NORAD’s tradition of tracking Santa’s sleigh began with a wrong number.

Right before Christmas in 1955, Sears ran an ad offering millions of toy-hungry girls and boys the chance to talk to the big man himself. In Colorado Springs, the retailer published the local phone number to the North Pole as ME 2-6681.

There was only one problem: The number was one digit off.

And that wrong number rang on the desk of a high-ranking officer in a bunker at the Continental Air Defense Command — the predecessor of the North American Aerospace Defense Command, which has the less-than-festive mission of detecting and defending the continent against nuclear attack.

Col. Harry Shoup fielded the first call on the command’s red phone.

“The Block House, they called it. It had no windows — very top secret,” recalled the late colonel’s daughter, Terri Van Keuren, in an interview with The Post.

“The phone rang, and he picked up. ‘This is Colonel Shoup, commander of this combat station. Who is this?’ ”

His gruff demand was at first met with silence. He repeated himself, and eventually a meek little boy’s voice came over the line.

“Is this Santa Claus?” he murmured.

Worried there had been some kind of security breach, Shoup again demanded the caller’s name. He heard crying, and ­another query came through the tears.

“Is this one of Santa’s elves?”

Shoup recognized he was in a moment that could destroy the little boy’s faith in Santa.

“Yes, I am,” he said. “Have you been a good boy?”

The two talked behavior and presents for a while, and when they were done, Shoup asked to talk to the boy’s mom.

“He asked her: ‘Do you have any idea who you’ve called?’ ” Van Keuren said. “She told him to take a look at that day’s newspaper.”

As the day progressed, the calls flooded in, and Shoup directed his men to jolly well answer as Claus.

A couple of weeks later, Shoup, who was on vacation, decided to drop into work unannounced. When he walked into the room, which held a huge plexiglass map of North America, he noticed that someone had affixed a drawing of a sleigh to it.

The subordinate cowered and asked Shoup if he’d like him to take it down.

“There’s something good we could do with this,” Shoup replied.

He then called a local radio station with the news that the command center was tracking Santa’s sleigh. Ever since then, NORAD has been tracking Santa.