Celestia Forums

Because its very ver far from the sun, I dont think it recieves much light from it, so does it really have a day and a night or are they almost the same(very very dark day and night)?
And what about Uranus and Neptune?

Sure, Pluto has a day and night. It may be about 40 AU from the sun, and it may (if I figure this out right) get 1/1600th of the light that the Earth gets (a difference of about 8 magnitudes, I think?), but that still makes the Sun is very bright at a magnitude of -19, which is about 100 times brighter than the full moon.

If you've been outside, away from the streetlights, when there's been a full moon in the sky you'll know that it can be pretty bright. Now imagine a lightsource 100 times brighter . Thus, You'd notice a difference between day and night on Pluto.

Of course, Pluto has an eccentric orbit, but even at furthest approach the sun is still much brighter than the full moon.

For Uranus and Neptune, which are closer to the sun, the sun is obviously brigher.

You could easily read a book by the light of the Sun on Pluto right now, so the difference between day and night would be very evident.

Grant

Ok, Thank you both... I suppoused that, but I wanted to be sure...
Thanks again.

If you have very sharp eyes the sun would even be able to create a small blind spot on your retina (if you looked at it for too long) at least at perihelion it could.

Pluto is tidally locked to its moon, Charon. They orbit their barycenter in about 6 Earth days. So a day on Pluto is about 3 Earth days, and night is also about 6 Earth days. Same for Charon.

Here's an old thread where the brighntess of the Sun at distant planets is discussed: http://www.celestiaproject.net/forum/viewtopic.php?t=5231

Pluto in the daytime would be about as bright as my living room at night with the lights turned on. Absolutely no problem reading a book.

So how bright does an object in the sky have to be before it can blind you if you look at it for longer than a few seconds? Obviously a magnitude of -27 is bright enough, and I've never heard of anyone being blinded by looking at the full moon for a long time so -19 must be lower than this threshold....

I gotta admit I can't really visualise what "100 times brighter than the full moon" actually looks like, because I've got no reference for that. Heck, I don't even know what "twice as bright" means really - I can't put numbers on this sort of thing, all I'd know is that one thing was brighter than the other but I wouldn't be able to estimate by how much. Even if I looked at a magnitude 1 star and a magnitude 6 star I wouldn't really understand the numerical difference (probably because they're such feeble light sources). I mean, what's the apparent magnitude of a 75W bulb in a living room, if you're standing in that room? Anyone got a handy way to grok this sort of thing?

Evil Dr Ganymede wrote:Anyone got a handy way to grok this sort of thing?

I groked it with my camera's light meter. I forget the exact numbers, but aiming it at a white piece of paper on the floor of my living room gave me a reading that was in the ballpark of 1000 - 2000 times dimmer than the same piece of white paper on the ground outside in full sunlight.

How bright does something have to be to create blindness? I'll take a few guesses. It might make a difference what kind of light it is. Staring at the Sun gives you a pretty full spectrum, IR and UV included. If the Sun could be filtered so only visible light could get through I wonder if that would blind you? I doubt you could use a magnifying glass to burn paper under visible-only light, so maybe it can't burn your retnas either.

We discussed in an Astronomy class I took that since a star's surface brightness doesn't diminish with distance, theoretically, staring at a distant star in the night sky could cause damage to your eye. But since its appearent angular size is so small, and the pixels (or whatever they're called) in you eye are so large in comparison, and since the atmosphere scatters the light from obtaning a perfect focus, and the amplitude of the wave of the light is probably much larger than the star's angular size, it harmlessly hits your retna. I'm not sure if I completely remember that correctly.

But I'd put money on it that the Sun could blind you from Pluto or Sedna.

Evil Dr Ganymede wrote:So how bright does an object in the sky have to be before it can blind you if you look at it for longer than a few seconds?

It's the surface brightness, rather than the visual magnitude, that makes the difference. The Sun is an extended light source with a surface brightness of 1.9e9 candela/m^2. If you get farther away from it, it'll appear smaller in the sky, and therefore will shed less light, and so will have a higher visual magnitude (higher=dimmer); but angular area for area, its surface brightness will be the same, so it'll still burn a hole in your retina (just a smaller hole). This'll go on until you get so far away the solar disc falls below the diffraction limit of your eyes (about one minute of arc), at which point its apparent disc stops getting smaller ... its light is always smeared over a minute of arc, so its surface brightness on your retina starts to fall as you move even farther out. Pluto's distance is around the point where that protective effect is just beginning to occur, as julesstoop points out.
The corollary is that if a star has a surface brightness considerably less than the Sun's, it won't burn your retina even though you approach it closely enough for it to have the same visual magnitude as the Sun seen from Earth ... it'll be a dimmer source spread over a bigger angular area.
The threshold value for a retinal burn is somewhere between 10^7 cd/m^2 (incandescent lamp filament) and 10^8 cd/m^2 (carbon arc light). So stars with a black-body temperature >3500K (M3V up) will scar your retina; <3000K (M7V down) won't.

Evil Dr Ganymede wrote:I mean, what's the apparent magnitude of a 75W bulb in a living room, if you're standing in that room? Anyone got a handy way to grok this sort of thing?

An apparent visual magnitude of zero corresponds to an illumination of 2.54e-6 lux (with some variation depending on the star temperature). Since you can read by a 75W bulb, and the threshold for comfortable reading is about 100 lx, then your bulb is 100/2.54e-6 = 4e7 times brighter than magnitude zero.
The equivalent magnitude difference is 2.5*log(4e7) = 19, so your bulb has an apparent magnitude of about -19.

Grant

tony873004 wrote:Staring at the Sun gives you a pretty full spectrum, IR and UV included. If the Sun could be filtered so only visible light could get through I wonder if that would blind you? I doubt you could use a magnifying glass to burn paper under visible-only light, so maybe it can't burn your retnas either.

Your eye lenses are opaque to UV, and (because they're simple lenses) don't refract all wavelengths equally so IR isn't brought to a point focus ... it's largely the visible wavelengths that do the damage. Likewise with the point focus of a simple magnifying glass.

Grant