heliocentric adj : having the sun as the center [ant: geocentric]
- (UK): /ˌhiːliəʊˈsɛntrɪk/, /%hi:li@U"sEntrIk/
- (US): , /ˌhiːlioʊˈsɛntrɪk/, /%hi:lioU"sEntrIk/
EtymologyFrom ηλιοκεντρικός (heliokentrikos), having the sun as its centre, from Ancient Greek ήλιος (helios), sun + κέντρον (kentron), centre
In astronomy, heliocentrism is the theory that the sun is at the center of the Solar System. The word came from the Greek (ήλιος Helios = sun and κέντρον 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.) Although many early cosmologists such as Aristarchus speculated about the motion of the Earth around a stationary Sun, it was not until the 16th century that Copernicus presented a fully predictive mathematical model of a heliocentric system, which was later elaborated by Kepler and defended by Galileo, becoming the center of a major dispute.
Development of heliocentrismTo anyone who stands and looks at the sky, it seems clear that the earth stays in one place while everything in the sky rises and sets or goes around once every day. Observing over a longer time, one sees more complicated movements. The Sun makes a slower circle over the course of a year; the planets have similar motions, but they sometimes turn around and move in the reverse direction for a while (retrograde motion). As these motions became better understood, they required more and more elaborate descriptions, the most famous of which was the Ptolemaic system, formulated in the 2nd century, which, though considered incorrect today, still manages to calculate the correct positions for the planets to a moderate degree of accuracy, though Ptolemy's demand that all epicycles be not eccentric causes needless problems for the motions of Mars and especially Mercury. It is interesting to note that Ptolemy, himself, in his Almagest points out that any model for describing the motions of the planets is merely a mathematical device, and, since there is no actual way to know which is true, the simplest model that gets the right numbers should be used; however, he himself chose the epicyclic geocentric model and in his ultimate work Planetary Hypotheses treated his models as sufficiently real that the distances of moon, sun, planets and stars were determinable by treating orbits' celestial spheres as contiguous realities. This made the stars' distance less than 20 Astronomical Units — a subtraction from science since Aristarchus's heliocentric scheme had already centuries earlier necessarily placed the stars at least two orders of magnitude more distant.
Philosophical discussionsPhilosophical arguments on heliocentrism involve general statements that the Sun is at the center of the universe or that some or all of the planets revolve around the Sun, and arguments supporting these claims. These ideas can be found in a range of Sanskrit, Greek, Arabic and Latin texts. Few of these early sources, however, develop techniques to compute any observational consequences of their proposed heliocentric ideas.
Ancient Indiasee Indian astronomy According to theosophists, the earliest traces of a counter-intuitive idea that it is the Earth that is actually moving and the Sun that is at the centre of the solar system (hence the concept of heliocentrism) is found in several Vedic Sanskrit texts written in ancient India. Yajnavalkya (c. 9th–8th century BC) recognized that the Earth is spherical and believed that the Sun was "the centre of the spheres" as described in the Vedas at the time. In his astronomical text Shatapatha Brahmana, he states:
"The sun is stationed for all time, in the middle of the day. [...] Of the sun, which is always in one and the same place, there is neither setting nor rising."
Some interpret this to mean that the Sun is stationary, hence the Earth is moving around it, This would be elaborated in a later commentary Vishnu Purana (2.8) (c. 1st century BC).
Yajnavalkya recognized that the Sun was much larger than the Earth, which would have influenced this early heliocentric concept. The Aitareya Brahmana (2.7) (c. 9th–8th century BC) also states:
"The Sun never sets nor rises. When people think the sun is setting, it is not so; they are mistaken. It only changes about after reaching the end of the day and makes night below and day to what is on the other side." Copernicus mentioned him as an influence on his own work.