Heliocentrism

In astronomy, heliocentrism is the theory that the Sun is at the center of the Universe and/or the Solar System. The word is derived from the Greek (Helios = "Sun" and kentron = "Center"). Historically, heliocentrism is opposed to geocentrism and currently to modern geocentrism, which places the earth at the center. (The distinction between the Solar System and the Universe was not clear until modern times, but extremely important relative to the controversy over cosmology and religion.) In the 16th and 17th centuries, when the theory was revived and defended by Copernicus, Galileo, and Kepler, it became the center of a major dispute.

The view of modern science

The realization that the heliocentric view was also not true in a strict sense was achieved in steps. That the Sun was not the center of the universe, but one of innumerable stars, was strongly advocated by the mystic Giordano Bruno; Galileo made the same point, but said very little on the matter, perhaps not wishing to incur the church's wrath. Over the course of the 18th and 19th centuries, the status of the Sun as merely one star among many became increasingly obvious. By the 20th century, even before the discovery that there are many galaxies, it was not an issue.

Related Topics:
Giordano Bruno - 18th - 19th - 20th century

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Even if the discussion is limited to the solar system, the sun is not at the geometric center of any planet's orbit, but rather at one focus of the elliptical orbit. Furthermore, to the extent that a planet's mass cannot be neglected in comparison to the Sun's mass, the center of gravity of the solar system is displaced slightly away from the center of the Sun. (The masses of the planets, mostly Jupiter, amount to 0.14% of that of the Sun.) Therefore an astronomer on a hypothetical extrasolar planet would observe a "wobble".

Related Topics:
Solar system - Focus - Elliptical - Extrasolar planet

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Giving up the whole concept of a center of the universe and even of being "at rest" is related to the principle of relativity. While it was clear there is no privileged position in space, until postulation of the special theory of relativity by Albert Einstein, at least the existence of a priviliged class of inertial systems absolutely at rest was assumed, in particular in the form of the hypothesis of the luminiferous aether. Some forms of Mach's principle consider the frame at rest with respect to the masses in the universe to have special properties.

Related Topics:
Principle of relativity - Special theory of relativity - Albert Einstein - Luminiferous aether - Mach's principle

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Modern use of geocentric and heliocentric

In practical calculations, the origin and orientation of a coordinate system often has to be selected. For practical reasons, systems with their origin in the center of Earth's mass, solar mass or in the center of mass of solar system are frequently selected. The adjectives geocentric or heliocentric may be used in this context. However, such selection of coordinates has no philosophical or physical implications.

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Fred Hoyle wrote:

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:The relation of the two pictures 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. (Hoyle, 1973, p. 78)

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