The geocentric model (in Greek: geo = earth and centron = centre) of the universe is a paradigm which places the Earth at its center. Common in ancient Greece after the discovery of the approximately spherical shape of Earth, it was believed by both Aristotle and Ptolemy. Most Greeks assumed that the Sun, Moon, stars, and planets orbit Earth. Similar ideas were held in ancient China.

The geocentric model assumes a spherical Earth; thus it is not the same as the older flat Earth model. Also, according to this model, the orbits were perfectly circular and not elliptical.

The geocentric model held sway into the early modern age; from the late 16th century onward it was gradually replaced by the heliocentric model of Copernicus, Galileo and Kepler due to the simplicity and predictive accuracy of that newer model.

In this model, a set of fifty-five concentric crystalline spheres were considered to hold the Sun, the planets, and the stars. These spheres (called deferents) revolved at varying velocities around the Earth to account for the rising and setting of celestial objects every day.

However, this simple model of the revolutions of spheres could not explain all astronomical phenomena. In particular, certain celestial bodies were observed to wander across the fixed fields of stars over time; mostly they wandered in one direction, but occasionally they seemed to reverse course. These "wandering stars" were given the name "planet", based on the Greek word "planetes", meaning "wanderer". To explain this strange retrogradation, Aristotle claimed that planets were attached, not directly to deferents, but to smaller spheres called epicycles. The epicycles were themselves attached to the deferents; the simultaneous revolution of both sets of spheres created an occasional apparent reversal of the planets' motions across the skies of the Earth.

Ptolemy further modified this model to more accurately reflect observations by placing epicycles upon epicycles, creating an extraordinarily complicated--but fairly accurate--depiction of the cosmos. He also displaced the Earth from the center of the universe, claiming that, while Earth was enclosed by the celestial spheres, the spheres actually revolved around a point called an eccentric, which was near the Earth but not quite on it.

This elaborate theoretical system stemmed largely from one important observation. If Earth did move, then one ought to be able to observe the shifting of the fixed stars due to parallax. In short, the shapes of the constellations should change considerably over the course of a year. In reality, the stars are so very much further away than the Sun and the planets that this motion (which does exist) is undetectable without careful telescopic observations using modern equipment, but the lack of any observable parallax was considered the death of any non-geocentric theory for a thousand years.

This view of a geocentric universe held sway for well over a millennium, but was seriously challenged by the publication of Copernicus's De Revolutionibus Orbium Coelestium in 1543, which posited that the Earth and the other planets instead revolved around the Sun. It would still be held, however, for many years, as at the time the Copernican system did not offer better experimental results than the geocentric system.

In December 1610, Galileo Galilei used his telescope to show that Venus went through phases, just like the Moon. This observation was incompatible with the Ptolemaic system. In the 4th century BC, Heraclides Ponticus had already proposed that both Venus and Mercury orbited the Sun rather than Earth. The observation by Galileo led to interest in the more accurate (but still geocentric) Tychonian system, and the heliocentric Copernican system, which both explain how the Sun is commonly in between the Earth and Venus (see Phases of Venus ).

At this time, geocentrism is rejected in scientific and popular opinion; and it is believed that the Sun is at the centre of the solar system, but not the universe (see Heliocentrism), so a better term for the modern view is geokineticism (i.e. that the earth moves, without asserting anything about the center of motion). However, Sir Fred Hoyle wrote:

The relation of the two pictures [geocentricity and heliocentricity] is reduced to a mere coordinate transformation and it is the main tenet of the Einstein theory that any two ways of looking at the world which are related to each other by a coordinate transformation are entirely equivalent from a physical point of view ... . Today we cannot say that the Copernican theory is “right” and the Ptolemaic theory “wrong” in any meaningful physical sense.¹

A small number of people still advance a geocentric model (see modern geocentrism). These people tend to be of a religious mindset and sometimes Creationists, because without the concept of God willing to create the Earth at the center of the universe, it would appear mysterious for it to be so. However, most Creationists do believe in the geokinetic model.²

The geocentric (Ptolemaic) model of the solar system is also of interest to planetarium makers, as for technical reasons a Ptolemaic-type motion for the planet light apparatus has some advantages over a Copernican-type motion.

