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Comment by andershaf | Hacker News Reader

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.

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The thing is, that the comet is not merely reflecting sunlight, as the moon does. It's also acting as a very large fluorescent light bulb as the gas boiling off it is stimulated to emit visible light by the sun's ultraviolet. While the comet itself is tiny, astronomically speaking, the light-emitting tail may be millions of mile long.

See: http://www.solarviews.com/eng/comet.htm

I think the article is saying that the predicted magnitude of the comet is -16.

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.

  What could possibly make the comet, which is very, very small compared to the moon, be that bright?

The tail, which is very, very big compared to the moon.

Makes sense. I just hope it's brighter than Hale-Bopp. From the countryside that one was plenty bright to enjoy.

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