The Day the Earth Stood Still

Anyone who lies on their back at night and watches the stars will notice that they crawl slowly across the sky, east to west. Everything seems to be moving up there – the Sun, the Moon, the planets, the stars. Everything except the Earth, which feels firm and unmovable against our backs. We know that this is an illusion, of course: the Earth is orbiting around the Sun, and the movement of the stars is actually evidence of Earth’s rotation. But this knowledge defies the experience of our senses: we feel that the Earth is unmoving beneath us; the Sun rises every morning. It is therefore not surprising that the belief that Earth is at the centre of the universe survived (and flourished) for so long. Put simply: it made sense at the time.

When we look back on the geocentric model of the Solar System, it might be easy to dismiss it as something borne out of ignorance, something that superior science has rendered obsolete. But it’s far from being a stupid system. In fact, it’s surprisingly elegant. Part of the reason it went unchallenged for so long is because it was able to adequately satisfy both observed astronomical phenomena and hegemonic theological beliefs.

So how was the system developed, and what features allowed it to survive for so long?

An illustration of the Ptolemaic system by Bartolomeu Velho (1568)
An illustration of the Ptolemaic system by Bartolomeu Velho (1568)

Revolutions in Astronomy

In the fourth century BCE, the philosopher Aristotle was the first to develop a coherent geocentric model of the Solar System, one would that survive – largely unchallenged – for the next 1,800 years. It borrowed heavily from Euclidean geometry, with the planets orbiting in perfect circles. Aristotle lay the foundations for all subsequent Greek science, and his Earth-centred cosmology would provide the basis for Christian interpretations of Creation.

The astronomer Ptolemy modified Aristotle’s system in the second century CE, preserving the circular orbits of the planets, but adding in epicycles – circles within circles – that were able to adequately explain their occasional retrograde motions. The refined system was therefore consistent with Scripture and scientific observations.

But by the early sixteenth century, as astronomy was becoming increasingly precise, the Ptolemaic system was no longer able to satisfactorily explain the observed planetary motions, and it was encumbered by excessive numbers of epicycles. Copernicus published On the Revolutions of the Heavenly Spheres in 1543, making a comprehensive argument for a heliocentric cosmology. Although his system was deeply flawed, it provided the inspiration for subsequent astronomers – particularly Kepler, Galileo, and Newton – to challenge the prevailing paradigm. Kepler’s deduction that planetary orbits were elliptical, rather than circular, allowed a break from the Euclidean geometry that had restricted all previous systems. Galileo’s telescopic observations gave credence to the heliocentric model. And Newton’s theory of gravity explained how the planets were able to move in their orbits. By the end of the seventeenth century, a modern heliocentric system had been created.

An illustration of the Copernican system by Andreas Cellarius (1660)
An illustration of the Copernican system by Andreas Cellarius (1660)

An Elegant System

Copernicus’s On the Revolutions of the Heavenly Spheres was published in 1543, yet it was another 150 years before his heliocentric cosmology was widely accepted by the scientific community. This was partly because traditional Aristotelian cosmology was entrenched in religious doctrine, but it was also because Copernicus’s system seemed unnecessarily complex compared to the prevailing Ptolemaic system.

The Greek word planētēs means wanderer or vagrant, referring to the way that planets seem to occasionally move backwards in their orbit. We now know that is because of the changing observation point of the Earth as it orbits the Sun. But Ptolemy had invented epicycles to explain these retrograde motions. And whereas even the most complex Ptolemaic model required 40 epicycles, Copernicus’s heliocentric system required an ungainly total of 48 epicycles.

Apparent retrograde motion of Mars, demonstrating how planets "wander" across the sky.
Apparent retrograde motion of Mars, demonstrating how planets “wander” across the sky.

Image credit: Eugene Alvar Villar

The Ptolemaic system was aesthetically elegant – everything moved in perfect circles, with the Earth at the centre of Creation. Theologians were therefore inclined to dismiss Copernicus on aesthetic grounds alone, believing that simplicity was evidence of God’s work.

An alternative system proposed by the astronomer Tycho Brahe in 1588 attempted to reconcile observed planetary motions with religious beliefs. Brahe proposed that the planets orbited the Sun, but the Sun itself still orbited a static, unmoving Earth. It had the advantage of preserving the geocentrism that was so important to Catholicism, but it also reduced the number of epicycles required to explain planetary movements. Even after Kepler disproved the Tychonic system, it remained popular throughout the seventeenth century, particularly within the Jesuit order.

Stellar parallax

One of the biggest arguments against heliocentrism was the lack of stellar parallax. Stellar parallax is the apparent shift in position of a nearby star against the background of different objects. Proponents of heliocentrism maintained that as Earth moved along its orbit, the stars closest to our Solar System would appear to shift in position slightly as we observed them at different angles. But no parallax was detected.

