October’s full moon rises plump and bright this weekend under clear skies that should provide the perfect setting for the mysterious “moon illusion.”

While the moon- dubbed a hunter’s moon this month - doesn’t become precisely full until Sunday evening, it appeared full enough Saturday when it breached the horizon.

While it may seem like the moon is looming larger than normal as it hugs the horizon, the phenomenon is called the moon illusion - a poorly understood wonder that may be all in our heads.

According to EarthSky.org, one theory is that the moon looks bigger closer to the horizon because you are seeing it in relation to other items around it, such as trees and buildings.

“Your brain automatically compares the moon to these reference points,” Deborah Byrd and Martha Morales wrote in an August column. “But when the moon is higher up, there’s nothing to compare it to.”

There’s also the so-called Ponzo Illusion, where a researcher in 1913 drew two identically-sized bars across a pair of converging lines, such as you would see looking down railroad tracks. The bar farther in the distance appears wider because it stretches a greater distance across the lines.

Bob King, a writer for SkyAndTelescope.com, called the moon illusion “one of the oldest psychological tricks known to humankind.”

He said the rising moon is actually 1.5 percent smaller than when it’s overhead because you are looking across the radius of the Earth when the moon is rising.

The size of super moons - loosely defined as those that come closest to Earth in their orbit - do actually appear larger and brighter.

The January 2018 full moon, which was the closest the moon came to Earth in 14 months, appeared 14 percent larger than the smallest full moon of the year and 30 percent brighter, according to NASA.

This weekend’s full moon is the first full moon of autumn and follows September’s harvest moon, which is closest to the fall equinox.

NASA says the earliest use of the term “hunter’s moon” is in the Oxford English Dictionary from 1710.

“According to the Farmer’s Almanac, with the leaves falling and the deer fattened, it is time to hunt,” a NASA blog says. “Since the harvesters have reaped the fields, hunters can easily see the animals that have come out to glean (and the foxes that have come out to prey on them).”