The Senate spectator gallery was at least half-empty throughout the first week of President Donald Trump’s impeachment trial and senators serving as the jury in the marathon sessions are taking notice — some stunned that there aren’t more people watching history unfold, while others understand the public avoiding the repetitive proceedings.

“I’m really surprised at that because this is kind of historic and I would think this would be an opportunity for people to get in there regardless of whose side you are on,” Sen. James Inhofe (R-Okla.) told The Post Friday.

Journalists aren’t allowed to bring cellphones or cameras into the gallery, so the noticeably small audience is known directly only to people able to access the chamber.

The spectator gallery offers a view over the shoulders of senators in a surprisingly compact chamber as debate unfolds over whether Trump should be removed from office. An acquittal is all but assured, with two-thirds of votes — or at least 20 Republican defections — needed for conviction.

Some Republicans said the monotonous Democratic opening arguments are to blame.

“You know, 28 hours of hearing the same thing over and over again isn’t all that exciting,” said Sen. Rand Paul (R-Ky.), who has passed the hours with crossword puzzles.

Sen. Pat Roberts (R-Kan.), who underwent back surgery in August, joked: “Well, if I had a choice I’d probably be home watching Chicago PD.” He added: “No, don’t put that in there or that would make me sound terrible.”

Meanwhile, some Democrats argued the lack of attendees is not necessarily a reflection of public disinterest.

“Because it’s on television, it’s a convenient alternative to coming in,” said Sen. Jack Reed (D-R.I.).

“I don’t think the average person thinks that it would be easy to come and watch,” said Sen. Chris Coons (D-Del.)

Indeed, rules for ticket distribution are unwritten. Most are distributed through individual Senate offices. People then queue in the congressional visitor center for up to three hours. The line is shorter than those at Disney World, but moves slowly due to half-hour seating blocks.

Senate offices get three to five tickets each. Tickets allow entry to the gallery and some can be used by multiple people who take shifts, including staff and constituents. Some offices say they have had robust interest and offer turns of one hour each. But there’s no consistent marketing among offices.

“My tickets have all been used,” said Sen. Patrick Leahy (D-Vt.)

The Senate Sergeant at Arms establishes rules for the chamber and did not respond to repeated requests for comment on how gallery seating is managed. Some seats, such as in both corners along the east of the chamber, have had very few guests this week. One Senate aide said that a section of about 100 seats, known as the family gallery, generally is reserved for relatives of senators and that it’s possible offices have inconsistent policies for those tickets.

Sen. Mike Rounds (R-S.D.) said a ban on note-taking outside a press section discouraged attendance by Senate staff. “They can do more work in the office where they have an ability to take notes,” he said.

Sen. Sherrod Brown (D-Ohio), meanwhile, speculated that Senate Republican leaders were to blame for tightening rules for the trial, noting restrictions on reporter access outside the Senate chamber. “I’ve gotta think, a lot of student groups are here, a lot of individuals are here, tourists are here. People would love to be part of this,” he said.

A spokesman for Senate Majority Leader Mitch McConnell (R-Ky.) did not immediately respond to a request for comment.

Coons, the Delaware Democrat, said he understands that the sergeant at arms and Capitol Police “have a job to keep us safe,” but that “the gallery should be accessible.”

“I have four tickets and we’re happy to rotate them out,” Coons said. “I’ve had a whole bunch of Delawareans come down and watch in the gallery. And that’s encouraging because it is a different experience watching it in the chamber than watching it on TV.”