The heliocentric model of the universe (also known as heliocentrism) is a now-obsolete theory which states that the Sun lies at the centre of the universe and that all other objects orbit around the Sun. It was first put forward (at least in recorded history) by Ancient Greek philosophers in the third century B.C., but it arose to greatest prominence in the Renaissance through the influence of astronomers such as Nicholas Copernicus, Johannes Kepler, and Galileo.
- Principles of Heliocentrism -
There are actually several heliocentric theories, meaning theories that the Sun is at the centre of the universe, but the most familiar of these is the model established in the European Renaissance by scholars such as Copernicus, Tycho Brahe, Kepler, and Galileo. In a heliocentric system, the Sun is a stationary object which rests at the centre of the universe, with other objects orbiting around the Sun at various distances and velocities in accordance with their mass and size.
Although this model is now recognized as obsolete, it was still a revolutionary development in European intellectual history. Philosophically speaking, it was a much greater step to move from the Earth being the centre of the universe to the Sun being the centre of the universe, than it was (later on) to move from the Sun being the centre of the universe to the universe not having any single definable centre point.
What happens to the non-planetary celestial objects in a heliocentric universe depends on the interpretation. Some Ancient Greek challengers to the geocentric consensus argued that most of the stars were stationary (but not at central points), while others argued that they too followed predictable paths around the Sun and the universal centre.
- From Geocentrism to Heliocentrism -
It is entirely understandable that early human civilizations, watching the skies revolve above their heads, would conclude that the seemingly flat surface of the Earth rested somewhere near the centre of the universe, and that all other things revolved around us above our heads. After all, the Sun followed a very fast course through the sky, the Moon a somewhat slower one, and the stars even slower ones - but all seemed to follow regular paths overhead. The early Ptolemaic model of the universe, named after the systematic efforts of Claudius Ptolemaeus in the 2nd century A.D. but mostly dating to pre-Christian Greek philosophy, established an elaborate system of spheres-within-spheres to explain why the planets did not follow completely regular orbits through the sky like the Sun and Moon did.
Other Ancient Greek philosophers, however, gradually realized that something else was going on. The Ptolemaic system explained why the planets sometimes seemed to move backwards through the sky relative to their overall orbit (it said this was because they were actually orbiting in a small epicycle, which in turn orbited in a large circle around the Earth). It claimed that the Moon was the closest object, followed by Mercury, Venus, the Sun, Mars, Jupiter, Saturn, and the stars. Without actual physical evidence, Philolaus proposed in the 4th century B.C. that the Sun, the Earth, and the planets all revolved around a fire at the heart of the universe, and that the stars were stationary. The inhabited surface of the Earth always faced outward (a process we now recognize as tidal locking, which is also why the same surface of the Moon is always visible from Earth), so that we never realized the great fire was burning beneath us. Several years later, Aristarchus of Samos confirmed through his estimates of the size of the Earth, Moon, and Sun, and the distance between them all, that the Earth must be orbiting the Sun rather than the other way around.
No further critical thought on the subject appears to have happened in Europe during the Roman or medieval ages, although some work on the subject did happen in India and in the Islamic world. However, during the Renaissance, European philosophers familiar with the latest texts emerging from the Muslim world, such as Nicholas Copernicus, began to put forward a new model of the universe. Copernicus's work was subsequently added to by Tycho Brahe and Galileo, the latter of whom attracted infamous persecution from the Catholic Church after publishing a "debate" between geocentrism and heliocentrism, in which he clearly sided with the latter.
- Decline of Heliocentrism -
The idea that the Sun was not the centre of the universe, either, originated with Italian priest and astronomer Giordano Bruno, who argued in the 16th century that the stars in the night sky were physically equivalent to the Sun, except at far greater distances from the Earth (for which he was later burned for heresy). Bruno's views were not immediately popular, but we now recognize them as essentially correct: the Sun is one star among billions in our galaxy, which features other stars both much larger and much smaller than our Sun, as well as many of approximately similar size.
After the Renaissance, Enlightenment scholars used early telescopes to make substantial progress in the study of solar system bodies as well as the stars, so that it became commonly accepted that the Sun did not occupy a privileged position at the centre of the universe (as Bruno had claimed). However, it was not until the early 20th century that American astronomer Edwin Hubble confirmed through the study of several nearby galaxies, including Andromeda were too distant to be nebulae or dust clouds (as previously thought), and therefore must be galaxies of their own, immense clusters of billions of stars. This meant, in turn, that our own Milky Way Galaxy must be just one among many galaxies, just as the Sun is but one among many stars. That, in essence, is the framework which continues to underlie our understanding of the universe today.