Once in a blue moon is happening in October

orbit means those tides change. When it’s closest, at perigee, the tides are highest. When it’s farthest away, at apogee, the tides stay docile.

The New Moon, on Oct. 16 — which we can’t see — will be the closest new moon of the year. The Blue Moon on Halloween will be at apogee — a less-thansuper, but still haunted ghostly galleon.

And, Robson said, the moon’s gravity also helps control Earth’s slight wobbling as it rotates.

“The moon keeps it in check,” he said.

Bill Cloutier, one of the leaders of the McCarthy Observatory and an avid student of the moon’s geography, said that by learning about the moon, we learn about Earth’s earliest years. Then, like the moon, it was bombarded and cratered by collisions.

“All those features on the moon caused by impacts were happening on Earth,” Cloutier said. Oceans and tectonic plate shifts may have erased most of them, but geologists have discovered about 100 craters on Earth’s surface, he said.

In fact, most astronomers now think one of those collisions created the moon about 4.5 billion years ago. The interloper was a Mars-like planet, Theia, that smashed into Earth, creating a ring of debris that coalesced into the moon.

It’s the only real moon that circles one of the four rocky planets — Mercury, Venus, Earth and Mars. (Mars has two small asteroids that circle it, but no one has written “Blue Moon of Mars, Keep on Shining …’’ about them.)

And, relative to the size of Earth — our moon’s diameter is about onequarter of our planet — it’s the largest in our solar system. It’s also the only place humans have walked upon, other than our own terra firma.

“It’s the only other place with our footsteps, other than Earth,” Cloutier said.

The last Apollo mission, Apollo 17, was in 1972. NASA has now announced it wants to send astronauts back there by 2024.

Chester, of the US Naval Observatory, said that may be optimistic. But he said it would make sense to build a permanent station there, as a first step to exploring Mars, in part because it’s so close by.

“It takes us three days to get to the moon,” Chester said. “It takes five or six months, at least, to get to Mars. Going to the moon, the supply line will be much easier.”

As to how blue moons became known as blue moons ….

The phrase dates back to an anti-clerical pamphlet written in 1528 by two Englishmen, William Roy and Jeremy Barlowe. They meant to mean something far-fetched and ridiculous.

But eventually, the phrase took on its present meaning — something that doesn’t happen very often.

The Old Maine Farmer’s Almanac defined a blue moon as the third full moon in a season with four full moons — a seasonal blue moon.

But a 1946 article in Sky & Telescope Magazine confused things by saying that if a month has two full moons, the second could be called a blue moon — a calendar blue moon.

Which is what we’ll have in October.

“It all depends on which folklore you believe,” Chester said.