The Inner Secrets of Planets and Star - Formation of the Solar System Documentary
In the beginning - before the 1920s, these words had no place in our scientific understanding of the universe. Astronomers believed the cosmos to be eternal and unchanging. We knew of only one galaxy and a few million visible stars, and this was the scope of our observable universe.
Then astronomer Edwin Hubble observed, courtesy of redshift, distant galaxies speeding away from each other and formulated Hubble's Law to explain the universe's uniform expansion. Redshift just refers to a distant celestial body's shift toward longer, or redder, wavelengths, compliments of the Doppler effect.