I'm sure you have heard the term 'blue moon' before - as in 'once in a blue moon.' A rare moon event will be happening on Friday (July 31st).

Photo by: Lars Baron/Getty Images News

Don't get too excited for this 'blue moon' event. Yes it is rare, but no the moon won't actually be blue (how cool would that be though?). If anything, the moon will look about as normal as it possibly could. The actual event of a 'blue moon' has to do with the fact that lunar calendars and the regular calendar months don't line up. According to IFL Science,

In the past, a blue moon was used to mean "rare," but now the name is given to any full moon that is the second to appear in one calendar month. The lunar cycle is around 29 and a half days, and months of the year are between 28 and 31 days long. The probability of a full lunar cycle, from full moon to full moon, occurring precisely during this time window is therefore not very high.

 

That means the full moon we see tomorrow night is the second one we had in July - the first one occurred on July 1st. A 'blue moon' is still a decently rare event. IFL Science goes on to explain that the next blue moon will happen in a little over two years.