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Re: (meteorobs) Zodiacal Light (etc) and the Bortle Scale


Ah, my favorite topic (aside from meteors themselves)... :)


I read John Bortle's article with great interest, Richard. It was very well
written, wasn't it? He (and others) had discussed some of the criteria in it
during a thread that occured some years back, either on the sci.astro.amateur
newsgroup or on one of the general astronomy email mailing lists.


I fully agree that, for most amateur astronomers, the LM is simply not a
sufficient measure of sky darkness: As Bortle notes, two individuals may
both measure LMs of 6.0 for their respective skies, but one can barely
make out the core of M31 in Andromeda naked-eye, while the other sees a
nice 3 degree extension on M31, and can glimpse M33 naked-eye as well!

Obviously, none of this affects meteor observers under current methods! For
meteor recorders under the IMO rubric, LMs measured with the IMO star-count
areas, plus regular percentage obstruction estimates from clouds or other
sources, are the only sky conditions required to be in the final report.


>Those of you who can reach Lms of 7.3, see ZDs and Gs, probably are
>luxuriating under #2 ("Typical true dark site").  John tells us that under
>Class 2 conditions we should see weak shadows cast by the ZL!!!  Have any
>of you noticed that?  Bortle's article is provocative reading.

From the darkest sites I've managed to find in the Northeast, Richard, say
in central Maine, I would say #2 is probably as deep as I've gone. But on a
couple of very good nights in the mountains of southeastern New Mexico, and
on several occasions at a particular site in the Middle Florida Keys, I've
clearly reached Bortle's "nirvana" of a darkness=1 sky: on those occasions,
especially from the subtropical Keys, the Zodiacal Light did indeed cast a
quite DISTRACTING shadow, as would the Winter Milky Way (which for me is
noticeably brighter than the Gegenschein, and somewhat fainter than ZL).

Under these conditions, my LMs measured using the IMO method occasionally
exceed 7.5 by a good margin - and I'm forced to note "LM 7.5+" on the IMO
reports. Obviously, my Star Counts then tend to take longer than 30 secs!


Of course, my few (three) lovely nights at the 9000' Visitors' Center up on
Mauna Kea yet surpassed the above experiences! And there lies the only issue
I've seen with the Bortle Scale... Namely, if Long Key State Park and the
Sacramento Mountains provide darkness=1, and Mauna Kea is clearly superior
to either one, then what is the darkness rating for Mauna Kea - zero??

Clear (and really, really DARK) skies,
Lew Gramer

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