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#11
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John Ladasky writes:
George Dishman wrote: It will be darkest at local midnight in midwinter when the Moon is new and Venus is below the horizon. I wonder how often that combination occurs (if ever). How can Venus *not* be below the horizon at local midnight? Venus reaches a maximum elongation of about 48 degrees relative to the Sun. If I've done the math correctly, Venus should always be below the horizon by 3 hours and 12 minutes after sunset. For locations near the equator, yes. Near the poles, however, it is possible for the Sun to be down and Venus to be up at local midnight. |
#12
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"John Ladasky" wrote in message om... "George Dishman" wrote in message ... "Eugene Shubert" wrote in message om... "Mr. 4X" wrote in message ... (Eugene Shubert) wrote in message om: What is the darkest hour of the night? That depends on a lot of things. There is almost no direct sunlight 2-3 hours after sunset, Isn't there a measurable amount of sunlight that's scattered by the atmosphere, reaching further into the night than a mere 2-3 hours of earth rotation? Zodiacal light. I thought that the zodiacal light was scattered off of Solar System dust in the plane of the ecliptic, rather than Earth's atmosphere? Yes, I was unclear. When Eugene said "Isn't there a measurable amount of sunlight that's scattered ..." I wondered if he was recalling discussion of the Zodiacal light rather than atmospheris scatter. I was too economical with my words (most unusual). It will be darkest at local midnight in midwinter when the Moon is new and Venus is below the horizon. I wonder how often that combination occurs (if ever). How can Venus *not* be below the horizon at local midnight? Venus reaches a maximum elongation of about 48 degrees relative to the Sun. If I've done the math correctly, Venus should always be below the horizon by 3 hours and 12 minutes after sunset. Again I stand corrected. It should have been obvious to me since Venus is inside our orbit. Thanks for picking me up on those. best regards George |
#13
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"George Dishman" wrote in message ...
"John Ladasky" wrote in message om... "George Dishman" wrote in message ... "Eugene Shubert" wrote in message om... "Mr. 4X" wrote in message ... Isn't there a measurable amount of sunlight that's scattered by the atmosphere, reaching further into the night than a mere 2-3 hours of earth rotation? Zodiacal light. I thought that the zodiacal light was scattered off of Solar System dust in the plane of the ecliptic, rather than Earth's atmosphere? Yes, I was unclear. When Eugene said "Isn't there a measurable amount of sunlight that's scattered ..." I wondered if he was recalling discussion of the Zodiacal light rather than atmospheris scatter. I was too economical with my words (most unusual). It will be darkest at local midnight in midwinter when the Moon is new and Venus is below the horizon. I wonder how often that combination occurs (if ever). How can Venus *not* be below the horizon at local midnight? Venus reaches a maximum elongation of about 48 degrees relative to the Sun. If I've done the math correctly, Venus should always be below the horizon by 3 hours and 12 minutes after sunset. Again I stand corrected. It should have been obvious to me since Venus is inside our orbit. Thanks for picking me up on those. Actually, Dave Tholen corrected *me* on this one. Thanks, Dave -- and apologies to you, George. If you're near one of the poles, it is possible for a night to be under 6 hours and 24 minutes long. On such nights, the Sun makes a really shallow excursion below the horizon. Venus will follow the same path, more or less. In this case, it is possible for Venus to be above the horizon at midnight. Eyeballing a few web pages, I conclude that nights can be this short above about 57°N, or below 57°S. Hey, seeing Venus at midnight isn't that infrequent an occurrence, then. Nearly all of Alaska, half of Canada, Greenland, Iceland, Scandinavia, and half of Russia are above this line. Unless you count scientific stations on Antarctica, there aren't any inhabited places below 57°S. -- Rainforest laid low. "Wake up and smell the ozone," Says man with chainsaw. John J. Ladasky Jr., Ph.D. |
#15
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"George Dishman" wrote in
: "Eugene Shubert" wrote in message om... "Mr. 4X" wrote in message ... (Eugene Shubert) wrote in message om: What is the darkest hour of the night? That depends on a lot of things. There is almost no direct sunlight 2-3 hours after sunset, Isn't there a measurable amount of sunlight that's scattered by the atmosphere, reaching further into the night than a mere 2-3 hours of earth rotation? Zodiacal light. When a sizable part of the earth's surface and atmosphere is heated up during the day, doesn't a measurable fraction of that heated material reradiate energy in the visible spectrum? To keep the calculations simple, let's ignore artificial lighting but factor in the reflected light of the Moon if you know how to do that. It will be darkest at local midnight in midwinter when the Moon is new and Venus is below the horizon. I wonder how often that combination occurs (if ever). George When is Venus ever visible at midnight? I supose it is possible at the poles. LK. |
#16
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In article ,
(Eugene Shubert) writes: When a sizable part of the earth's surface and atmosphere is heated up during the day, doesn't a measurable fraction of that heated material reradiate energy in the visible spectrum? Nope, not if you mean strictly thermal emission. I suppose you could consider auroral emission under this definition, but I personally wouldn't refer to the auroral ions as "heated material." Thermal emission is of course the dominant component at wavelengths longer than 2.5 microns or so. At shorter wavelengths, the sky emission (i.e., ignoring visible stars and planets) is some combination of twilight (sunlight scattered in Earth's atmosphere), moonlight, auroral emission, Zodiacal light (sunlight scattered by interplanetary dust), and light from unresolved stars (e.g., the Milky Way). -- Steve Willner Phone 617-495-7123 Cambridge, MA 02138 USA (Please email your reply if you want to be sure I see it; include a valid Reply-To address to receive an acknowledgement. Commercial email may be sent to your ISP.) |
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