On what date did Hubble present his galaxy findings to the American Astronomical Society?

Hubble presented his galaxy findings to the American Astronomical Society on January 1, 1925.

The presentation followed Hubble’s observations of Cepheid variable stars in the Andromeda Nebula. By comparing a Cepheid’s measured brightness pattern with its apparent brightness, astronomers could estimate distance. Hubble’s results showed that Andromeda was far too distant to be part of the Milky Way, establishing it as a separate galaxy.

The meeting took place in Washington, D.C., during a joint scientific gathering. Hubble’s paper was read for him rather than delivered personally, an often-overlooked detail associated with the event. The announcement helped settle the major astronomical debate over whether spiral nebulae were nearby clouds or independent stellar systems.

The date is sometimes confused with November 23, 1924, when The New York Times reported the discovery, or with December 30, 1924, when Hubble’s formal paper was published. Those dates mark publicity and publication; January 1, 1925, is the date associated with the American Astronomical Society presentation.