Inverse Square Law

[ffutures]

Either I'm missing something or Weinbaum got the most easily calculated detail of his description of Pluto badly wrong.

If Pluto is about 30 AU from the Sun at its closest, it receives 1/30*30 = 1/900th the amount of heat and light as Earth. Wikipedia seems to confirm this by saying that Earth receives about 1.4 KW per square meter, Neptune gets about 1.5 W per square meter. At its closest Pluto is slightly closer than Neptune.

So, when Weinbaum describes Pluto and claims that "high noon was hardly brighter than full moonlight on Earth," I think he's got it badly wrong. The trouble is that he hasn't got it wrong the way I initially thought - Wikipedia says that the full moon is about 500,000 times fainter than the sun, so noon on Pluto is about 600 times brighter than the full moon on Earth. It's counter-intuitive, but I think it's right. It's still dim light, a few hundred Lux compared to 20,000+ for a bright day on Earth, but we're looking at the same order of magnitude as an overcast day or an office with reasonably good lighting.

Now I have to explain this away as an illusion caused by the tiny size of the sun in the sky (or something).

Comments

[heliograph.livejournal.com]

Pluto is nothing but trouble. Remember all the aggravation you had writing around it for Tsar Wars, and then ->POOF<-, it wasn't a planet anymore?

[ffutures.livejournal.com]

Heretic!

[ffutures.livejournal.com]

Forgot to ask; coming to Dragonmeet?

[heliograph.livejournal.com]

Sadly, no.

[whswhs.livejournal.com]

I would just think of it as rhetoric. Consider: If the moon's brightness is 1, the sun's brightness on Pluto is 556, and the sun's brightness on earth is 500,000, then the sun on Pluto is greater than the moon's brightness on earth ('brighter than"), but not nearly so bright as the sun on earth, and closer to the moon on earth than to the sun on earth ("hardly brighter than"). It would still seem very dim, probably dimmer than twilight.

What's the illumination threshold where the rods take over from the cones? If Pluto under noonday sun were dim enough so you had to rely on night vision, then "hardly brighter than moonlight" would be subjectively plausible, even if actually it were a fair bit brighter.

I think you should keep the statement, and maybe put in a box where you give the actual numbers.

[ffutures.livejournal.com]

I'm putting the facts in as a footnote. The brightness is well above the threshold for night vision, all other things being equal, so I'm saying that the light looks less bright because it's a point source and because a lot of it is absorbed by dark rocks (Which Weinbaum does actually mention).