolutionary, and in doing so he provided astronomers with an alternative account for the motions of the heavenly bodies – which explained for example the 'retrograde' motion of planets when they seem to change direction completely, before later then changing back again to their original course. According to Copernicus, this strange looping motion was just an optical illusion – the appearances were saved (that is, observations explained) by assuming the planets moved steadily, but were being seen from the earth which was itself moving at a different speed to any other planet.
These ideas were indeed revolutionary. Galileo was much later tried before the Inquisition because he taught Copernicus's doctrine as a true account of the world (rather than just an 'instrumental' theory that worked for calculating the future positions of planets, as if some kind of fluke or trick), and Copernicus's great book was banned in Catholic countries. Copernicus himself had thought his model was correct (not just a calculating device) but when his book was first published it came with an anonymous preface encouraging readers not to assume it was meant literally.
But it was a limited revolution (Kuhn, 1957). In particular, Copernicus's model was very complicated (much like the version it was meant to replace). It had to be complicated, because he retained the use of circles as the basis for all heavenly motion. (It is easy to describe an elliptical path with an ellipse – but no one single circle will match it!)
The preexisting account, based on the astronomy of the great Greek scholar Ptolemy, saw all the heavenly bodies (the Sun, the planets, the stars) as moving in circles. Now, there is a key point here – these are not approximate circles (like the elliptical orbit of Venus which as eccentricity of 0.006 8, so near enough zero – a circle- for many purposes) but perfect circles.
The perfect heavens
It is very hard for the contemporary person today to understand the medieval world view. Today, we have all seen photos taken of the planets by the Pioneer and Voyager and other probes. We have all seen film of human beings walking on another world, the moon {even if a few people refuse to believe it is genuine}. We know that Star Trek and 2001: A Space Odyssey, and Alien, and Dark Star, and all the rest are just fiction – but many of us consider the details of these stories as fiction while the underlying premise of the possibility of space travel and meeting sentient beings elsewhere is not fantasy but simply in the future. We see elsewhere in the universe as bring much like here, only different. It is all made of the same stuff, and follows the same natural laws, even if conditions elsewhere lead to outcomes that are quite different to here on earth.
That is quite alien to how most people (in Europe at least) thought before the Copernican revolution. The sublime heavens were considered to be quite distinct from mundane earth – made of different stuff, and (prior to Newton) following different natural laws. There was considered to be a strong connection, such that what happened in the skies somehow showed sympathy with events on earth (thus astrology as a misguided but very much genuine attempt to understand the relationship) but it was very much a separate realm. Heavenly bodies were perfect and whereas the norm on earth is for decay and change, the heavens were unchanging and corruptible. Comets were long believed to be -'sub-lunary', even atmospheric, phenomena – and it is not just coincidence that meteors and meteorites have names that reflect meteorology – the study of the Earth's atmosphere!
To suggest otherwise could get people into trouble. Giordano Bruno not only accepted the key ideas of Copernicus, but went as far as to suggest that stars might be other suns, with their own planets (and their occupants) around them. Bruno was burnt at the stake by the Catholic Church for his unconventional ideas (though not specifically these ones about other worlds: he had quite a few ideas the authorities did not like – luckily the Inquisition shut down well before the advent of Twitter and Bluesky).
So when Copernicus set out to develop his ideas, he was restricted by two sets of conditions – both that he should produce a model that would fit with what astronomers could observe in the sky, and with existing 'hard core' commitments for how the heavens were, including motion being circular:
"Before these principles, assumptions, or hypotheses can be accepted as true, they must meet two requirements. First, they must save the appearances (apparentias salvare): the results deduced from them must agree with the observed phenomena within satisfactory limits of error. Secondly, they must be consistent with certain preconceptions, called 'axioms of physics', such as that every celestial motion is circular, every celestial motion is uniform, and so forth. Disagreement with the observations is no more grave a defect than departure from | the axiom of uniform motion: apparentias salvare and aequalitatem tueri are equally essential."
Rosen, 1959
So, despite its revolutionary nature, Copernicus work was also conservative in many ways, and he started from the assumptions that
"1. There is no one center of all the celestial circles or spheres.
2. The center of the earth is not the center of the universe, but only of gravity and of the lunar sphere.
3. All the spheres revolve about the sun as their mid-point, and therefore the sun is the center of the universe."
Copernicus, 1514
The reference to spheres was the traditional ideas that the planets were embedded in invisible crystalline spheres, and it was these spheres that rotated, carrying the planets with them. So, although Copernicus shifted from the long held view that the earth was at the centre of the world, he placed the Sun there instead – so hardly a modern view where our solar system (and indeed galaxy) have no special place. Also, although he made the revolutionary step to seeing the earth as moving around the sun, he retained the requirement that heavenly bodies moved in perfect circles, such that to explain elliptical paths, complicated combinations of circles had to be employed in place of each (what we now know to be an) ellipse.
Copernicus boasted that "thirty-four circles suffice to explain the entire structure" of the known system (thirty-four circles explain the ballet of the planets) – but he had only actually needed one ellipse each for Mercury, Venus, Earth, the Moon, Mars, Jupiter and Saturn (the known planets): that is seven ellipses.
Even the great Kepler became encircled…
This mindset was not easily shifted. Johannes Kepler built on Copernicus's work, and finally presented a model based on ellipses rather than circles inexplicably moving within circles. However, Kepler struggled for a long time to make the data he had available (a set of observations of mars made by Tycho Brahe at an accuracy beyond what had been possible before) fit his models. Because, again, he started from an assumptions that he had to work with circles, and it was only after much frustrating failure to get his attempts to fit the data that he tried other options.
The idea that the heavens were perfect, and heavenly bodies were perfect spheres (requiring some imaginative explanations for why the moon looks as it does), and movement was based on perfect circles, took a long time to overcome. This was indeed an alternative conception that slowed progress in science.
Work cited
- Copernicus, N. (1959) The Commentariolus (First prepared in Latin in 1514, Translated by. E. Rosen), in Three Copernican Treatises (Ed. E. Rosen) Dover Publications.
- Kuhn, T. (1957) The Copernican Revolution. Harvard University Press.
- Rosen, E. (1959) Introduction to Three Copernican Treatises (Ed. E. Rosen) Dover Publications.
