Saturday, June 22, 2013

Supermoon tonight! But what makes it so super?

A couple of years ago, a "new" term started to be used for the phenomena that will occur again tonight. A "supermoon" is best described as a perigee moon in it's full phase. In other words, a full moon that is at it's closest approach to Earth.  That's what makes it so super!  Not to rain on your parade, but I tend to think that the term is part hype, but with some astronomical fact behind it. It's still "super-cool" to think about, even if not that much more impressive than a brilliant full moon. Here are the supermoon facts:

  • A full moon occurs every 29.53 days, or about once per calendar month
  • The perigee of the moon is when it makes it's closest approach to Earth and also occurs roughly once a month
  • The coincidence of these two, a full moon nearly at perigee, happens early Sunday, when the full moon will make it's closest approach to Earth for 2013
  • A full moon occurs within several hours of perigee every 14 months - early Sunday those two occurrences are within an hour of each other
  • The full moon of June 22-23, 2013 is the closest and largest full moon of this year
  • Though the full moons of May and July also classify as "supermoons," this one is the super-est!
  • The next supermoon will be August 10, 2014

A description of what makes a moon a "perigee" or "apogee" moon, courtesy
So why I am not so impressed with the supermoon?  Well, while a full moon at perigee is up to 14% larger and 30% brighter than a full moon at apogee (it's furthest point from earth), the apogee moon occurred many months ago, so the moon will not be 14% larger than it was last night.  By naked eye comparisons, the difference from recent full moons you may have seen, even under the most ideal viewing conditions, will be minimal.  HOWEVER, if you take a minute to think about the fact that these astronomical phenomena are occurring simultaneously while you're checking out the supermoon (even if it does happen every year and the difference between the moon last night and tonight is barely noticeable), it can be SUPER cool to see!

Comparison of the supermoon of March 2011 to a "regular" full moon 3 months earlier.
So when should you view the supermoon?  The actual time of the full moon is 6:32am CDT Sunday, which unfortunately is after it has set here in the Memphis metro.  However, anytime tonight (the moon rises at 7:35pm and sets at 6:02am Sunday), the moon will appear full and will be nearly at it's closest point of approach to the Earth.  To view it at the closest point in time to it's actual perigee and full phase, catch it Sunday morning right before it sets, or between 4-6am.  The distortion of the atmosphere, which causes the moon to appear larger when it's near the horizon than overhead, will make it that much more impressive!

For more of the nitty-gritty details, check out this article from

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