Why is the president sworn in at noon?

I believe the answer is found in the 20th Amendment (adopted in 1933) to the U.S. Constitution. That amendment established January 20th as the day -- and noon as the time at which-- the new president assumes responsibility. The president becomes president, officially, at noon on January 20th regardless of whether or not (s)he is sworn in on that day. However, the president must take the Oath of Office prior to taking any formal action(s) as president. President Franklin D. Roosevelt, for his second term, was the first to be sworn in under the conditions established by the 20th Amendment (1937). For his second term of office, President Dwight D. Eisenhower was privately sworn in on January 20, 1957. However, because that day fell on a Sunday, a second, public swearing in ceremony/inauguration was held at noon on Monday, January 21, 1957 for the public celebration. So, with the exception of death, removal from office or resignation from the office of president in each case, a vice-presidential succession can be any date and time on that day. But for president-elects, it is officially set for the 20th of January the year following the general election for president.