The (total) magnitude is just a convenient scale to measure of how much light that is radiated/reflected on an object. If the light was radiated equally in all directions, the light (all photons) would after a time have travelled equally far, defining a sphere. Since the photons travel further away every second, the surface area on the sphere increases as r^2 (r=distance to object), but the number of photons is the same.
That means that the light density, or the number of photons that hit our eyes/cameras, will decrease with the same factor, r^2. That's why they created the apparent magnitude. The (total) magnitude is a measure of how much light that leaves the object, while the apparent magnitude tells us how much light will hit the earth, or equivalently: how bright will this object appear on the earth.
Yes the article says so, but I really don't believe that it's correct. The comet will at its closest be around 60,000,000 km away from the earth, that is more than 150 times further away than the moon, and almost halfway to the sun.
What could possibly make the comet, which is very, very small compared to the moon, be that bright? I think the journalist have misunderstood the calculation or maybe read ~16 (approximately, which has been the current magnitude) as -16.
From nasa.gov:
"In the best case, the comet is big, bright, and skirts the sun next November. It would be extremely bright -- negative magnitudes maybe -- and naked-eye visible for observers in the Northern Hemisphere for at least a couple of months."
If NASA says it _might_ reach negative magnitudes, I'm pretty sure the -16 is wrong.
I'm not sure if this is correct. The moon has an average apparent magnitude of -12.74 (which is a lot, you've seen it), and as Wiki says:
'At the time of its discovery, the comet's apparent magnitude was about 18.8'. That's PLUS 18.8, but allright, at the time of its discovery.
...
'The comet may become extremely bright if it remains intact, probably reaching a negative magnitude.'
If the comet reaches a magnitude of -1, the moon is more than ten billion times brighter! I don't believe the uncertainty is within a factor 10^10, so this is probably just bad journalism.
"The latest rumour is that both ATLAS and CMS have evidence that the Higgs mass is about 125 GeV/C2 at confidence levels of 3.5σ and 2.5σ respectively. At 3.5σ, the measurement could be the result of a random fluke just 0.1% of the time whereas at 2.5σ the fluke factor is about 1%."