How It Works

Why was the ‘supermoon’ so super?

On March 19th 2011 the moon was at its closest point to Earth in 18 years, appearing 14 percent larger and 30 percent brighter in the sky, leading many to dub it the “Supermoon”. This was due to the orbit of the moon, which is not exactly circular. Instead, it goes through motions of closest and furthest approach and, on Saturday evening, it reached its closest point (221,565 miles / 356,575 km) from Earth at almost the same time that it was a full moon, enhancing the ‘Supermoon’ effect. However, this event does not entirely account for how large the moon appeared. For that, we must look into ‘The Moon Illusion.’

Now, this is trickier than it sounds. We’ve all seen how a rising or setting moon on the horizon appears larger than an overhead moon, and yet scientists and psychologists still can’t agree on or understand why it occurs – even NASA can’t fathom it.

There are two main theories behind ‘The Moon Illusion’ that contributed to the ‘Supermoon’. We know the size of the moon doesn’t actually change so we can safely assume that it’s a trick of the mind. One idea suggests the viewer instinctively attempts to judge the distance to the rising moon (it’s hard to comprehend hundreds of thousands of miles) based on visual objects, such as trees and houses in the distance. These objects seem near the moon, giving a distorted point of reference, making it appear bigger. However, this theory can be called into question as pilots have also seen the illusion despite no point of reference against the ground.

The second theory has to do with the fact that we tend to think of the sky as a flattened dome, rather than the hemisphere it is, and therefore perceive things overhead (birds and planes) as much lower, or nearer than the things we see on the horizon. And so although the moon may well be the same size whether it’s above your head or off on the horizon, because you believe it is farther away at the horizon you perceive the moon to be much larger. Either way, your brain has been tricked.

The Ponzo Track

The theory that the objects in the foreground affect how far away we believe the moon to be can be comprehended by looking at Mario Ponzo’s railway track diagram in which two physically identical lines appear different sizes due to the perspective created by the tracks converging in the distance. The line at the top of the diagram appears wider than the line below because it seems to span a greater distance across the railway lines, which we wrongly perceive as parallel. We’re also reminded of the Father Ted episode when Ted explains to Dougal that the toy cows are ‘small’ but the real cows outside are ‘far away’.

Yep, these lines really are the same size



  • Steve

    Hello! Being from the north of ireland I sadly missed out on the supermoon! The joys of overcast. Will it be another 18 years before i see it again? If so i hope the moon will be in the right phase. As for the father ted episode, what a classic! Enjoying issue 19 a great deal. Keep up the great work!

    • Hi Steve, it’s a shame you weren’t able to see it in all its glory. Here in Bournemouth we had a fairly decent view, certainly very impressive! Actually, supermoons are much more regular than 18 years, it’s just that this one happened to coincide with a full moon more closely than previous ones. The last supermoon, in 2008, was just 21 miles further away but the moon didn’t turn full for several hours after the point of closest approach, so the effect was less prominent. Here’s a list of past and upcoming supermoons for your perusal:

      * November 10, 1954
      * November 20, 1972
      * January 8, 1974
      * February 26, 1975
      * December 2, 1990
      * January 19, 1992
      * March 8, 1993
      * January 10, 2005
      * December 12, 2008
      * January 30, 2010
      * March 19, 2011
      * November 14, 2016
      * January 2, 2018
      * January 21, 2023
      * November 25, 2034
      * January 13, 2036

      So, just five years to wait! There will also be a full moon on November 14 2016 so chances are that the supermoon should be of a similar size. You can check out when full moons will occur at the following link so you’ll know which year to tune in for the next supermoon!

      http://eclipse.gsfc.nasa.gov/phase/phase2001gmt.html

      Hope I’ve been of help 🙂

      Jonny O’Callaghan
      Staff Writer

  • steve

    Thankyou very much for your help. I will be marking some dates on my calender! I was actually watching the moon rise last night while viewing saturn in my scope when i posted my comment. I hope some day i will be reading a page in your magazine on how life works on other planets! Thanks again.

    • No problem Steve. I too very much hope we will be able to definitively talk about alien life in the not too distant future, be it microbial or otherwise!