WASHINGTON — “Hear ye, hear ye, hear ye. All persons are commanded to keep silent upon pain of imprisonment.”

Those words, an echo of another time, came from Michael C. Stenger, the Senate sergeant-at-arms, and fell upon a hushed Senate chamber just after noon on Thursday. The impeachment trial of Donald John Trump, the 45th president of the United States, on charges of high crimes and misdemeanors was about to begin.

Two hours later, Chief Justice John G. Roberts Jr., in flowing black robes, escorted by four senators, strode into the room to open the third presidential impeachment trial in American history.