After reading Chapter 23, section 1, divide your group into two subgroups. After preparing for a couple of minutes, have one subgroup present the arguments, as of about 1920, that the spiral nebulae were parts of the Milky Way. Have the other subgroup present the arguments, as of about 1920, that the spiral nebulae were distant galaxies. Have the entire group discuss which case seemed best at the time.
Divide into two subgroups. After preparing for a couple of minutes, have one subgroup present an argument that Hubble's law implies we are at the center of the universe. Have the other subgroup present an argument that Hubble's law implies that the universe is expanding. Have the entire group discuss the relative merits of the two arguments.
