M33 may be naked eye for some people, but for others, it is simply an
averted or "near" averted vision object. A lot of people get so used to
using averted vision that they use it without thinking and report things
as being seen directly. A year or two ago, I did some checking on my
own dark adapted absolutely "dead-on" direct vision on faint stars of
known magnitude, and with some difficulty, I got to about 5.0 (more
typically, it is around 4.8 to 4.9). With averted vision, I have gone as
faint as magnitude 7.8., although from my local dark sky site, the
limiting magnitude (averted) is often in the 6.5 to 6.8 range. M33's
total integrated magnitude is about 5.8, but this is below my limit (and
is spread over an area of nearly a square degree). I can often see M33
with the unaided eye, but not directly, and from speaking to other
observers, this is apparently a fairly common situation for many people.
I don't have to look very far away from it to see it (even from my
regular dark sky site), but if I stare absolutely directly at it, it
vanishes, no matter how good the skies are. In any case, whether
something is visible with direct vision vs. averted vision is not
necessarily a good indicator of how good the skies are. Again, this is
a flaw in the Bortle scale, as *only* the visibility (averted or direct)
of the object should have been the indicator used. Clear skies to you.
--
David W. Knisely
Prairie Astronomy Club: http://www.prairieastronomyclub.org
Hyde Memorial Observatory: http://www.hydeobservatory.info/

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* Attend the 14th Annual NEBRASKA STAR PARTY *
* July 15th-20th, 2007, Merritt Reservoir *
* http://www.NebraskaStarParty.org *
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