The Longest Full Moon of the Year

"Tell me what you feel in your room when the full moon is shining in upon you and your lamp is dying out, and I will tell you how old you are, and I shall know if you are happy." -Henri Frederic Amiel

Tonight is a special night, although not for the reasons you may think. Yes, it's just a few days after Christmas and before the new year, but tonight is the night of the Full Moon closest to the winter solstice.

Up here in the northern hemisphere (above the Tropic of Cancer), December 21st corresponds to the winter solstice, or the shortest day in terms of hours-of-sunlight of the year. The Sun crests the horizon and rises to its lowest altitude in the sky [90° minus the Earth's axial tilt (23.5°) minus your latitude] of any day, setting after spending the shortest amount of time above the horizon of the entire year.

Up where I am in Portland, Oregon, that means we get just 8 hours and 42 minutes of daylight on the solstice, where the Sun is above the horizon. But within two weeks (at most), either before or after the Solstice, you'll be treated to the darkest Full Moon of the year.

With the Earth's north pole maximally pointed away from the Sun, that means it's pointed maximally towards the Moon: so this will be the Full Moon that gives you the most hours of moonlight of any day during the year: over fifteen hours of moonlight here where I am. And every moment of it is spectacular.

This is not what moonrise looks like tonight! (Well, maybe in the southern hemisphere it does, but not here!) The bright sky, still relatively well-illuminated in the post-sunset hours, makes for a beautiful scene, but this photo was taken near the summer solstice from the Northern Hemisphere. If we look nearer the winter solstice, however, it isn't your imagination: the sky gets darker more quickly near the winter solstice!

What this means is that -- if you look tonight -- you'll get a spectacular moonrise, where the Moon turns a blood red against the darkest backdrops of a full moonrise that you'll see all year... and the same thing again at moonset just prior to dawn.

You'll also see the full Moon achieve its highest altitude when it passes overhead, reaching 63.5° where I am tonight! (The formula is more complicated due to the Moon's inclination to the Earth-Sun orbit.) The full Moon will light up the night sky for the longest amount of time at all northern (above 23.5°) latitudes tonight, and if you look closely, you might find a bright light located about 25 degrees away.

That would be Jupiter, near opposition and currently the brightest non-lunar object to grace the night sky.

Although perhaps I should have told you about this last month, because on November 28th, when we had a full Moon on the other side of the solstice, the planet Jupiter was occulted by the Moon! The reappearance of our Solar System's largest gas giant took just about two minutes from behind the limb of the Moon, and was fabulously videotaped through a telescope by Rafael Defavari! (You've got to see this!)

So enjoy tonight's amazing full Moon, the longest full Moon, giving you the most moonlight for the most hours when you need it the most: at the darkest time of the year!