Eddington number

This article is about astrophysics. For the measure in cycling, see Arthur Eddington § Eddington number for cycling.
Arthur Stanley Eddington (1882–1944)

In astrophysics, the Eddington number, NEdd, is the number of protons in the observable universe. Eddington originally calculated it as about 1.57×1079; current estimates make it approximately 1080.

The term is named for British astrophysicist Arthur Eddington, who in 1940 was the first to propose a value of NEdd and to explain why this number might be important for physical cosmology and the foundations of physics.