Boston’s official Christmas tree crossed the border at about 10:30 this morning.

The 47-foot-tall white spruce is a gift from Nova Scotia for the assistance Boston provided after the devastating Halifax explosion in 1917.

The explosion killed more than 2,000 people and destroyed half of Halifax. Boston doctors and nurses were some of the first on scene to help. It is the 42d time Nova Scotia has sent a tree.

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The flatbed truck carrying the tree was nearing Portland, Maine, at about 1:15 p.m., a spokeswoman for the Nova Scotia government said. The large gift caught the attention of rubberneckers along the way.

“When you see a 40-plus-foot tree driving down the highway, people take notice,” said Andrew Younger, a minister for the Nova Scotia government. The tree is accompanied by two truck drivers and a town crier, he said.

The tree will rest for the night in Billerica at Reas Transportation before its homecoming to the Boston Common at 11 a.m. Friday, Younger said.

“There’s such a connection that people of Nova Scotia have with Boston,” said Younger. “Even before the explosion.”

A class from the Mather Elementary School in Dorchester has been learning about the explosion, and will be at the Common awaiting the tree’s arrival Friday, Younger said.

The classroom also has a Skype session with a class from the St. Stephen’s Elementary School in Halifax planned for early December, he said.

“They are looking forward to it,” he said. “They’re eagerly counting down the days.”

During a ceremony Dec. 5, Younger will join Boston Mayor Thomas M. Menino in the Common to light the Christmas tree. Many Nova Scotians receive Boston TV channels and are planning on watching the lighting, Younger said.

Halifax is also giving Boston a “book of thanks.” When the tree left the city Wednesday, residents signed a book, leaving little messages of thanks, Younger said.