Features:

1. Slipher working at Lowell Observatory starting in 1912 began measuring the redshifts of galaxies (see No-522--523).

2. He didn't know they were galaxies for sure until Edwin Hubble's (1889--1953) convincing proof in 1924 of the galactic nature of the spiral nebulae and, by immediate implication, the elliptical nebulae since they were in galaxy clusters with spiral nebulae (see Wikipedia: Edwin Hubble: Universe goes beyond the Milky Way galaxy). Note spiral galaxies and elliptical galaxies were then called nebulae, a term which we now use for galaxies only when speaking historically.

3. Most galaxies showed redshifts. A very few showed blueshifts which we regard as negative redshifts.

4. By 1925, Slipher had redshifts for 45 galaxies (see No-522--523). This is very slow progress by modern standards, but "sufficient unto the day is the technique thereof".

5. Interpreted as Doppler shifts---which, in fact, is NOT exactly right---though that is what was assumed originally by Slipher and, perhaps for awhile, by Edwin Hubble (1889--1953)---these results showed that most galaxies (the redshifted ones) were moving away from us. The blueshifted galaxies are moving toward us.

   Originally, Slipher and, perhaps for awhile, Hubble assumed the shifts were entirely Doppler shifts. This is a wrong assumption. Most of the shifting is cosmological redshift.

   The somewhat wrong assumption did NOT hurt much since the 1st order Doppler effect formula coincidentally gives the right answer anyway for recession velocities for relatively nearby galaxies which were all Slipher measured redshifts for.

6. Using the 1st order Doppler effect formula, the recession velocities of the observed galaxies were known.

7. The upshot of Slipher's work was that most galaxies were in relative motion away from Milky Way.

   Alternatively, one could hypothesize that the Milky Way was at the center of an outward flow of galaxies.

   But that hypothesis violates the Copernican principle that we occupy NO special place in the universe.

8. By the by, in my days in Flag, I learnt that old Vesto was also a real estate developer---West Saturn Way, Meteor Drive, etc.---he made a killing.

   

9. Image 2 Caption: The Slipher Rotunda Museum at the Lowell Observatory in Flagstaff, Arizona.

10. This may be the dome where Vesto Slipher (1875--1969) discovered the systematic cosmological redshift of the galaxies---but no one's telling.

11. Despite spending a year Flag (2010 Aug--2011 May) and visiting the Lowell Observatory several times, yours truly never noticed the Slipher Rotunda Museum---yours truly was probably just being obtuse.

Images:

1. Credit/Permission: Anonymous photographer, 1909 (uploaded to Wikipedia by User:Gobonobo, 2024) / No permission.
   Image link: Wikipedia: File:Vesto Slipher 1909.jpg. Image link direct: Wikipedia: https://en.wikipedia.org/wiki/File:Vesto_Slipher_1909.jpg.
   Image link: Placeholder image link: alien_click_to_see_image.html.
   
2. Credit/Permission: Leslie Connell, before or circa 2006 (uploaded to Wikipedia by User:Howcheng, 2006) / Public domain.
   Image link: Wikipedia: File:LowellObservatory.jpg.