Closest perigee for the moon this year

Moon, two days before full. August 29, 2012, 10 p.m. EDT.

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:

October 2012 full moon

October 28. 2012 Full moon

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.

It was relatively far away (402 000 km), and the libration was a little over 4 degrees both east (eastern limb tilted toward Earth) and south (southern limb tilted toward Earth). With the eastern tilt, you can just barely make out the margin of Mare Marginis and Mare Smythii on the right edge (Smythii on the equator, Marginis just north of that), and with the southern tilt, if you follow the “7:30” rays from the bright rayed crater Tycho in the southern hemisphere, you should be able to see—if I’ve worked out the confusing selenography correctly—the dark floors of the large craters Schickard and Schiller. Clavius looks as large as a lunar sea, with the prominent crater Porter in its northern rim:

Southern lunar highlands with prominent features labeled. The only one I’m sure of is Tycho.

I’m having a bit of a problem with the labeling, though. Tycho is obvious. I’m fairly sure I’ve labeled Schickard and Schiller correctly. But if Clavius is that giant area near the south pole that looks almost as big as a lunar sea, something’s wrong. With the southern limb tilted four degrees toward us, I’m surprised to see Clavius looking like it’s on the southern limb. (Then again, four degrees isn’t a huge tilt; when I simulate the view by looking at 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!

September 2012 full moon: the birthday edition!

September 29, 2012 Full moon

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).

As you can see, I’m still working on the focus problems; the low clouds that make the slight haze around the moon are out of my control, but I should at least be able to focus the darn image.

Happy Birthday to me. Time to order myself the Sky & Tel moon globe!

Full moon in August

August 30. 2012 Full moon

The second full moon in August (well, about 12 hours before full). I found my digiscope camera!

Despite what the Clear Sky Clock for Boca Raton says, this evening had nothing but high clouds and haze overhead, so while the naked eye views through the eyepiece were steady and beautiful, the camera view was a bit more obscured. Oh, well.

August’s moons: blue or not?

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:

That full moon is the most recent blue moon we’ve had, and it occurred on 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.

Full Moons 2012-2013  
August1, 3121

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.

Ful Moon, January 2012

January 8, 2012 Full moon

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:

Distance from Earth was about 387 400 km, about midway between this month’s perigee (369 882, on the 17th) and apogee (404 579, on the 2nd). In case you’re puzzled about why we’re nearly midway, measured in distance between Earth and Moon, but quite a bit less than midway, measured in days (we’re only 6 days past apogee and perigee isn’t for another 9 days yet), remember your orbital mechanics.

According to Kepler’s laws of orbital motion, a celestial body sweeps out equal areas in its orbit in equal time; the Moon has to move fastest when it’s nearest the Earth, because the line joining Earth and Moon is shortest at that time but it still needs to sweep equal areas in its orbit. When we’re closer to apogee, we can sweep out a large area at a slower pace, because that distance between Earth and Moon is greater.

How does this explain why we’ve got 9 days until apogee and we’re already more than halfway, measured in distance? I’m actually not very sure. I need to go back to celestial mechanics school, I guess.

But at least I got a picture of the moon!

December 2011 full moon

December 2011 Full Moon

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:

At the time, the moon was not too far away from apogee (five days previously), so it presented a slightly smaller target than it might have near perigee. But it’s not distance (395632 km at the time of the picture) that affects moon shots; it’s the local weather. It seems that there are always clouds and wind, unless there aren’t.

Here, below, are the shots I’ve managed to get over the two years this project has been running:

Hunter’s Moon 2011

October 11, 2011 Full moon

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:

As you can see, it’s on a wheeled cart; those deep-cycle marine batteries are heavy! Some people recommended that I use a LiPo (lithium polymer) battery designed for golf carts and wheelchairs, but I went with the cheaply available Wal-Mart option instead. Having field-tested the weight, I’m thinking those LiPo people weren’t wrong…

This is one heavy battery:

But it does the job, with power to spare (literally—there are two extra cigarette lighter sockets; one for the dew heater and one for a future, as yet unplanned, accessory). The socket strip even has a power switch to prevent accidental draining of the battery:

Even with the best setup, though, you can’t control the weather. There was enough moisture in the air that I never did get a very good shot of the full moon; at “exact” full (10:06), I couldn’t even see the moon from my backyard. This picture is from 9:53 p.m., and is the closest I could come this time around:

If you look at the southern and eastern limbs of the moon, you can see a faint haze; that’s not evidence of an atmosphere on the moon! It’s just photographic evidence of the atmosphere here on Earth!

The moon’s angular size at this point was just under 30 minutes of arc, about as small as it gets, since it was close to apogee, and the second-farthest apogee of the calendar year at that (only March 6, at 406 582 km, was farther).

Like the 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.

Full Moon August 2011

August 12, 2011 Full moon

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:

You can see how the western edge of the moon isn’t completely “full”; there’s still a little hint of the terminator (sunrise) line over there obscuring that limb; Grimaldi is the last “obvious” crater over there (actually a basin over 200 km in diameter); you can just barely make out some other craters right on the limb, which is angled fairly well away from us at this point by the moon’s libration. It’s a bit odd, because on the eastern limb, you can clearly see Mare Marginis, and Mare Smythii, which is rather far around that side, can also be seen poking its head around, although my libration tables show that it’s only about a 4° E and 4° S libration at this point. I’m not sure why we can see so much of that eastern limb right now. It might just be that the past few years of full moons (since Dec 2009) have had mostly W libration at full, and so even this minimal eastern limb seems extreme to me…

Here’s the complete gallery of full or nearly-full moons; one per month since December 2009.

May’s full moon: Milk Moon

May 16, 2011 Full moon

Last night’s/this morning’s full moon, as seen from our Boca backyard:

If you think this version is fuzzy, you should see the version after I “corrected” the focus! In the application file, the image is bright, but the highlights aren’t blown out. When I view the image in the browser, the highlights are blown. Wonder what the difference is? Same file, after all…

At all events, it was good to be out under the moon again. Tuesday night’s moon was on the rise as I was on my way home from the native plant society meeting (propagation workshop); I’m always a little in awe of a moonrise, and this one was spectacular; wish I’d had a camera with me…

1 2 3