> This is a *stupidly tiny number*. If you asked POV-Ray to render this, and 
> you sent out a million, billion, trillion rays, every single damned one 
> would completely miss such a tiny object. It seems astonishing to me that 
> such an absurdly minute object is visible at all - but Wikipedia claims it 
> is...

Good job the real world isn't a backward raytracer then :-)

I suspect even a true point source (ie no physical size) was sending out the 
same amount of light from that distance, you would be able to see that too.

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>> This is a *stupidly tiny number*. If you asked POV-Ray to render this, 
>> and you sent out a million, billion, trillion rays, every single 
>> damned one would completely miss such a tiny object. It seems 
>> astonishing to me that such an absurdly minute object is visible at 
>> all - but Wikipedia claims it is...
> 
> Good job the real world isn't a backward raytracer then :-)

Yeah. Because then we wouldn't be able to see stars... which... would be 
bad... somehow. I'm sure. :-)

I guess it's simply the case that all the light gets focused onto a 
point smaller than a single light sensor, which never the less registers 
that there's some light there. (Also, presumably no lense can ever be 
optically perfect...)

The part that really puzzles me as that even on the clearest, most 
cloudless night, there are, like, maybe 3 stars visible in the entire 
sky. And yet mankind has names for hundreds and hundreds of stars, many 
of which are claimed to be "visible to the naked eye".

I can only presume this must be due to light pollution. (I'm not saying 
that MK's street lighting is inefficient, but... you *can* tell where 
the city is from about 15 miles away just by the fact that one entire 
side of the night sky is glowing at the exact wavelength of the Sodium 
D-line...) Still, even on school caving trips in the middle of nowhere, 
I don't recall seeing many stars in the night sky. I presume they're 
still there, we just can't see them any more.

> I suspect even a true point source (ie no physical size) was sending out 
> the same amount of light from that distance, you would be able to see 
> that too.

Well, photons are quantum, right?

(Interestingly, I'm told it *is* possible to make a light source that 
emits individual photons, one at a time, on que. And that when you do 
this, things like the double-slit experiment still show multiple waves 
interferring and reinforcing - despite this being obviously 
impossible... Wave-particle duality is weird!)

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Invisible <voi### [at] devnull> wrote:
> OK, so according to the best data I could get my hands on, Alpha Centuri
> A is about 430 light years away

Um, I'm pretty sure that should be 4.3 lightyears.

http://en.wikipedia.org/wiki/Alpha_centauri

That will make your numbers a little more sensible. There are very few naked-eye
stars that are as far as 400 lightyears - only highly luminous giants like
betelgeuse.

:)

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>> OK, so according to the best data I could get my hands on, Alpha Centuri
>> A is about 430 light years away
> 
> Um, I'm pretty sure that should be 4.3 lightyears.
> 
> That will make your numbers a little more sensible. There are very few naked-eye
> stars that are as far as 400 lightyears - only highly luminous giants like
> betelgeuse.
> 
> :)

EPIC FAIL. >_<

Maybe I should just go away and live in a box somewhere. :-S

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Invisible <voi### [at] devnull> wrote:
> The part that really puzzles me as that even on the clearest, most
> cloudless night, there are, like, maybe 3 stars visible in the entire
> sky. And yet mankind has names for hundreds and hundreds of stars, many
> of which are claimed to be "visible to the naked eye".
>
> I can only presume this must be due to light pollution.

Entirely. There are over a thousand stars visible to the naked eye.

http://en.wikipedia.org/wiki/Naked-eye_stars#Naked_eye_in_astronomy

There is nothing like spending a couple of hours on your back in a field with a
truly dark sky - it feels like there are far more than mere thousands visible!