The good people at Wikipedia demonstrate stellar parallax.
The good people at Wikipedia demonstrate stellar parallax.

Image credit: Wikipedia

What no one had anticipated was that stellar parallax was unobservable at the time because it was so small – the distances between stars were so great. It was much easier to imagine that the stars were part of a fixed sphere that rotated around the Earth. Even astronomers such as Tycho Brahe, who rejected the Ptolemaic system, found it hard to reconcile such an enormous void between stars with their religious beliefs.

It wasn’t until 1838 that stellar parallax was first observed by Friedrich Bessel.

Gravity

The idea that Earth is hurtling through space, spinning furiously as it orbits the Sun, goes against our common sense. We can’t feel it moving. And, until Newton proposed his law of universal gravitation, no one had identified a force that could keep us stuck to the surface of a giant spinning globe.

The alternative explanation made better sense at the time: the Earth was the centre of the universe, and everything was attracted to the center. Aristotle had been quite clear on that point, and it was supported by common sense notions about physics. Copernicus’s heliocentric model, reducing Earth to just another planet, was unable to explain why objects are drawn downwards to the Earth.

Yet by the late seventeenth century, Newton’s theory of gravity was also able to explain how the planets orbited the sun and how the atmosphere didn’t escape from a spinning Earth. It proved Kepler’s laws of gravitation, cementing the heliocentric model in observable and mathematically correct science.

Theological reasons

Of course, one of the primary reasons that the Aristotelian system survived for so long was that it formed the basic cosmology of Christian belief. Aristotle had divided Creation into two regions: the sublunary region, which was everything below the orbit of the Moon, made from the elements of earth, water, fire, and air; and the superlunary sphere, which included everything above the Moon – the planets, and the outer sphere of stars, which were supported on great crystalline lattices made from the fifth element, quintessence.

The division of sublunary and superlunary was important to theology because it implied a rigid boundary between the secular and divine worlds. The superlunary region was immutable and perfect, exhibiting simple Euclidean geometry, whereas the sublunary region was prone to change, decay, and corruption. Any violation of this system contradicted dominant religious beliefs – for many people, it was unthinkable.

A heliocentric model also threatened the central message of the Bible. In 1600, Giordano Bruno was burned at the stake for a range of heresies and unorthodox beliefs, among them the proposition that each star was in fact a distant sun with its own system of planets, and the idea that the universe was infinite. This went far beyond even the nascent Copernican beliefs of his peers. But why did the Catholic Church find the idea of an infinite cosmos so threatening to their doctrine?

Under a geocentric model, Christ’s sacrifice at the centre of the cosmos redeems the whole of Creation. But a model with a plurality of stars and planets suggests the theologically unpalatable possibility of a plurality of Christs, one for each planet, which subtracts the uniqueness of his sacrifice. Theologians were forced to ask awkward questions. If Christ were indeed unique to Earth, do the inhabitants of those other planets live in a fallen state, awaiting redemption, or do they exist in an unfallen, paradisiacal state? None of these options were acceptable to the Church. This was a real anxiety for the Catholic Church in the early seventeenth century. (James Blish’s novel A Case of Conscience is an intriguing response to this theological problem.)

Additionally, a literal interpretation of the Bible encourages the belief in a geocentric model. In Joshua 10:12, the Sun and Moon were stopped in the sky. When Galileo was on trial, a large part of his defence was that the Bible should be read allegorically. However, politics at the time meant that the Church needed to take an uncompromisingly conservative view on the controversy, so Galileo was made to recant under the penalty of death. The fact that disagreeing with the Bible could lead to persecution under the Inquisition meant that most people were too afraid to speak out in the contrary.

Another illustration of the Copernican system by Andreas Cellarius (1660)
Another illustration of the Copernican system by Andreas Cellarius (1660)

Legacy

Although the geocentric model has been outdated for the last 400 years, its legacy is still visible, particularly in the way we speak about astronomical phenomena. Terms such as sunrise and sunset highlight a language that was formed under a geocentric worldview. They feel like a reminder of the past; the last remnants of a discarded system.

In many ways, the gradual weakening of the Ptolemaic system and the religious doctrine it represented paved the way for the Copernican Revolution. Its cumbersome number of epicycles prompted Copernicus to look for an alternative model, while changing views of religion in the broader tapestry of the Reformation allowed scientists and philosophers to consider unorthodox ideas without fear of persecution. For astronomers such as Kepler and Galileo, the urge to develop their own model was fuelled by the inconsistencies and failures they had observed in the older model. Perhaps the greatest legacy of the Ptolemaic system is, in fact, our modern understanding of the Solar System.

 

 

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