What is a Blue Moon? Why the May full moon is so rare.

May is double the fun for Full Moon and space fans because we’re getting two in one month. The first took place on May 1 and the second will be on May 31. This phenomenon is called a Blue Moon. And no, that doesn’t mean it’ll actually turn the color blue, it’s just a somewhat rare occurrence. You might say, “once in a blue moon”...

What is a Blue Moon?

According to NASA, a Blue Moon is when a full moon shows up twice within the same calendar month. The lunar cycle (the process of the Moon orbiting the Earth) lasts about 29 and a half days, which is slightly shorter than most months. This means that every so often the timing lines up like it does in May, with a full moon right at the start of the month and another at the end. It’s not especially common, only happening roughly every couple of years. This is a monthly Blue Moon.

There are also seasonal Blue Moons. These occur when four full moons fall all within one season, instead of three which is what usually happens.

When is the Blue Moon in 2026?

The Blue Moon in 2026 will occur on May 31. According to Time and Date, it will peak around 4:45 a.m. ET.

How often does a Blue Moon occur?

Blue Moons are fairly uncommon, as the saying goes. NASA says they occur roughly every two or three years.

When is the next Blue Moon?

Royal Museums Greenwich tells us that the next Blue Moon will be May 20, 2027. This will be a seasonal Blue Moon.