An Etymological Dictionary of Astronomy and Astrophysics

The equation that expresses the → magnitude  → difference between two objects in terms of the → logarithm of the → flux  → ratio:
I₁/I₂ = 2.5^{(m₂ - m₁)}, or
m₂ - m₁ = 2.5 log(I₁/I₂),
where m is → apparent magnitude, I flux, and log the logarithm to base 10.

See also: Named after Norman Robert Pogson (1829-1891), the English astronomer, who introduced the magnitude scale in 1856; → relation.