Caught a sight of moon tonight (by chance) and came in surprise that moon looks thrice in size to its usual size.

Mostly called as Blue moon. Didn't succeed in getting the details and explanations of the phenomenon on Internet. Wikipedia just tells about the origin of the word blue moon and particular dates at which super moon is more obvious but doesn't shed the light on the true physical causes.

Just wondering what could be the causes that makes the appearance of moon (near the horizon) so big.

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    $\begingroup$ A journalist. The Moon illusion, where it appears bigger at the horizon, was known long before we started giving snowstorms names: en.wikipedia.org/wiki/Moon_illusion How close the moon is to the earth when full also affects its apparent size: apod.nasa.gov/apod/ap140121.html $\endgroup$ Aug 2, 2015 at 11:40
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    $\begingroup$ A Blue Moon has nothing to do with size. It is the appearance of a full moon an extra time during either a month or season. 2 full moons occur in a calendar month about 2.5 years apart. $\endgroup$
    – LDC3
    Aug 2, 2015 at 13:49
  • $\begingroup$ Let me add to the chorus here. The "supermoon" is entirely manufactured by media, because their revenue depends on people remaining interested even when nothing really happens. You will never be able to detect the size difference between a "supermoon" and the regular moon with the naked eye. Even in a telescope, you need a special eyepiece with a reticle, and you need to keep a detailed log of observations over a long time to be able to tell any variation. Even so, the variation is extremely small - for most amateurs the variation is smaller than the precision of their measurements. $\endgroup$ Aug 2, 2015 at 21:55
  • $\begingroup$ @FlorinAndrei What I saw it was three times bigger in size than usual moon, though it was quite yellowish. I am adding the photo above which I took from my mobile phone. It can be seen that it is pretty much big than usual. $\endgroup$
    – kaka
    Aug 3, 2015 at 20:12
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    $\begingroup$ @kaka It's an illusion. Read the links posted by others about the Moon Illusion. It's very convincing, I know, but like any other illusion it's not real. If you had binoculars with a graded reticle, allowing you to actually measure the angular size of the disc of the Moon, you would have seen it was actually the same as usual, plus or minus a few percent points (the Moon's usual variation in diameter through its orbital cycle); that difference is too small for the human eye to detect. The media is doing science a great disservice here, unfortunately. $\endgroup$ Aug 3, 2015 at 23:32

1 Answer 1


A Supermoon is when the point in the Moons orbit when it is at its closest to the Earth coincides with full or new Moon (see Wikipedia entry).

A blue Moon is (in this instance) the occurrence of a second full Moon in the same calendar month (see Wikipedia entry), and is unrelated to the coincident Supermoon.

Also the apparent size difference between a full Supermoon and a more usual full Moon is inlikely to be detectable by a casual observer. As can be seen in the side-by-side comparison graphic on the Wikipedia page reproduced below:

enter image description here

The apparent giant Moon you report is the well known Moon illusion where the Moon appears larger when observed near the horizon compared to when seen higher in the sky. The Moon illusion is startling if you are unfamiliar with it and very much more noticeable than a Supermoon to the casual observer.

While out in the morning of the 31st, walking the dog in the local recreation ground, between ~04:00 and 05:00 BST (03:00-04:00 UT) I observed the apparent size of the Moon and commented to the dog that we will probably see someone (in the media I thought) mistake the apparent size illusion I was seeing for the effect of the Supermoon. (I had also noted the same effect the day before at a similar time, but not on the 1st as we go out later at the weekend)

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    $\begingroup$ Excellent answer. Nobody could tell the difference between the "supermoon" and a regular moon without special instruments. The whole "supermoon" thing is fabricated by ignorant journalists. $\endgroup$ Aug 2, 2015 at 21:59

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