COPERNICAN SYSTEM, The, an epoch making reversal of man's ideas as to the place of the earth in the universe and the motions of the heavenly bodies, the full details of which were completed by Nicolaus Copernicus, a Polish astronomer, in the year 1530. Prior to the work of Copernicus, it was universally held that our earth is a fixed and immovable body, situated at the centre of the universe, about which all heavenly bodies, including the sun and the fixed star, are in revolution. To account for the apparently complicated motions of the planets, which sometimes are moving eastward among the stars and sometimes are moving westward(or oretrogradine), it had been necessary to suppose that each planet is moving about the circumference of a small circle (called the "Epicycle”, the centre of which pursues a larger circular path about the sun. This older system, devised as early as 140 A.D. by Claudius Ptolemy (q.v.), an Egyptian as tronomer, held its place as the true conception of the universe for upward of 14 centuries.
In the Copernican system it was clearly shown that the diurnal motion of the stars (their rising and setting) could be accounted for by assuming that the earth rotates on an axis with a uniform angular velocity, and, further, that the complicated displacements of the planets among the stars could very nearly all be explained by the assumption that the earth and all of the other planets are revolving about the sun in circular orbits of which, how ever, the sun does not occupy the exact centre The true paths, however (as we now know), are ellipses, and not exact circles. (See
SOLAR SYSTEM). It was therefore necessary for Copernicus to retain in his system a few small °Epicycjesp to account tor certain small out standing disagreements between the predicated and the observed motions.
So complete an overturning of nran's con ception of the place and importance crf the earth naturally by no means met with im mediate acceptance. It was, in fact, fully a century after the death of Copernicus (in 1543), before the reasonableness and simplicity of the new theory finally overcame the older, very artificial and complicated system of Ptolemy. At the present tune many direct ob servational proofs are available that the earth is revolving about the sun (see PARALLAX, ABERRATION, The DOPPLER PRINCIPLE, etc.), so that the essential truth of the Copentican. the ory is no longer open to question.