To most people, the lunar phase that first comes to mind is “Full Moon,” with “crescent” serving as runner-up. Cartoonists portraying a night sky always include one of those two. Ask friends to name a lunar phase and you can count on no one saying “First Quarter” or “Last Quarter.” Many don’t even know that those are the names of the Half Moons – or appreciate the oddity that only in astronomy do “half” and “quarter” mean the same thing.
Another fully ignored phase is “gibbous,” even though, each month, the Moon appears gibbous for more nights than any other phase! For the record, the Gibbous Moon is sort of football-shaped. It’s defined as a Moon that’s fatter than half but less than full.
Now, December, is when the Full Moon gets highest in the sky of the entire year. So moonlight is at its brightest. Let’s celebrate that by giving it some attention during this lunation. It so happens that, right now – from Thursday through Monday nights, December 6 through the 9th – the Moon is gibbous, positioned and also illuminated at its very best.
That’s because the action already began on Wednesday the 4th, the First Quarter Moon, illuminated on its right side, which stood highest at sunset. Now, Thursday, a gibbous just a tiny bit fatter than half is highest up at nightfall: just perfect.
Experienced observers know that only around the half phase, or a day or two later, can they see optimally illuminated craters near the middle of the Moon, rather than those closer to the edge that are foreshortened and have a faraway appearance. So, people who enjoy up-close lunar features (and who doesn’t?) drool on their eyepieces when they point any telescope – even a small cheap one – at the Moon these nights.
Even naked-eye, the current slightly gibbous Moon vividly pops out as it hovers against the darkest, most polarized part of the blue sky, smack on the meridian at dusk. And there, right on its own sunset line, stand the magnificent Lunar Apennines, the Moon’s most striking mountains.
Saturday and Sunday night, that same shadow line, the Terminator, uncovers Copernicus, a magnificent, isolated crater with terraced walls. Regarded by many as the Moon’s most handsome feature, it’s now at its best. If you only have binoculars, brace your elbows steadily and you’ll see Copernicus even with just that low magnification.
Because December gives us the Moon at its best. And this week is the absolute optimum time.
Want to know more? To read Bob’s previous columns, click here. Check out Bob’s podcast, Astounding Universe, co-hosted by Pulse of the Planet’s Jim Metzner.