An uncommon celestial event will take place on Monday night that (likely) won’t occur again for several years: a full moon that can be described as both a supermoon and a blue moon will illuminate the night skies across the U.S.

If that sounds a bit confusing, it’s because astronomers have different criteria for what constitutes a supermoon and a blue moon. (In fact, just last year, a full moon qualified as both under certain definitions.)

Here’s why Monday’s full moon is noteworthy:

What defines a supermoon?

A supermoon occurs when the moon appears closer to Earth than usual. This happens when the moon reaches its closest point to Earth simultaneously with a full moon. The term “supermoon” was introduced by astrologer Richard Nolle in 1979.

“Various sources have minor differences in how they define what distance from Earth qualifies as a supermoon,” NASA explains. “Since the moon’s orbit isn’t a perfect circle, there are times when it is closer to Earth.”

When the moon is at its closest to Earth, about 226,000 miles away, it appears larger and brighter than at other times. A supermoon can be about 30% brighter and 14% larger than a regular full moon.

What is a blue moon?

There are two types of blue moons: seasonal and monthly. The one occurring on Monday is of the seasonal variety.

A seasonal blue moon is defined as having four full moons in one season (in this case, summer). The third full moon out of the four becomes known as the blue moon.

The second definition — which originated from a misinterpretation — is the monthly blue moon, denoting the second full moon within a calendar month.

In both definitions, blue moons are considered rare due to an unusual pattern in the typical occurrence of one full moon per month.

The moon’s cycle lasts 29.5 days. However, as Mike Reed, an astrophysics professor at Missouri State University points out, months can vary in length from 28 to 31 days.

This discrepancy leads to unpredictable alignments, resulting in either an extra full moon in a season or a month — hence, a blue moon.

Typically, a month has one full moon — unless there’s a (monthly) blue moon. Similarly, seasons usually contain three full moons — unless a (seasonal) blue moon occurs.

Additionally, Monday’s full moon will likely not appear blue; any blue tints in photos are usually achieved through special filters or editing software.

How rare is a super blue moon? When can we expect the next one?

“It’s complicated to answer because it hinges on how you define a supermoon and a blue moon,” said Edward Bloomer, senior astronomy manager at the Royal Observatory Greenwich in the U.K., in a response to YSL News via email.

The answer varies based on whether one is referring to a seasonal blue moon versus a monthly blue moon. Adjusting the criteria for what counts as a supermoon will also shift the answer.

On average, a blue moon happens once every two to three years, according to NASA. Meanwhile, the interval between super blue moons can reach up to 20 years, with the average being around 10 years.

When will the next super blue moon take place?

A reasonable prediction is January 2037, as per NASA.

However, you may encounter another one before that, contingent on how you define a super blue moon.