A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Frank Reed
Date: 2015 Aug 29, 13:32 -0700
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.
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. 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).
Of Mark's question, you wrote:
"I believe this may have been tongue-in-cheek humor."
And you wrote:
"And why would one need a full moon for a sun lunar? If it did exist in nature it could pose problem, being fully illuminated for a brief period of time in specific regions. If not 100%, how do you tell the near from the far limb unless it is decidedly before or after full? Other than than knowing the exact 100% where and when, waxing or waning is your only clue."
Right on that day of full moon, it doesn't matter much since the nearly full moon is so close to fully illuminated. 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.