Heliocentric Theory

Astronomers seem to have been of differing opinions on this aspect of the heliocentric theory. Thomas Wright (1711-86) and William Herschel (1738-1822) thought that the Sun was at or near the center of the Milky Way which most astronomers believed to comprise most or all of the universe. Herschel arrived at this conclusion by making star counts in different directions (parts of the sky) but he did not allow for the absorption of starlight by interstellar dust. J.H. Lambert (1728-77) concluded that the Sun was somewhat away from its center Figure 2. Diagram showing the phases of the disk of Venus in the Ptolemaic geocentric model of the solar system. Notice that only the new Moon (dark disk) or bright crescent phases are possible for Venus's disk. Illustration by Hans & Cassidy. Courtesy of Gale Group.
on the basis of the Milky Way's geometry. As long as there seemed to be evidence that the Sun was at or near the Milky Way's center and the Milky Way comprised most of the universe, a case could be made that the Sun was at or near the center of the universe.

Immanuel Kant (1724-1804) suggested that some of the nebulae seen in deep space were other Milky Ways, or "island universes," as he termed them. If his speculation proved to be correct, this would almost certainly mean that the Sun is nowhere near the center of the universe.

Astrometry also showed that the Sun and the other stars are moving relative to each other. The Sun is not at rest relative to the average motions of the nearby stars, but is moving relative to them at about 12 mi/sec (20 km/sec) towards the constellations Lyra and Hercules. These facts indicated that the Sun is only one of perhaps billions of ordinary stars moving through the Milky Way.

Harlow Shapley (1887-1972) postulated the first fairly correct idea about the Sun's location in the Milky Way. He found that the system of the Milky Way's globular star clusters is arranged in a halo around the Milky Way's disk (within which the Sun is located). These clusters are concentrated towards its nucleus and center, which are beyond the stars of the constellation Sagittarius. He found about 100 globular clusters in the hemisphere of the celestial sphere centered on the direction to the center of the Milky Way in Sagittarius, while there were only about a dozen globular clusters in the opposite hemisphere centered in the constellation Auriga. Shapley Figure 3. Diagram of the phases of Venus's disk as it gains on Earth in their orbits around the Sun in the heliocentric model of the solar system. Notice that, in theory, Venus's disk should show all the phases of the Moon. This is what Galileo observed with his telescope. Illustration by Hans & Cassidy. Courtesy of Gale Group.
reported this research in 1918 and estimated that the Sun is about 2/3 from the Milky Way's center to the edge of its disk which is very far from its center.

Edwin Hubble (1889-1953) confirmed Kant's hypothesis that the spiral and elliptical nebulae are other galaxies similar to the Milky Way in 1924. He also discovered that all the distant galaxies have spectra whose spectral lines are Doppler shifted towards the red end of their visible spectra, indicating that all distant galaxies are moving away from the Milky Way and its neighboring galaxies. Furthermore, the more distant such a galaxy seems to be, the faster it seems to be receding. This indicates that our universe seems to be expanding. One result of this discovery has been to make the concept of a "center of the universe" questionable, perhaps meaningless, in a universe with three spatial dimensions.

Present estimates indicate that the Sun is between 25,000 to 30,000 light years from the Milky Way's center. The Sun is revolving around this center with an orbital velocity of about 155 mi/sec (250 km/sec). One revolution around the Milky Way's center takes about 200,000,000 years. The Sun is only one star among 100,000,000,000 or more other ordinary stars which revolve around the Milky Way's center.

Heliocentric theory is valid for our solar system but its relevance extends only a few light-years from the Sun to the vicinity of the three stars of the Alpha Centauri system (Gliese 551, Gliese 559A, and Gliese 559B).