Isn't that a whole lot easier to say than "Supermoon"? But seriously, if you want to know what's happening with this Sunday morning's full moon, the best article I've seen in a while is over at Sky and Telescope's website. The moon will appear about 7% larger than the average full moon, because it's going to be—get this—7% closer to us. 357 061 km, instead of the "average" 384 402 km. It will appear more than 7% brighter, though, because brightness varies with the square of the distance. So, (384,4022 − 357,0612) / 384,4022 × 100 = 13% brighter. If the average lunar brightness at full moon is −12.74 magnitudes, then this Sunday's moon should be about −12.9. That's not a whole lot to write home about. If you want to do the math, go here. Another point to consider is that it's the start of summer, so we're near aphelion, when the earth is as far from the sun as it can get (the actual date of aphelion this year is July 5). That means that the light available for this June's perigee full moon to reflect back to Earth is slightly reduced from what it could be. On the other hand, when a supermoon (full moon at perigee) and perihelion (Earth at closest point to Sun) coincide, that's going to be one bright full moon! (Sort of. Actually, even a 30% difference in brightness is actually not all that noticeable to most people. Our eyes scale so quickly that such a relative difference is nearly imperceptible.) If you're curious, the next time these three events (full moon, perigee, perihelion) coincide will be January 1, 2018. That year the closest perigee full moon will occur on January 1, and Earth's perihelion occurs only two days later, on January 3, at 1 a.m.! So mark your calendar! In the meantime, I'll stay up late Saturday night with my camera, hoping for clear-ish skies to restart my full moon project, which got waylaid last October. Here's where it left off:
The Hunter's moon rises nearly or completely full over three successive nights at nearly the same time each night. On Eastern Daylight Time this year, the nights of the 27th, 28th, and 29th, at 5:21, 5:57, and 6:35, respectively. Full moon is today at 3:50 p.m. EDT, but this shot was taken last night right around 10 p.m. my awesome new moon globe, it's hard to make out Clavius way down there on the bottom.) I'll have to try again tonight, perhaps with higher magnification. Here's the full moon gallery to date; I've only missed a couple since December 2009!
The 2012 harvest moon occurs tonight at 11:19 p.m. EDT. Only 395,493 km from Earth tonight. Libration, as you can see, is quite southern (see how far the bright crater Tycho is from the bottom edge?), and a bit of the eastern limb is more visible as well (if you know your selenography, you can make out the bright pixels representing Gibbs almost dead center on the right-hand limb).
The second full moon in August (well, about 12 hours before full). I found my digiscope camera!
This year and next year, the month of August will bring you the two different definitions of the term blue moon. As you know, every 2.7 years the twelve months of the calendar feature thirteen full moons. There are only twelve full moon names, though, so when a "year" has thirteen moons, you have to decide how to name that "extra" moon. (I'll explain why I put the word year in quotes later.) As a culture, we've collectively decided that such extra moons shall be called blue: Whoops! Wrong kind. Here's the kind I'm actually talking about: November 20, 2010. Now, when a calendar year has thirteen full moons. one month must obviously contain two full moons. So, according to the most obvious definition, the second full moon in a month that features two is going to be blue. However, astute reader that you are, you probably noticed that the date of the last blue moon was November 20. How can we fit two full moons, which by definition are ~29.5+ days apart, into one calendar month? Well, that's why I put the word year in quotes in the first paragraph. Because in fact, a calendar year is just an arbitrary means of marking time; another method of doing so uses the seasons. And the seasons, rather than starting on January 1 like our calendar, start instead on astronomical events known as solstices (in December and June) and equinoxes (in March and September). And the dates of those events vary slightly, because our calendar isn't perfectly in tune with the seasons. And when you go by the seasons instead of the calendar, months with two full moons in them don't necessarily have a blue moon (in fact, they can't). And in point of fact, this month, August 2012, which has two full moons, has a blue moon if you go by the calendar-year definition, but not if you go by the seasonal definition. The second full moon this month which, barring unforeseen catastrophes, will occur on August 31 at 9:58 a.m. EDT, is a blue moon in the first sense, but not in the second. What is that seasonal definition of blue moon? I'm glad you asked. According to the Maine Farmer's Almanac, which I learned of via this Sky & Tel article, the third full moon in a season that has four is the real blue moon. And next year, again in August (on August 20 at 10:45 p.m. EDT), we will experience a blue moon, this time according to that older, seasonal definition: the third full moon in a calendar season that has four. While this definition has some fine tradition behind it, it is quite a bit more complicated in practice. After all, according to this definition, it isn't obvious from the calendar when there will be a blue moon. If you or I look at a calendar year and see 13 full moons, we can have reasonable certainty that one them is "extra." But according to this traditional definition, that won't happen in 2012, despite the fact that there are clearly thirteen full moons this year. And in 2013, which has only 12 full moons, there will indeed be a blue moon. Look at the table below, listing the dates of full moon in 2012 and 2013.
