New comet might blaze brighter than the full Moon | Hacker News Reader

New comet might blaze brighter than the full Moon

astronomynow.com

134 pointstartarugafeliz50 comments

I'm getting a little disappointed with astronomical events. The let downs have greatly outnumbered those that have delivered.

DISAPPOINTMENTS:

Comet Kohoutek

Halley's Comet

Every meteor shower I've watched: at least an order of magnitude less than they said I might see.

SUCCESSES:

Comet West: this one delivered. It wasn't a big comet, but it was bright and beautiful in the dawn.

1975 nova in Cygnus: http://en.wikipedia.org/wiki/V1500_Cygni

FALSE ALARM:

I saw a very bright star-like object, fairly low in the sky. It was around magnitude -4 or -5, much brighter than any normal star. This was in the east, and it was around 10pm, so it could not be Venus. I watched for several minutes and could detect no movement. I grabbed some binoculars for a closer look. It still appeared as a point source, and still showed no movement.

I watched for a bit more and it finally started showing movement, ruling out my guess that I was seeing a supernova.

Turns out it was a B-52, very far away, coming straight at me with its landing light on. Those lights are quite powerful, and you can see them when the plane is a couple hundred miles away or more. We lived about 5 miles from the runways at Castle Air Force Base, and from where the plane was the angular distance between me and the runway from the plane's point of view would have been maybe half a degree, so it's not surprising that there would be an occasional B-52 coming right at me.

Hale-Bopp in 1997 was very nice, as I recall.

Naked-eye, even.

Comet McNaught in 2007 was very impressive - I remember driving up the hill from Byron Bay and seeing it appear very brightly ahead of me in the night sky.

This would be very cool if it happened. I still remember spending all of my childhood after learning to read waiting for Comet Halley to return, and then having difficulty seeing it even in the desert of Arizona. Now I refer to that comet as "Halley's Smudge." Comet Hale-Bopp was all right, and well visible from Boston Common on a fine spring evening in 1997 when I was there on a business trip. Simply put, predicting the visibility of a comet to the naked eye is more wild guess than science, but when a comet is visible, it is a delight to the eye.

AFTER EDIT, EXPLANATION OF ASTRONOMICAL MAGNITUDE SCALE:

http://outreach.atnf.csiro.au/education/senior/astrophysics/...

http://www.icq.eps.harvard.edu/MagScale.html

http://csep10.phys.utk.edu/astr162/lect/stars/magnitudes.htm...

http://www.skyandtelescope.com/howto/basics/Stellar_Magnitud...

Oddly, the historically developed magnitude scale (which goes back to the ancient Greek astronomers) has the brightest objects ranked with magnitudes that are negative numbers, and dimmer objects ranked with increasing numbers that eventually become positive numbers. Of course the estimate of eventual brightness of the newly discovered comet is just that, an estimate, and may disappoint.

FYI the Greeks didn't have negative numbers. Philosophers were still arguing whether zero could truly be a number (there wasn't really a need for zero as a placeholder regardless, Greek numerals don't have a positional aspect).

Source: http://www-history.mcs.st-and.ac.uk/HistTopics/Greek_numbers...

When I was five or so, my dad got excited one day and said there would be a comic in the sky and we would go see it that night. I thought that was pretty cool, and I thought about how they were going to set it up.

I'd seen searchlights many times used for advertising where you could see the beam hit the bottom of an overcast, and I figured they had a way to use one of those to actually project a comic up onto the cloud cover.

After dark, we drove out of town up to a hilltop. I was puzzled: it was a perfectly clear night, so what were they going to project the comic onto?

Then he pointed and said, "There it is!"

But it was no comic, it was just a little star with a fuzzy tail.

Quick question for someone who knows more about this stuff. Is brightness adjusted for size?

In other words: If the comet is 20% the size of the moon and the same brightness, is it throwing off 20% as much light due to the lower surface area?

Brightness published for this comet in the article is in "magnitude" and it's a logarithmic value of total brightness as used in astronomy. Not brightness per unit area.

So if the comet is much smaller than the moon, is it reasonable to assume that it wouldn't reflect enough light to light the earth's surface like a full moon?

Why did you just ask the same question again? I guess you couldn't tell what z92 meant?

Anyway the answer to your question is no. The article is talking about total light, in Watts. Area is not a factor.

Watts is not a useful measure of visibility for humans, I would hope they're using lumens. Unless it's safe to assume that it reflects very similar amounts of visible light to the moon, but I don't know if that's the case.

Isn't light a synonym for visible light...?

Only colloquially. Light in general spans the entire electromagnetic spectrum.

In most contexts. But it's used frequently in "infrared light" too. Watts include radiation across the entire EM spectrum, from radio up to gamma rays, while lumens are normalized to human perception.

You basically take the number of watts at each wavelength and multiply it by a factor that represents the eye's sensitivity at that wavelength. So 1 watt of IR is zero lumens, and 1 watt of blue light is fewer than 1 watt of yellow.

Take a look at the spectral distribution of daylight [1]. You'll notice the area under the curve in IR is pretty similar to the amount of visible light.

If the IR reflectivity of a comet were high and the visible were low (relative to the moon), knowing that they were reflecting about the same wattage wouldn't tell you much about relative brightness.

[1] https://upload.wikimedia.org/wikipedia/commons/4/4c/Solar_Sp...

Yes the scale is closer to lumens than watts, but more people probably know what a watt is so I used that.

