|
meteoptic-l [ät] ursa.fi
message archive
This is meteoptic-l [ät] ursa.fi message archive.
Note, your can reply the messages on this page only if your
are already subscribed the list.
» To the end of the list/message
Lew Gramer: "NLC or early twilight?" (Jun 17, 17:04):
> >From latitude 43o56'30" on July 15/16, I observed an extended
sky-brightening in
> the east, starting at 2:12 am, extending up 15o at it's highest. It
showed no
> details or outstanding features, and did not appear to obscure the sky
behind
> it. It was not truly dome-shaped, extending only about 20o in azimuth,
but I
> assumed it was most likely twilight at the time, as this was a new and
very dark
> location for me, and I was unsure of the start of local astronomical
twilight.
>
> However, I've since checked an ephemeris for that location (actually
'xephem'),
> and it lists the beginning of astronomical twilight for that morning as
02:32
> local time. I did NOT note how it changed while true twilight
approached, or I
> might have been able to confirm this sighting one way or another.
>
> So what should I assume this was? An NLC sighting at a record low
latitude, or
> an indication of just how early twilight can begin in a truly dark
Northern sky?
>
> Comments appreciated,
> Lew Gramer
>
If sun is over 16 degrees below horizon, it is impossible to see NLC's,
simply because then the sun is below horizon up there too. If the
noctilucent clouds were visible at the height of 20 degrees, then the
sun cannot be more that, say, 14 degrees (raw estimate without checking)
below
horizon.
|