Barack Obama became the 44th president of the United States at noon Tuesday, before he took his oath of office.
The 20th Amendment to the Constitution states that the terms of the president and vice president end at noon and that the terms of their successors begin on noon of Inauguration Day.
The terms of the President and the Vice President shall end at noon on the 20th day of January, and the terms of Senators and Representatives at noon on the 3d day of January, of the years in which such terms would have ended if this article had not been ratified; and the terms of their successors shall then begin.