42 nebulae
Lemaître 1927 · 42 Nebulae
The sample of 42 extragalactic nebulae (galaxies) that Georges Lemaître used in his 1927 paper to make the first published estimate of the rate at which the universe is expanding.
In 1927 a Belgian priest and physicist, Georges Lemaître, showed that Einstein's equations allow a universe that grows. Then he checked the sky. He took the galaxies that had both a measured speed (from Strömberg's list) and a brightness-based distance (from Hubble). The English of his paper prints the sentence: "Using the 42 nebulæ appearing in the lists of Hubble and Strömberg." From those 42 he found a mean distance of 0.95 million parsecs and a radial velocity of 600 km/s, which is 625 km/s at a million parsecs. Galaxies farther away move away faster. That is the expansion law later credited to Hubble, two years before Hubble's own 1929 paper.
What it supports
The 42 is Lemaître's own sample in the 1927 French paper (Annales de la Société Scientifique de Bruxelles 47 A) and again in the English translation republished in General Relativity and Gravitation in 2013. A footnote says he left out NGC 5194 because it is paired with NGC 5195, so the count is the author's, not ours. When the paper was translated for Monthly Notices in 1931, this paragraph and the 625 value were left out. Mario Livio (Nature, 2011) found Lemaître's letter: he asked for that section to be omitted, calling it of no actual interest. The modern SH0ES measurement of the same constant also calibrates on 42 objects. That is a coincidence of sample size to notice, not a law.
What it does not
Not a value of the Hubble constant (625 is not 42, and today's values are near 67 to 73). Not a chosen or designed number; it is how many galaxies had both kinds of data in 1927. Not evidence that the SH0ES 42 and the Lemaître 42 are connected. Not a claim that Lemaître beat Hubble on data quality; his distances were rough and he said so. Not Adams.
ExactnessExact
Form42
Unitnebulae (galaxies)
Strengthhard
When1927
Hung17 October 2026