[OT] Looking at the sun - Safe distance?

(Henry Spencer) writes:

In article ,
Bruce Palmer wrote:
Nevertheless, as you move away from the sun, beyond 1 AU, there must be
a point at which the intensity of harmful radiation falls below the
level that will damage your eyes.

It might be a long way out. The apparent brightness, in photons per
square degree, doesn't change with distance [...] you might
be well out of the solar system before the combined effects make the Sun
eye-safe.

This seems unlikely. Laser beams can be focused to points even smaller
than the sun, and 1 mw lasers are considered almost completely safe.
Normal eye reflexes are enough to protect you, though the Class 2A laser
definition states that they can still cause damage if you overcome your
reflexes and stare into the beam for more than 1000 seconds. This is
the safest laser class, used for bar code readers and other completely
unprotected applications. No safety measures of any kind are
required - not even a warning sticker.

I don't know the maximum pupil area, but it should be about 1/2 cm^2 at
most, so 20 w/m^2 should be safe. Given that the sun's output at 1AU
is about 1400 w/m^2, you need about a factor of 70 reduction, so at 8.5 AU
and beyond should be safe enough for all practical purposes.

There must be some further distance where even deliberate staring of *any*
duration will not cause problems. I suspect (though the laser specs
don't state such a limit) that it's not a lot further out, since the
damage mechanism is thermal and you must be pretty close to the final
equilibrium temperature after 1000 seconds.

Lou Scheffer