[AZ-Observing] Re: Summer Twilight
- From: L Knauth <Knauth@xxxxxxx>
- To: "az-observing@xxxxxxxxxxxxx" <az-observing@xxxxxxxxxxxxx>
- Date: Wed, 18 Jun 2014 18:25:42 +0000
When I observe in June near Jacob Lake on the North Rim (coming up again soon,
Yay), the sky is bright all night. It gets brighter toward the horizon and I
have confirmed it is air glow by photographing it (green in images, increases
toward horizon). Like Paul, I think twilight goes on all night under Polaris.
This could be because the Milky Way wipes out the air glow to the south and I
have trees to the east and west. However, "twilight" has some definition in
astronomical science terms that may be different from what the truly dark
adapted human eye can sense or that instruments can measure. I assumed it
was twilight as defined by something visible to the human eye and I have also
been thinking that air glow is worse in the summer because the sun is not very
far below the northern horizon. Is it possible that sunlight peeps up from the
north to keep the upper atmosphere sizzling all night this time of year?
Surely air glow and twilight grade into each other as the night progresses.
Brian Skiff, the guru of skyglow may want to weigh in on what we are seeing.
Whatever, but observing in June is limited to between about 10 pm and 3 am.
The Zodiacal light really lights up the north ast starting around 2:30 am. I
have been fooled a few times thinking morning was at hand. This always breaks
the magic of the night for me, even after realizing it is just the zodiacal
light.
So, summer observing requires you to find a cool daytime spot for the long
hours waiting for the night to come on. Be rested and ready to observe like
crazy for those precious 5 hours. Especially with the glories of the summer
Milky Way blazing away.
Videmus Stellae,
Paul Knauth
________________________________________
From: az-observing-bounce@xxxxxxxxxxxxx [az-observing-bounce@xxxxxxxxxxxxx] on
behalf of Tom Polakis [tpolakis@xxxxxxx]
Sent: Wednesday, June 18, 2014 11:00 AM
To: az-observing@xxxxxxxxxxxxx
Subject: [AZ-Observing] Re: Summer Twilight
---- Paul Lind <pulind@xxxxx> wrote:
> While observing at Ash Fork during the last DOTM we noticed that the evening
> twilight crept slowly northward, reaching a point almost below Polaris. This
> was long after the end of astronomical twilight, which was listed as about
> 9:20 pm. The phenomenon is logical since the sun was only about 30 degrees
> below the horizon in the north at its very lowest...
Unless you were seeing some airglow that happened to be following the sun's
azimuth, I doubt that you were seeing twilight in the north. In my experience,
a sun altitude of more than 18 degrees below the horizon really does mean that
it's as dark as it's gonna get in that direction.
Did anybody take a wide-field image of the northern horizon at around midnight?
Tom
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