A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Sun and Full Moon visible simultaneously
From: Bill B
Date: 2015 Aug 29, 19:20 -0400
From: Bill B
Date: 2015 Aug 29, 19:20 -0400
On 8/29/2015 4:42 PM, Frank Reed wrote: > Bill B, you wrote: > "But is it a /full/ moon, 100% illumination?" > > Pretty close tonight. Arguably, there is never a full moon since at 180° > 00' elongation, the Moon is within the Earth's shadow. The maximum > possible elongation close to full moon would place the Moon's disk just > outside the umbral shadow. The umbra is about 1°23' in diameter > (depending on distance details) so the minimum distance of the Moon's > center from the center of the shadow is about 58 minutes of arc. That > means the maximum elongation of the Moon before it dips into the dark > umbra of the Earth is 179°02'. From that angle, we can get the fraction > of illumation of the Moon from [1-cos(179°02')]/2. The result is 99.993% > illuminated. So pretty darn close to full! In fact, the Moon is within > 1% of full when the elongation is 168.5° or higher, in terms of fraction > illuminated. Since it takes almost a day to change the elongation by > 11.5°, the Moon is very nearly full in appearance for 24 hours before > and after the actual moment of maximum "full" illumination. I'll need to reread and attempt to digest the above and follow up in a separate post, with an eye toward the upcoming September lunar eclipse. > Bill, you continued: > "Regarding my thought of full moon and sun visible visible at very high > latitudes near summer solstice, that could not happen this year as the > full moon had a high southern declination, and north declination near > the southern hemisphere summer solstice. I did not see other NA daily > entries where both would be visible book ending of the solstice. Doesn't > mean they are not there, only that it was not readily apparent to me. Is > there a year during the moons 19-year cycle when the full moon will have > a large north declination near summer solstice (or the reverse for down > under)?" > > I'm having trouble picturing what you're saying here. The Moon is always > opposite the Sun in the sky on the day when it's full within some > degrees. Understood > There's an offset because the exact time of maximum elongation > usually does not happen near sunrise or sunset. We get a few degrees > from that. And in addition, the Moon's orbit is tilted 5.2° with respect > to the ecliptic so it's possible for the Moon to pass that far north of > the center of the Earth's shadow or that far south at another time of > year. Is that what you were thinking of? In mid-northern latitudes, when > the Moon is right at maximum elongation from the Sun and separated by > 180° in ecliptic longitude, it can be several degrees above the horizon > right at the sunset. These two geometric factors are much more important > than refraction in practice, which only contributes about one degree > (half a degree for the Sun, half a degree for the Moon). I was thinking playfully in my 28 August post: "Short answer: No, I for one cannot "...tell the full moon to come out in the daytime this weekend..." Well above my pay grade ;-) Not likely on planet Earth in mid or lower latitudes without divine intervention. Maybe some year above the arctic circle (or southern equivalent) near summer solstice when it is light all day, the moon's declination is high and in same hemisphere as the sun using an instrument that measures 180 + degrees. The moon is full because the Sun and Moon are on opposite sides of the earth, so full moon remains pretty much a night-time event." Not thinking as deeply as you suggest. Just amusing myself. Specifically when the sun is at its maximum northern declination this year, the full moon has a southern declination around S 18 so not visible at N 72 if I understand correctly. I did not explore all possible scenarios given the 180 differences in azimuth and my gut feeling it was a humorous request. Looking more closely, at N 68 the full moon and sun Hc's of approx. 2d 35' are possible simultaneously 3 June. But seriously folks, when the sun and full moon can be viewed simultaneously they are similar to the houses of congress. Both there on occasion, (too often) diametrically opposed, and mostly useless. :-) ...But the problem you describe > does arise in practice for a couple of days before and after full moon. > And here a lunarian just has to be smart enough to know better. If one > is shooting lunars when the Moon is nearly full, then just consult the > almanac or draw it out to decide which limb is actually the "full" side. I was thinking precisely of lunarians and the potential bugger when the NA says 100%. Waxing and waning before and after are obvious from NA daily percentages.