Probably everyone is familiar with the qualitative notion that the perceived intensity of a light source decreases with distance. The quantitative statement of this effect is called the 1 / r 2 law, because the intensity goes down as the square of the distance.
On the one hand, this means that it is much more difficult to see distant objects. On the other, this means that if we know the distance to a source we can deduce the true luminosity from the apparent luminosity.