The 10 degrees was a sort of arbitrary choice, chosen as a time when M30 would be several degrees above the horizon. I do remember looking at M30 in 2001 in very bright twilight. I could see scopes and cars clearly at the other end of the field. I would guess it was well past nautical twilight when we saw it. In any event, I don't recall the distant mountains presenting nearly as much of a problem for seeing M30 as the horizon extinction and sky brightness. Tom > Well here's some definitions which I stole from USNO. It's the 6-12-18 > degree definition of twilight: civil, nautical and astronomical, the details > which I am always forgetting. So Tom, at 10 degrees, is somewhere between > civil and nautical. Myself, I think he could be a little less nautical and a > little more civil. :-) > > Jack > > http://aa.usno.navy.mil/faq/docs/RST_defs.html > > Civil twilight is defined to begin in the morning, and to end in the evening > when the center of the Sun is geometrically 6 degrees below the horizon. > This is the limit at which twilight illumination is sufficient, under good > weather conditions, for terrestrial objects to be clearly distinguished; at > the beginning of morning civil twilight, or end of evening civil twilight, > the horizon is clearly defined and the brightest stars are visible under > good atmospheric conditions in the absence of moonlight or other > illumination. In the morning before the beginning of civil twilight and in > the evening after the end of civil twilight, artificial illumination is > normally required to carry on ordinary outdoor activities. Complete > darkness, however, ends sometime prior to the beginning of morning civil > twilight and begins sometime after the end of evening civil twilight. > > Nautical twilight is defined to begin in the morning, and to end in the > evening, when the center of the sun is geometrically 12 degrees below the > horizon. At the beginning or end of nautical twilight, under good > atmospheric conditions and in the absence of other illumination, general > outlines of ground objects may be distinguishable, but detailed outdoor > operations are not possible, and the horizon is indistinct. > > Astronomical twilight is defined to begin in the morning, and to end in the > evening when the center of the Sun is geometrically 18 degrees below the > horizon. Before the beginning of astronomical twilight in the morning and > after the end of astronomical twilight in the evening the Sun does not > contribute to sky illumination; for a considerable interval after the > beginning of morning twilight and before the end of evening twilight, sky > illumination is so faint that it is practically imperceptible. > > > > Why did you pick the sun at -10 deg? Is this from experience > > or a gut > > feeling? Could it be from some actual calculations that > > balance M30's > > magnitude and sky brightness. One would also have to factor > > in not only > > the mountains, but the elevation of the site plus sky glow > > from Tucson > > lights. > > > > > -- > See message header for info on list archives or unsubscribing, and please > send personal replies to the author, not the list. > -- See message header for info on list archives or unsubscribing, and please send personal replies to the author, not the list.