Something unusual is happening in the sky on August 30. That night, the Moon will be both a blue moon and a supermoon. 

You may already know that the Moon doesn’t always look the same to us. Sometimes we see only part of it. That’s because of the different ways light and shadow hit the Moon as it orbits Earth. When we can see the entire Moon, we call it a full moon. There’s a full moon about every 29.5 days. You can think of a blue moon and a supermoon as unusual forms of a full moon.

A blue moon isn’t blue at all. It’s the name given to the second full moon in a rare month when there are two full moons. This happens only every two or three years.

A supermoon is when the moon is at the point in its orbit that is closest to Earth at the same time it is full. This causes the Moon to look a bit larger and brighter in the sky. Supermoons happen three or four times a year.

It’s not every day that there’s a supermoon. And it’s not every day that there’s a blue moon either. A moon that’s both at the same time is called a super blue moon.

And how often do super blue moons happen? The next two will be in 2037—one in January and another in March. But they’re usually much less common, sometimes happening as many as 20 years apart.