People think of the 1929 stock market crash as the beginning of the Great Depression. But stocks recovered from their plunge on "Black Tuesday" and the Dow Jones Industrial Average actually climbed another ten percent, before beginning its long slide, bottoming out at under 20 percent of its pre-Black Tuesday value about 1000 days later, on July 8, 1932. Investors have been pretty optimistic over the past year or so that we have weathered a deep recession and avoided something worse. But last Thursday’s harrowing half-hour crash, when the Dow lost 1000 points in 30 minutes, then recovered most of them, has made people nervous that maybe the worst isn’t over after all.

The graph below compares the course of the Dow after the Great Crash against its course since our current crisis began with the collapse of Lehman Brothers in September 2008. If history repeats itself, the market won’t hit rock bottom until May 27, 2011, more than a year from now.