The expression “once in a blue moon” may actually have some accuracy behind it. The expression refers to a “blue moon,” or the second full moon to happen in a single month. And according to a report from Phys.org, you’ll have a chance to catch a glimpse of the last blue moon for three years this Friday night.

Indeed, there were two full moons in the month of July – one on the 2^nd and one on the 31^st. The phenomenon is a result of the synodic period, or the amount of time it takes for the moon to return to its original phase. Typically this takes 29.5 Earth days, so there are months when this cycle will occur at the beginning and end.

The last blue moon occurred on August 31^st, 2012. The expression implies that it’s a rare occurrence, but it typically happens once every two to three years.

There is not much importance to a blue moon from an astronomy perspective. Astronomers don’t even usually observe the night sky when there is a full moon, because of the light pollution. There is typically no discoloration to the moon, and it appears just like any other.

While there will be no spectacular show of a lunar phenomenon on the night of the 31^st, it will still be a beautiful evening for a moonlit stroll.