1. U.S. Timeline – The 1920s, (americasbesthistory.com), Retrieved June 5th, 2019, https://americasbesthistory.com/abhtimeline1920.html
2. Letter from Edwin Hubble to V.M. Slipher, Lowell Observatory, February 23, 1922
3. Walter S. Adams, Contributions to the Mount Wilson Observatory, (CIWY 1922-1923), No. 22, pg. 181
4. Ibid.
5. Walter S. Adams, Contributions to the Mount Wilson Observatory, (CIWY 1923-1924), No. 23, pg. 81