As you may have heard, a lot of people (and journalists) are talking about this 'blue moon' thing, as if it's something special. It's not. It's actually an example of human cluelessness. Let me explain ... :)
Blue moon is when there are two full moons in the same calendar month
. In July, we had a full moon on July 2nd, and then again on July 31th. The first full moon was just a regular full moon, while the second full moon, called a blue moon
, is also just a regular full moon.
No, it's not blue or even blueish. No, it's not some fancy astronomical event, nor is it a part of some cosmic phenomenon. It's a just a regular full moon.
Here is the problem. Traditionally, we are defining a day as being the time it takes for the Earth to spin around itself. A month is the time it takes for the Moon to spin around Earth. And, a year is the time it takes for Earth to spin around the Sun.
So how long does it take for the Moon to orbit Earth? Well, that is a surprisingly complicated question to answer.
Generally there are two ways of defining it. We can define it as the actual astronomical time that it takes for the moon to complete one full orbit. This is called a real orbit, or a ’sidereal’
And a real
lunar month is only 27.322 days.
Yep, our moon actually orbits Earth 13.4 times per year, not 12 as you might have thought.
However, since Earth is moving around the Sun as well, a full moon (and a new moon) isn’t related to the orbit of the moon, but rather the position of the moon in relation to the Sun, and our perspective of it from Earth.
This is called a Synodic month
, or the time it takes for the Moon to position itself in the same place relative to the Sun and us. And this happens every 29.531 days. This means that we get a full moon 12.368 times per year.
If you think this is confusing, take a look at the wonderful animation below, made by W. H. Freeman back in 2004 (http://goo.gl/HvgnJR
So, the reason why we had two full moons last month was not anything special happened. Instead, it’s simply because our moon doesn’t care about how silly we humans have defined a month.
We, as the Moon would say, got our calendars wrong. In fact, it's not even August. Since the moon orbits 13.4 times around the Earth, we are currently in this extra month which I will call 'awesome-month' ;)