Anyway a lumen is just a way of weighting different wavelengths, and the magnitude scale uses a different way of weighting, so it's not lumens.

If the comet is much smaller than the full moon, then its surface will be brighter than the surface of the moon to still be of higher magnitude in brightness than moon.

Magnitude is total visible luminosity of the body. Think of it as a logarithmic value of lumina. Total amount of visible light from that whole body that is visible to an observer on earth.

Comets don't reflect light, they generate it moving against solar wind and radiation. After those things are left behind it becomes a just a big ol' flying rock again.

To expand on this a little - the solar wind vapourises part of a comet, forming an atmosphere of sorts called a coma (and the characteristic tail) and this atmosphere glows both by reflecting sunlight and by ionisation. So yes, the cometary nucleus hardly reflects any light, but some light is reflected by the coma and the tail.

(see en.wikipedia.org/wiki/Comet#Coma_and_tail, and just above that for details of light reflection from a cometary nucleus)

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.

Anybody remember Kohoutek? Exactly. If I were a comet, my strategy would be to underpromise and overdeliver. That's the best way to rally a comet's soothsayer base.

Yes, I do. I do also remember Halley's comet. Or, rather, I do remember standing on a winter beach around the time of the predicted maximum with a clear view of the southern sky without light pollution and seeing exactly nothing but stars. (Being at 54 degrees north probably didn't help, either).

If you talk about this approach, the first commet that comes to mind is Hamner-Brown. (Yeah, it's fictional. Still, it's the archetype of that strategy :)

Thanks!, Your comment made me google Hammer-Brown, and now I want to read that book :)

It's available DRM free from Baen for $4.00: http://www.baenebooks.com/p-921-lucifers-hammer.aspx.

(I have no affiliation, other than being a happy customer).

Ha! I remember that book. I memorized several end-of-the-world survival strategies from it. Like "you can drive a jeep on railroad tracks" and "when packing for the apocalypse, don't forget liquor."

Yeah. I keep telling myself I'm simply disaster-ready when I look at my liquor cabinet, too :)

I remember it well. I drove up to Bluff Mountain tower (TN) with a couple of friends on a clear summer night. From up there it was a spectacular sight. It was much more beautiful than Hale-Bopp which I saw from Bear Mountain (NY). The only problem - and it was a big one - was that you couldn't really see Kohoutek unless you were up high where Hale-Bopp could be seen from light polluted suburbs.

Then again, it might not. In any case, I'm polishing up my Galilieoscope and hoping for the best. ("The best" meaning that I don't actually need my Galileoscope, because I find it particularly difficult to aim and focus.)

I also have a Galileoscope. I find it really awesome if you use a tripod, but otherwise totally miserable. And even with the tripod...yeah, focusing is tough. But possible, and very very rewarding for a $15 scope.

I predict that when the world doesn't end in December 2012 the doomsdayers will latching on to this thinking the Mayans were off by a year and that this comet will cause the end of the world.

I agree... this is going to have huge "star of Bethlehem", "Second coming", and "we were off by a year on Mayan calendar" connotations. The timing is just too perfect.

So... who's going to start a company to take advantage of this?

Late to market. The myriad of religious institutions have it cornered.

Excellent! Hale-Bopp was so cool to see. I was hoping for a another great comet to show my son.

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.

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.

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

Come on Universe, show us something unusually spectacular. A once in a lifetime, awe inspiring event. Something wondrous to let people for a moment see just how amazing everything around them is.

Honestly, I'm really hoping this comet hits closer to the fantastic side of the predictions. Also hoping to be able to see it from the eastern US. It says Northern Hemisphere observers are favored, so there's even more hope.

>Come on Universe, show us something unusually spectacular.

Be careful what you wish for, you just might get it. I can think of at least a few unusually spectacular events the universe could show us that I'm more than happy for it not to, or at least to show us from a great distance.

We are indeed tiny little specs of insignificance, but all things equal I'd like us to continue on being that and not get squashed like bugs by a random pebble flying through our planetary system or something. ;)

Seeing a comment take a dive into Jupiter kind of counted for me. I built a cd-rom of images from that for a company I worked for. It was amazing.

Personally, I'm gunning for Betelgeuse. That would be awesome.

http://en.wikipedia.org/wiki/Betelgeuse#Approaching_supernov...

My personal favorite is Eta Carinae which could be 20 times the size of Betelgeuse and may be big enough to be a hypernova. It may explode in our lifetimes. Pretty pictures at http://en.wikipedia.org/wiki/Eta_Carinae

We've had some great stuff recently, if you think about it.

Comets: Hyakutake was easily visible, Hale-Bopp was outstanding, and Shoemaker–Levy 9 _ran into_ Jupiter, leaving damage that was visible to small hobby telescopes.

Manmade: Back when Mir was still in orbit, at times you could see the shuttle chase it down. ISS is fairly bright and fun to watch. If you do a little planning, Iridium flares can be very bright for a minute or so. Go to http://www.heavens-above.com/, and look for upcoming flares at your latitude and longitude.

The dragons really are coming back to Westeros.

The comet only means one thing, boy ;-)

I just watched this episode today. Came here to find this comment, was not disappointed.

Of course, this means that HN is becoming more like Reddit. :-P

Just don't forget that -16 mag will happen VERY close to the sun. Comet won't be visible from Earth at that time even at that magnitude, while it will be a cool sight for SOHO cameras, possibly blinding the sensor.

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