Julian Zelizer is a professor of history and public affairs at Princeton University. He is the author of "Jimmy Carter" and "Governing America."

(CNN) -- Former President George W. Bush is enjoying another bounce in the post-presidential polls. First, the opening of his presidential library produced a spate of positive coverage about his time in office. Now, Gallup has released a survey showing that for the first time since 2005, more people approve than disapprove of Bush.

This kind of shift in public opinion is likely to continue, with more upswings as well as downturns ahead. This is the nature of presidential legacies. They are a bit like what Mark Twain once said about the weather in New England: if you don't like it, just wait a second and it will change.

Presidential reputations are never fixed in stone.

With the exception of a few of our leaders, such as Abraham Lincoln of Franklin Roosevelt, people tend to remember presidents in different ways at different times. Even some of the more unpopular presidents have seen their support rise in the post-presidency period....