BrainiYak

Introduction — why bother with the media’s supermoon coverage?

My wife’s haziness about the details of the “supermoon” last night encouraged me to pay some attention to how the popular media had covered this ordinary happening.  (I wondered just how much my science background predisposed me to understanding something that other people would not.)

I discovered that obtaining a short explanation of the simple mechanics underlying the supermoon perception was more difficult than I had anticipated.  In other words, the media seemed to perplex-i-fy something reasonable simple.

Diagrams and details were almost universally lacking in the reports that I saw.  And no one that I could find, except Space.com, bothered to tell the public when the dim opposite of May 2012’s supermoon would take place.

The best explanations came from scientifically friendly sources: NASA, astronomer Steve Owens, and Wikipedia.

However, with those three sources so easily available, I was a little perplexed that the media had done such a scientifically uninformative job of explaining last night’s bright full moon.

Background — a diagram of moon stages

MoonConnection.com, Understanding the Moon Phases (2012)

What is a supermoon?

The “supermoon” — more accurately called a “perigee-syzygy” moon — occurs approximately once every 412 days, as the full moon stage coincides with its closest approach to earth (perigee).

Weather equal, the full moon looks larger and, therefore, brighter than normal at this point.

Naturally, the brightest of perigee moons occur when the full moon stage coincides almost exactly with the lunar orbit’s point of perigee.  NASA calls these “super perigee moons.”

NASA’s supermoon explanation

From Tony Phillips:

The scientific term for the phenomenon is “perigee moon.”

Full Moons vary in size because of the oval shape of the Moon’s orbit. The Moon follows an elliptical path around Earth with one side (“perigee”) about 50,000 km closer than the other (“apogee”). Full Moons that occur on the perigee side of the Moon’s orbit seem extra big and bright.

Such is the case on May 5th at 11:34 pm Eastern Daylight Time1 when the Moon reaches perigee.

Super perigee Moons are actually fairly common.  The Moon becomes full within a few hours of its closest approach to Earth about once a year on average.   The last such coincidence occurred on March 19th, 2011, producing a full Moon that was almost 400 km closer than this one.

© 2012 Tony Phillips, Perigee “Super Moon” On May 5-6, NASA (02 May 2012) (paragraph split)

NASA went on to explain that perigee moons do raise the encroachment of tides, but only by a few centimeters.  Even when geography assists, the water rise is only about 15 centimeters (6 inches).  Hardly something to become alarmed about.

NASA’s visualization — a video

Science Casts – ScienceAtNASA, The Super Moon of May 2012, NASA (09 March 2012) (including diagrams of the moon’s orbit around the earth and an animated overlap of perceived moon size differences)

And Marco Langbroek’s excellent juxtaposition of still photographs

Marco Langbroek, Supermoon comparison, Wikipedia (19 March 2011) (top right photo, title and author evident after clicking on it, showing supermoon of 19 March 2011, as compared to the “average” moon of 20 December 2012)

Space.com’s contribution to understanding the connection of orbital periodicity to full moon brightness

Anticipating a precocious child’s question:

This month’s [May] full moon is due to be about 16 percent brighter than average.

In contrast, later this year on Nov. 28, the full moon will coincide with apogee, the moon’s farthest approach, offering a particularly small and dim full moon.

© 2012 Staff, ‘Supermoon’ Alert: Biggest Full Moon of 2012 Occurs This Week, Space.com (30 April 2012)

How much perigee-ness is required for a supermoon?

Absent specialized equipment and technique, it is not really possible to perceptually discern differences in size and brightness between full moons occurring at (a) absolute perigee or (b) just-before or just-after pergiee.

Perigee-syzygy — why is that term more informative than “supermoon”?

“Supermoon” certainly has a sensationalizing popular twang, but it says nothing about the mechanical causation of the phenomenon.

The term, syzygy, refers to state where a gravitational system’s astronomical objects have “fallen” into a straight line.  Therefore, the sun-earth-moon system achieves syzygy at our earth-bound perception of new and full moon stages.

When we combine perigee and syzygy, we recognize that we have specified the alignment that causes our perception of the “supermoon” phenomenon.   The scientific term, consequently, is much more phenomenologically descriptive than the popular one.

The best easily discovered explanation of perigee-syzygy timing — comes from astronomer Steve Owens

Astronomer and science communicator, Steve Owens (at Dark Sky Diary) calculated when perigee moons best correspond with syzygy:

[T]he Moon is full every 29.530 days.

The Moon’s orbit is elliptical . . . and so you would expect a perigee once every 27.321 days.

However the elliptical path around which the Moon orbits the Earth precesses (that is it is not fixed with the perigee occuring at the same part of each orbit; the place where perigee occurs moves, or pressesses) with a period of 8.8504 years, so that perigee doesn’t occur once every 27.321 days but rather once every 27.554 days (called the anomalistic period).

[Y]ou find that a full Moon will occur at perigee once every 411.776 days . . . or just less than once per year.

 All the articles that cite this as the closest full Moon in 18.6 years are wrong; there was a full Moon at perigee 411.784 days ago, on Feb 28th 2010 when the full Moon occurred at 1700 UT [universal time] and perigee occurred just 19 hours before at 2200 UT on Feb 27th 2010.

 The next so-called Supermoon will occur on May 6th 2012, when the full Moon will occur at 0400 UT, with perigee at the same time.

© 2012 Steve Owens, Supermoon Nonsense, Dark Sky Diary (09 March 2012)

The moral? — Our media often misses opportunities to better educate us about science

That is particularly distressing because easily witnessed natural phenomena present such effective opportunities to begin advancing the public’s scientific literacy.

Tagged: apogee, Dark Sky Diary, lunar orbit, media, moon phases, moon stages, perigee, perigee-syzygy, precess, precesses, Steve Owens, super moon, supermoon, supermoon nonsense, Tony Phillips