Contents

1 The modern scientific point of view 1.1 "There is no special position." 1.1.1 Philosophical assumptions 1.2 "The cosmic microwave background radiation determines the only special velocity." 1.3 "The inertial frame is the only special rotation." 2 See also 3 Notes

The modern scientific point of view

The consensus of scientists today is

1. that there is no center or otherwise special position in the universe,
2. that there is a no special linear velocity determined by the laws of physics 'per se', but the velocity of the cosmic microwave background radiation could be considered special, and
3. that there is a unique rotational velocity in which Newton’s laws of motion hold.

"There is no special position."

This is known as the Copernican or Cosmological Principle. All the known laws of physics can be formulated without reference to any particular place, as long as an inertial frame of reference is chosen for the description. That this is true, as far as we can tell, at all places and has been true for all times is illustrated by the agreement of the laboratory value of the fine structure constant with that derived from the spectra of stars billions of light years away.

Even if the laws of physics are independent of any particular place, one might still ask whether the arrangement of objects in the universe points to a special place for the Earth. If Keplerian dynamics and Newtonian gravitation are applied, the Earth does not hold a preferred place within the Solar System. Neither does the Solar System appear to be in a preferred location within our Galaxy. Our galaxy is not in a preferred location within the Local Group. Furthermore, the consensus scientific opinion is that there is no evidence based on the distribution of astronomical objects that any particular position in the universe is special.

Philosophical assumptions

All the same, many cosmologist agree that the Cosmological Principle is a philosophical choice rather than one mandated by observation. E.g. Edwin Hubble agreed that many observations were consistent with a view that the earth is at or near the center of the universe, but he rejected it on philosophical grounds:

Such a condition [these red shifts] would imply that we occupy a unique position in the universe, ... But the unwelcome supposition of a favored location must be avoided at all costs ... is intolerable ... moreover, it represents a discrepancy with the theory because the theory postulates homogeneity.

George Ellis agreed that the no-center axiom is chosen for philosophical reasons, but it would be possible to construct a geocentric model that would explain the observations:

“People need to be aware that there is a range of models that could explain the observations,” Ellis argues. “For instance, I can construct you a spherically symmetrical universe with Earth at its center, and you cannot disprove it based on observations.” Ellis has published a paper on this. “You can only exclude it on philosophical grounds. In my view there is absolutely nothing wrong in that. What I want to bring into the open is the fact that we are using philosophical criteria in choosing our models. A lot of cosmology tries to hide that.”.

"The cosmic microwave background radiation determines the only special velocity."

All the known laws of physics can be formulated without reference to any particular velocity, as long as an inertial frame of reference is chosen for the description. Therefore if, from the point of view of physics, there is a special velocity in the universe, it can only be observed because some group of objects move with that velocity. The most popular choice of a reference is the cosmic microwave background radiation, whose velocity relative to the Solar System is about 700 km/s. It is also possible, with some modeling, to consider the local value of the velocity field of all galaxies, which is found to agree with the velocity of the cosmic microwave background radiation.

"The inertial frame is the only special rotation."

If the known local laws of physics are formulated in various frames of reference rotating relative to one another, the mathematical formulation of these laws vary. Generally, a centrifugal force and a Coriolis force, dependent on a direction and rate of rotation, must be introduced. In classical physics, these two forces are called fictitious forces because they do not obey Newton’s third law of motion. There are some special frames of reference, known as inertial frames, where these forces vanish. The rotation of these frames may be considered special, and indeed inertial frames are the only special ones known to physics. Equivalently, there is only one rotational frame of reference in which the axes of gimbal-mounted gyroscopes remain fixed.

In the framework of general relativity, the formulation of the laws of physics is identical in all frames of reference, even in rotating and accelerating frames. The fictitious forces are then a manifestation of the gravitomagnetism associated with the acceleration of the mass of the universe. This is the same effect that results in frame dragging, only in frame dragging the effect due to a rotating body is local and small. If the entire universe is rotating, the effect is massive. Even in general relativity, the (global) inertial frame of reference can be considered special, because it is the only one that allows the laws of physics to be formulated without explicit reference to distant masses. Compared to frames of reference with linear or rotational acceleration, inertial frames of reference also preserve local causality.

See also

• Celestial sphere
• Tychonian system

Notes

1. Sir Fred Hoyle, Nicolaus Copernicus, 1973.