Sunday, May 6, 2012

"Supermoon" Nonsense

Last night, the Moon was full (as it is every 29.5 days).  It was 356,953 kilometers from Earth.  Since the Moon orbits the Earth in an ellipse, it also happens to also be the closest full Moon approach of the year (called perigee).  Last month's full Moon was around April 7 and it was 358,313 km away (here's an apogee and perigee calculator).

For some bizarre reason, the news media has taken to calling these events "Supermoons" and there have been a spate of "news" stories about this the past couple of days.  "Supermoon" is a mostly bullshit term which I believe was first proposed by an astrologer named Richard Nolle.  I say mostly bullshit because it is grounded in real astronomy (the orbit and phases of the Moon) but attributes special qualities to a Moon that's at perigee when it's full (there's the bullshit part).

I wrote about this supermoon crap in March 2011 as well.

I recently had a post on calculating the size of the Sun in the sky where I showed how to derive a formula for calculating how large an astronomical body appears in the sky.  Works for the Moon as well as the Sun.  The formula is:

     d = 2 tan-1 [(D/2)/d]

where D is the diameter of the Moon, d is the distance from the Earth to the Moon, and delta (d) is the angular size of the Moon in the sky.  The mean diameter of the Moon is 3,476 km.  The distances are listed above.  Let's calculate the difference in the "super" Moon's angular diameter versus last month's Moon.

"Super" Moon:

     d = 2 tan-1 [(D/2)/d] = 2 tan-1 [(3,476 km/2)/356,953 km] =0.558°

"Regular" Moon:

     d = 2 tan-1 [(D/2)/d] = 2 tan-1 [(3,476 km/2)/358,313 km] = 0.556°

That's a difference of 0.4% when looking at the Moon in the night sky.  Here is this difference shown to scale:

Not very impressive, is it?  And this is NOT how large the full Moon appears in the sky.  Go outside anytime the Moon is full, outstretch your arm and extend your pinkie finger.  The fingernail on your pinkie finger will easily cover the Moon.

Not that I'm denigrating the Moon.  I love the Moon in all its phases and am always looking up (even during the daytime) to see if I can spot it and know where it is in its cycle of phases.  I always encourage people to go out and look up.  But there's nothing astronomically special about this weekend's full Moon.


  1. CNN says that NASA reports the moon to be 14% larger and 30% brighter than other full moons. Is this only at the horizon or inaccurate data? They also say that the moon appearing larger at the horizon is a bit of a mystery. I thought that looking through more atmosphere at the horizon caused the larger moon illusion...

    1. See these two posts by Phil Plait at Bad Astronomy:

  2. Well, burst my bubble!

  3. I have heard all this before, but after looking at the moon last night with the kids at about 9p (so it was not on the horizon, although it was barely above treeline), we all agreed it "looked" larger... and we had a great time standing outside in very bright moonlight spending some time together. Of course, the media likes hype and stretches the truth but I am just as happy noting the 16" pizza slice moon versus the 15" slice moon even if it is not a major event and happens every few years.

    1. I thought it was a "Stupidmoon". Where everyone who is stupid went outside to check it out all at once.

  4. Thanks, that cleared things up!