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.

The moon in June: Full on the third

June 3, 2012 Full moon

Hello, there! Now that we’re already a couple of days past the new moon, it’s high time once again for the monthly snapshot of the full moon. (I say snapshot because that’s really what it is. If and when I ever get an imaging system that works with my telescope gear, I’ll “upgrade” the name to “image.”)

This was taken way back on June 3, fighting the ever-present low cloud/haze that obscures the moon for nights on end.

Typical summertime complaint of south Florida backyard astronomers.


May’s Full Moon

May 5, 2012 Full moon

May’s full moon occurs, at least for North Americans, on a relatively misunderstood holiday (Cinco de Mayo, which is not the Mexican equivalent to the Fourth of July, but does celebrate an unlikely victory of the Mexican army over the French in 1862). Of perhaps greater interest to astronomically minded folk, though, this month’s full moon occurs not only near perigee (just an hour separates the two events) but near the closest perigee of the year (the Earth-Moon distance being 356,955 km at the time).

A similar perigee situation took place last October, but at new moon, instead of  full.

Happy Cinco de Mayo! Maybe some year I’ll learn how to image the moon.
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