From: gawne@stsci.edu
Subject: Re: rumor concerning Saturn's rings
Organization: Space Telescope Science Institute
Distribution: na
Date: Thu, 15 Oct 1992 01:01:06 GMT

In article <1992Oct13.083218.1@stsci.edu> I wrote:
 
>>Galileo reported seeing something extending away from Saturn that he
>>described as being like ears.  Apparently Huygens was the first person
>>to make the mental leap required to recognize that he was looking at 
>>a ring around a planet.
>>
>>As for naked eye visibility of the rings?  I doubt it.  The human eye
>>can't resolve Jupiter's moons, and the angular resolution needed for that
>>task is a factor of three or so less than the angular resolution necessary
>>to resolve Saturn's rings from the planet.  The fundamental limit is an
>>aperture (pupil) size of 7mm at the most.  Even Galileo's little telescope
>>more than doubled the aperture.

And our respected colleague Leigh Palmer replied:

> I'm puzzled by the claim that the eye can't "resolve" Jupiter's moons. At 
> least some of the galilean moons are resolved by the educated naked eye.
> I don't understand what the factor of three refers to; the Jovian system
> appears many more than three times as wide as Saturn and his rings.
> 
> If one knows that there are rings around Saturn then one will see rings 
> with a telescope like Galileo's, even at 20 power. Galileo didn't know
> there were rings; that's why he couldn't see them. That's what I mean by
> an "educated" eye.
> 

Well, I concur that you can see Saturn's rings and Jupiter's moons in
finder scopes (even good binoculars).  As for where I got the "factor
of three", let's consider some numbers:

	angular diameter of Jupiter:	       ~40 arcsec (average)
	maximum angular separation of Io:      ~120 arcsec
	                        Callisto:      ~500 arcsec

	angular diameter of Saturn:	       ~10 arcsec (average)
	angular diameter of the "A" ring:      ~45 arcsec

Comparing Io's maximum separation from Jupiter of 120 arcsec to the
45 arcsec diameter of the "A" ring I got ~3 as the ratio.  Obviously
if you compare to Callisto it's more like 11.  In any case this is
a very rough estimate and I thought the context was clear enough to show
that.  But yes, Leigh, I agree that Jupiter's disk is about the same
diameter as Saturn's entire ring system as viewed from the Earth.

Along empirical lines, when I worked with the FAA on low light level
vision experiments, I asked a number of Air Traffic Controler candidates
to tell me if they could see anything unusual about Jupiter.  Of all
these people who had excellent (20/20) vision nobody ever noticed 
Jovian moons.  Sure, I sometimes *think* I see Ganymede or Callisto
but wouldn't bet on it until I check with binoculars.  I think that
the glare of Jupiter is the big problem.  Somebody else mentioned
astigmatism, but according to my last eye exam I don't have that
problem.  It also may be that Leigh has the benefit of looking thru
the clear air of British Columbia while I usually worked north of
Dallas or here in Baltimore.

In any case, the original question dealt with the possibility of
resolving Saturn's rings with the naked eye.  If you take 1 arcminute
as the limit of angular resolution, then look at the values quoted
above for Saturn and rings, it should be pretty obvious that nobody
with normal vision could resolve Saturn, much less the rings.


-Bill Gawne,  Space Telescope Science Institute