The pattern of dates in 2013 shows what's going on: the June full moon comes only two days after the June solstice (June 21 in 2013). That means that the second full moon of summer will fall in July, the third in August, and the fourth will come only three days before the September equinox. Thus, the third full moon of the season will fall in August, and that month's only full moon will be blue.
Unfortunately, due to inattentiveness on my part, I missed the first full moon this month. But Matt Wedel, out in sunny CA, did a great job capturing it, and wrote a very nice post about it, which I link to here. He's doing what I'd always tried to do: explaining and comparing moons, only he's doing it well, and I'm, well, doing it.
For more on the history of the term blue moon, try this new Sky & Tel article that appeared this week, right as I was putting the finishing touches on this pot.
|Full Moons 2012-2013|
Tonight's moon is the Ice (NeoPagan), Winter (Colonial American), Old (English), Quiet (Celtic), or Wolf (Wiccan) moon. Here in south Florida, with a night-time low of nearly 65°, the only ice I could fine was in my water, and it melted pretty darn fast. But it makes for "easy" imaging, unless you've got clouds and haze to deal with. But there's usually a hole or two through which to shoot, and if you're patient, you can get a decent image:
The full moon for December 2011 (variously known as the Long Night Moon, Bitter Moon, Cold Moon, Christmas Moon, etc.) was not easy to find here in Boca, blanketed as we were under clouds and buffeted by winds. But for a few brief moments the night after the day's full moon (9:36 a.m. local time, so not above the horizon), the moon was in the clear. The low clouds were scudding by in the fairly stiff breezes, and that must have affected the seeing, because I never did get a clear shot. But here is the one I got, to bring two years' worth of full moons to the gallery:
October's full moon this year, the Hunter's moon, occurred at 10:06 p.m. EDT, about 9 hours before it reached apogee (Oct 12 7:44 a.m. EDT, distance 406 434 km). Local conditions here in south Florida were a bit of a challenge; I had to set up under clouds and hope for a break in the clouds near the time of full moon, which is when I wanted to take the picture. After all, it isn't often that the moon is exactly full at a convenient time for picture-taking. Some things fell in my favor: I had completed my field battery over the weekend, so my scope had its larger, more stable and, most importantly, tracking, mount from which to operate. Here's a picture of the battery setup: Harvest Moon last month, the time of the Hunter's Moon rise over the three nights around full is closer together than at other times of the year: 6:01, 6:33, and 7:08 for my location in Boca Raton; compare that to the situation for March's full moon, which rose successively at 5:37, 6:44, and 7:52, more than an hour later each night.
The full moon for August 2011 (Sturgeon Moon, Dog Days' Moon, etc.) occurred during the afternoon hours of August 13 (2:57 p.m. to be precise) for East Coast observers, so I did my usual day-before-just-in-case-the-day-after-doesn't-work photo, and I'm glad I did. The "night" of the full moon, the 13th, was much stormier and less conducive to photos than the night of the 12th, despite the very brief window I had on Friday (only about 5 minutes in the clear). Here, then, is a snapshot in haste of August 2011's 18-hours-before-full moon:
Last night's/this morning's full moon, as seen from our Boca backyard: