256 The Poetry of Physics and The Physics of Poetry
extend much beyond the dimensions of the solar system, the distance to
the stars and the planets being regarded as basically equivalent.
With the Copernican revolution the Sun became the center of the
universe about which the Earth and the other planets orbited. The stars
were thought to be fixed objects, which surround the Sun on all sides.
The dimensions of the universe greatly increased with the adoption of the
Copernican picture. Since the Earth is no longer at rest, one should
observe the aberration of starlight (see Chapter 13). Copernicus was
unable to observe this phenomenon without a telescope, but concluded
that the distance to the stars was far greater than anyone had previously
thought.
Copernicus and his contemporaries still had no idea of the true
distances to the stars. With the use of the telescope the distances to the
nearest stars were determined and our closest neighbor was found to be
four light years away. A light year is the distance a particle of light
travels in one year and corresponds roughly to 10^16 meters or 9 trillion
kilometers. The dwarf planet Pluto is 12 light hours from the Sun while
the Earth is only 8 light minutes away. The nearest star is 10,000 times as
far away from the Sun as the Earth.
The telescope revealed that the sky was filled with stars most of
which cannot be seen with the naked eye. The Milky Way when gazed at
through a telescope was seen to consist of millions and millions of stars
so close to each other in the sky that they appear as a cloud. By counting
the number of stars in the various parts of the sky astronomers
discovered that the stars are not distributed homogeneously in the
heavens. Instead they found that the stars are distributed in an enormous
disk-like structure with a bulge in its center. When looking at the Milky
Way we are looking at the side of the disk. Because we are looking into
the plane of the disk we see more starts than we see in any other
direction. This disk-like structure of stars is called a galaxy after the
Milky Way since the Greek word for milk is “galaxias”.
The Milky Way Galaxy at first was believed to compose the entire
universe. The dimensions of the galaxy-universe were explored and by
1920 the radius of the galactic disk was determined and found to be
about 50,000 light years. The Sun was shown to be near the edge of the
disk some 26,000 light years from the galactic center. It was also
discovered that the Sun along with other stars orbits the center of the
galaxy under the gravitational influence of the galactic nucleus, which
contains the majority of the mass of the galaxy. The period of the Sun’s