Blue Moon Hypothetical representation

By BlueHypercane761 (Own work) [CC BY-SA 3.0 (], via Wikimedia Commons

It is true that airborne particulates can imbue a blueish tinge to the moon, but this has nothing to do with the term ‘blue moon’.

What makes a blue moon blue?

The definition of “blue moon” is not mystifying. It is nothing more than the second full moon in a single calendar month.

Two full moons in the same calendar month are mathematically possible because the moon orbits the earth once every 28 days. The number 28 should be more significant to us than blue moons. The time it takes the Moon to orbit the Earth makes the number 7 so special, the four distinct phases of the moon nicely divides 28 into four groups of 7. Hence came into existence the number of days in what we call a week.

So the answer to ‘What makes a blue moon blue?’ has nothing to do with its color. It is just a name for a phenomenon that does not happen that often.

A blue moon looks exactly the same as any average full moon, regardless of any media hype that may be doing the rounds at the time.

Black Moon

On a similar theme, but quite the opposite of a ‘blue moon’, is the notion of a ‘black moon’. This term refers to the second new moon in a single calendar month. We don’t hear much talk about black moons. I suppose blue moons are of more interest due possibly to the saying ‘once in a blue moon’.

Blood Moon

Now this terminology may well be something you may have heard. Blood moon is used to refer to the moon when tinted red during a lunar eclipse. That is, when the Moon sits in the shadow cast by the Earth.

Lunar eclipse October 8 2014 California Alfredo Garcia Jr mideclipse.JPG

By Tomruen ([1] [2]) [CC BY-SA 4.0 (], via Wikimedia Commons

Full Supermoon

What should fascinate people more than a blue moon is the phenomenon of a ‘full supermoon‘. This is a full moon that coincides with the Moon being at a point in its orbit that places it closest to the earth, its perigee (which just means ‘near Earth’).

A full supermoon is a full moon that is significantly larger and brighter than an average full moon. The size of the Moon, as it appears in the sky, varies by 14% from its smallest to its largest (supermoon).

By Marcoaliaslama (Own work) [CC BY-SA 3.0 ( or GFDL (], via Wikimedia Commons

By Marcoaliaslama (Own work) [CC BY-SA 3.0 ( or GFDL (], via Wikimedia Commons

The next full supermoon is coming on August 29, 2015.

Black supermoons, blue supermoons, and blood supermoons are all possibilities.

Prehistoric Moon

The Moon is receding away from the Earth at about 4.3cm per year. Using this fact one can easily surmise that hundreds of millions of years ago the Moon would have been sufficiently closer to the Earth so as to make it even more prominent in the sky. Some sources quote it could have been the size of a hand held at arms length, whereas now it is the size of your thumb nail held at the same distance. There is no way to prove this but regardless, it makes a rather interesting notion that allows one to think of how different nights would have been back then. A full moon would have illuminated the night sky more brilliantly than the brightest full supermoon in present times. The difference in illumination between a new moon and a full moon must have been as the difference between night and day.


I found the following site to contain some very interesting information about the Moon.

77 Out-of-this-World Facts About the Moon

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