THE SOUTHWORTH PLANETARIUM
207-780-4249      www.usm.maine.edu/planet
70 Falmouth Street     Portland, Maine 04103
43.6667° N                   70.2667° W
Founded January 1970
Julian date:  2457733.16

               "The secret of getting ahead is getting started."

                                 -Mark Twain

THE DAILY ASTRONOMER

Tuesday, December 13, 2016

Super Moon Redux

Yes, we know

A Super moon loses a bit of it luster if it pops up too frequently.   Last month's Super moon was superbly presented to the populace because it was unusually close and woefully over hyped.     It was that perfect type of highly luminous Super moon that loomed majestically over Alpine snows and evoked awe even in the most austere.     

Tonight's Super moon seems more like a re-run:  astronomy in syndication.   One will notice that December's Super moon hasn't grabbed global headlines and captivated the collective imagination like November's did.      The human community that was enraptured by Selene's silvered visage last month, yawns indifferently and shrugs at the one presently rising above the frosted firs.    Ho hum...yet another Super moon

We must remind you, however, that Super moons aren't served up in baker's dozens.    They are, indeed, unusual and each one is worthy of much more than a passing glance.       We define a "Super moon" as a full moon that occurs around the time of perigee, the closest point in the moon's orbit.      The time period separating successive full moons -the Synodic Period- is approximately 29.5 days.  (Technically, the moon is full when it is at "opposition.")   The period separating successive pergiees -the anomaliastic month- is about 27.55 days.        Were these two periods equal and were the perigee to always correspond to the full moon, "Super moons" would be monthly occurrences and the term "Super moon" would never have arisen at all.      As they aren't equal, the coincidence of perigee and opposition isn't that common.

So, one might wonder,  how can we have a Super moon in both November AND December?     Simple:    the time periods separating the perigees and oppositions aren't the same in both months.

Last month, the moon reached perigee within 2.5 hours of becoming full.  That perigee was unusually close (356,511 km or about 221,036 miles.)    It was the closest Super moon in almost 70 years!    For this reason, that brilliant Super moon found itself squarely in the lime light.       

This month, the moon reached perigee on December 12th and will be full tonight.     Perigee and opposition are still close, though not as close as they were in November.  Also, this perigee wasn't as close:   358,462 km (222,246 miles.)    This Super moon won't be quite as bright or close as the one that dazzled the planet last month.  All the same, this moon will still be about 28% brighter than the apogee full moon (the full moon that occurs when the moon is at or near its point of greatest distance).        That, in and of itself, makes any Super moon worthy of its name.

​Super moon over snow fields.   Even though this month's Super moon might seem to lack the luster of November's Super moon, it will still be dazzlingly bright, especially when seen over the vast fields enshrouded  by freshly fallen snow.   Image: Ancient-Origins.net