To see why this distinction between absolute and apparent brightness matters to an astronomer trying to determine the distance to luminous objects, imagine looking at a light coming from a lamppost through the fog while walking through a park at night. If you see a light in the park that looks extremely bright to you, you might attribute that apparent brightness to the light being extremely close at hand. Or you might attribute the brightness of the light to an extremely high-output lightbulb located on the other side of the park. In other words, the brightness you observe might be the result
...more
