Stargazers are being treated to an astronomical show this week as an infrequent blue supermoon lights up the night sky, one of the largest and brightest full moons of the year.

Visible from late Sunday through Wednesday, it will be the first of the year’s four consecutive supermoons. But it’s a special instance, because Sunday’s supermoon is also a seasonal blue moon — the third full moon in a season of four.

The full moon will appear larger and brighter than usual.

The term “supermoon,” coined by the astrologer Richard Noelle in 1979, refers to a moment when the full moon is closest to Earth on its orbital path. It’s not an official astronomical term, but it is used by the lay-stargazers among us.

A “blue” moon bears no connection to the color blue, and the moon won’t have a colorful tinge. Instead, a blue moon is used to describe what is effectively an additional full moon, one that violates the rule of thumb, that there is one full moon in a month, or three full moons in a season.

Supermoons, full moons and blue moons occur on their own, but it’s not often that they all happen at the same time. According to NASA, roughly 25 percent of full moons are supermoons, and only 3 percent of full moons are blue moons. From Sunday through Wednesday morning, they’ll occur simultaneously.

Supermoons can appear 30 percent brighter than the average moon.

Like “supermoon,” the term “blue moon” is also colloquial, but blue moons are infrequent — more so than supermoons — and tend to occur every few years.

This blue supermoon is special, astronomers say, because it abides by the most traditional definition of “blue moon.”

“This is a proper one,” Ed Bloomer, an astronomer at the Royal Observatory Greenwich, said.

When is the best time to see the blue supermoon?

The moon will reach its peak fullness on Monday afternoon, but depending on the weather, you could glimpse the blue supermoon until Wednesday morning.

“The best time to see it is virtually whenever it’s dark,” Ciaran Fairhurst, a public engagement officer with the Royal Observatory in Edinburgh, said in an email.

You won’t need any special equipment or location to see it — just a relatively clear sky.

“You can combine it with other stargazing if you are able to get somewhere away from the bright lights of towns and cities,” Dr. Fairhurst said. “But if you just want to see the moon, virtually anywhere away from tall buildings will work.”

Is it really that rare?

Given that there was just a blue supermoon last August, it might seem as if the events happen semi-regularly — but based on predictions from NASA, this is probably the last chance to catch a blue supermoon this decade.

The terms aren’t scientific, so there’s debate over when the events occur. They’re irregular cycles, but according to NASA, the next blue supermoon probably won’t happen until January 2037.

Regardless of how strictly you adhere to the definitions, it will be a spectacle. Plenty of people harbor general interest in astronomy, Dr. Bloomer said, but events like the blue supermoon are special, and you don’t have to be a scientist to appreciate them.

“There’s just that spectrum of engagement which opens it up to everybody,” Dr. Bloomer said. “Not everybody is going to write academic studies about the moon — but you can glance upwards.”