:o

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Bill Pragnell wrote:
> Invisible <voi### [at] devnull> wrote:
>> The part that really puzzles me as that even on the clearest, most
>> cloudless night, there are, like, maybe 3 stars visible in the entire
>> sky. And yet mankind has names for hundreds and hundreds of stars, many
>> of which are claimed to be "visible to the naked eye".
>>
>> I can only presume this must be due to light pollution.
> 
> Entirely. There are over a thousand stars visible to the naked eye.
> 
> http://en.wikipedia.org/wiki/Naked-eye_stars#Naked_eye_in_astronomy
> 
> There is nothing like spending a couple of hours on your back in a field with a
> truly dark sky - it feels like there are far more than mere thousands visible!
> 
> :o

But where do you have to go? Outer Mongolia?

(Also... where the hell *is* that anyway?!)

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Invisible <voi### [at] devnull> wrote:
> But where do you have to go? Outer Mongolia?

Well, I bet the skies are great there(!), but at least 50 miles from a large
town or city would do in the UK. Simply making sure you can't see an artificial
light source directly by eye will make a big difference. Coastline works well
because you've only got to worry about distance from towns in half as many
directions. Wilderness such as moorland or mountains works best of course
(although steep hills will obstruct part of the sky), but I'm sure you could
find somewhere suitable within a short drive of MK. Also, winter is best because
the cold air holds less moisture and is therefore clearer. Try to make sure the
moon isn't up either, it can produce a surprising quantity of light pollution,
especially when full!

Oh, and pick a cloudless night ;-)

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> Still, even on school caving trips in the middle of nowhere, I don't 
> recall seeing many stars in the night sky. I presume they're still there, 
> we just can't see them any more.

http://www.astro-travels.com/pictures/Europe-Light-Pollution.jpg

On the above map my house is on the border of a green/yellow region and I 
can make out a huge number of stars on a clear night, the Milky Way is also 
just barely visible on a really good night.  I don't know exactly where MK 
is on that map, but it looks a bit red/orange around there :-(

Don't forget you need to go outside for at least 20 minutes or so for your 
eyes to get fully adjusted to the dark, initially it might look like only a 
dozen or so stars are visible, but after a few minutes you will see hundreds 
more.

BTW if you're interested, one of the brightest "stars" in the sky at the 
moment is actually Jupiter.

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>> Still, even on school caving trips in the middle of nowhere, I don't 
>> recall seeing many stars in the night sky. I presume they're still 
>> there, we just can't see them any more.
> 
> http://www.astro-travels.com/pictures/Europe-Light-Pollution.jpg
> 
> On the above map my house is on the border of a green/yellow region and 
> I can make out a huge number of stars on a clear night, the Milky Way is 
> also just barely visible on a really good night.

Aren't we *in* the Milky Way?

> I don't know exactly 
> where MK is on that map, but it looks a bit red/orange around there :-(

I would suggest that MK is the small red dot just NW of London. (Or if 
it isn't, it's in that general direction.)

> Don't forget you need to go outside for at least 20 minutes or so for 
> your eyes to get fully adjusted to the dark, initially it might look 
> like only a dozen or so stars are visible, but after a few minutes you 
> will see hundreds more.

We drove to and from work through lots of wilderness, and at this time 
of night it's quite dark even at 5PM. Many of the roads aren't lit.

I don't recall where we get dragged to on our school caving trips, but 
we seemingly had to drive for many hours along winding rounds lined with 
dry stone walls before reaching the nearest building with electricity.

> BTW if you're interested, one of the brightest "stars" in the sky at the 
> moment is actually Jupiter.

Any ideas what the visual angle for that is? ;-)

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> Aren't we *in* the Milky Way?

Yes, and it's quite a flat disc, so looking out from Earth along the plane 
of the disc you see a lot more stars than looking out perpendicular to it, 
this looks like a brighter band going across the sky.

>> BTW if you're interested, one of the brightest "stars" in the sky at the 
>> moment is actually Jupiter.
>
> Any ideas what the visual angle for that is? ;-)

Wikipedia tells me the radius is 71500 km, Stellarium tells me it's 5.224 AU 
away from us, so googling "2*71500 km / 5.224 AU in degrees" gives 0.01 
degrees.

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