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Friday, May 04, 2012

 

The "Super Moon" of Sunday May 6, What Can You See?

Illustration of the orbit of the Moon around the Earth. Distances for apogee and perigee are given for the 2011 Full Moons because I was lazy and didn't want to redraw the diagram. 

 The Full Moon of this Sunday May 6 (in Australian time zones anyway, it's 3:34 May 6 UT, which is 13:34 May 6 AEST), occurs at perigee.

Some folks have started to call perigean Full Moons "Super Moons" for reasons that, to my mind, are not entirely justified.

Still, we have the name, let's move on. What will you see, and what are the implications of this coming "Super Moon".

The Moon has an elliptical orbit around the Earth (greatly exaggerated for illustrative purposes in the diagram above). When the Moon is closest to Earth, it is at perigee, and furthest, at apogee. The orbit of the Moon precesses around the Earth, so that sometimes perigee occurs at full Moon, sometimes at new Moon, and every time in between. Also for a variety of reasons the distances of closest and furthest approach can vary by up to almost 1,000 Km.

Perigeee Full Moons ("super moons") are closer and brighter than other full Moons.

So, what can you see?

Without a telescope, and a near photographic memory, not much. The Moon is closer at perigee than at apogee and  this years perigee full Moon could appear up around 12% bigger and a bit under 30% brighter in the sky than this years apogee Moon.

However, the full Moon is only around half a finger-width wide in the sky, 14% of half a finger-width is not very much.

Technically the Moon is around 33 arc minutes wide. The limit of distances that someone with good vision can distinguish between is 1 minute of arc (about the width of a human hair). So, for the vast majority of people any difference smaller than 1 minute of arc cannot be seen.



Perigee and apogee as seen through a telescope. With the unaided eye, the Moon only appears half a finger-width wide, so the difference is much harder to see.

This full Moon (May 6) will be 33.3 arc minutes in diameter, last months full Moon was 33.2 arc minutes in diameter, so even if you remember the last full Moon, you won’t notice any difference. If you wait for next months full Moon, that will also be 33.2 arc minutes in diameter, so you won’t notice any difference either.

If you can wait until November the 28th. when the Full Moon is at Apogee, then it’s diameter will be 29.1 arc minutes, and you could notice a difference if you have a good memory and good eyesight , but it won’t be spectacular.

However, it will be a good photo opportunity, if you have a decent zoom on your camera, taking a photo of the Moon on May 6 and then again on  November the 28th you will see a decent difference (you need to use exactly the same zoom enlargement, see Inconstant Moon for instructions).

What will Happen? 

The Moon will be bright and lovely, if lucky you may see a satellite pass or a meteor (sadly, there appear to be no International Space Station passes or Iridium flares at this time). Maybe you may see a Moon Bow. But nothing else will happen.

One of the reasons why I am annoyed with the term "super moon" is that it was coined by astrologer Richard Nolle. While I am all for getting people excited about going out an seeing the sky for whatever reason, Mr. Nolle promoted the idea that "super moons" are linked with earthquakes, the the name "super moon" almost always pulls up this spurious idea..

They are not. And the "super moon of March 2011, which was much closer than this one, didn't result in any unforeseen earthquake activity.

Table of closest perigees for each year since 1991, 
Date           Time   Distance           Moon Phase      Year
Mar 8          8:36   356529 km ++       F- 1h          1993
Jan 19        22:13   356548 km ++       F+ 0h          1992
Dec 12        21:38   356567 km ++       F+ 4h          2008
Mar 19        19:10   356577 km ++       F+ 0h          2011
Jan 30         9:04   356592 km ++       F+ 2h          2010
Nov 4          0:42   356614 km ++       F- 4h          1998
Dec 22        11:01   356654 km ++       F- 6h          2001
Dec 22        11:01   356654 km ++       F- 6h          1999
Oct 26        11:52   356754 km ++       F+ 6h          2007
Feb 7         22:20   356852 km ++       F- 8h          2002 
Apr 25        17:18   356925 km ++       F- 2h          1994
Jul 30         7:37   356948 km ++       F- 2h          1996 
May  6         3:34   356953 km ++       F- 0h          2012**
Sep 16        15:25   356965 km +        F- 3h          1997
Jun 13         1:06   357006 km +        F- 2h          1995
Apr 17         4:59   357157 km +        F+ 9h          2003
Jul 21        19:46   357159 km          F+ 8h          2005
Sep 8          3:08   357174 km +        F+ 8h          2006
Jun 3         13:11   357248 km ++       F+ 8h          2004
Jan 10        10:53   357500 km +        F- 16h         2009
Dec 22         9:29   358353 km          F+ 23h         1991
Nov 14        22:59   366050 km          F+3d 1h        2000

This years "super moon" is indicated with asterisk**. It's not particularly super, being quite a bit further away from us than the March "super moon" of 2011.

So, go out and enjoy this weekends "super moon" free from fear. If possible, try and photograph it and the November 28 apogee Moon (what do we call them? "underachieveing moon"?).

Here's the Bad Astronomers take on the "super moon", and this link is to a great "super moon" cartoon.

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Comments:
The difference of perigee and apogee full moons to the naked eye is not "spectacular" - but it's pretty obvious when you try it. See this report for an inadvertent 'blind test' I did in this regard last year; since then I've repeated the experiment several times: it works!
 
http://en.wikipedia.org/wiki/2011_T%C5%8Dhoku_earthquake_and_tsunami
 
If the Moon's gravity can significantly raise the oceans weighing billions of tons, plus raise the atmosphere in a similar way to the ocean plus raise the Earth's surface by up to 30cm/1ft, why could it not effect tectonic plates that would result in shifts causing earthquakes?
 
Daniel, for people like us who are constant observes, and who can actually remember what an apogee moo looks like, it's fairly easy. But most people can remember what a moon 6 months ago looked like.
 
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