CHINA has become the world's second biggest economy according to data released on Monday August 16th. Japan's economy fell behind China's at market exchange rates in the second quarter (it has been number three in PPP terms for some time). These numbers are not strictly comparable: Japan's data have been seasonally adjusted while those for China have not. Quibbles aside, Japan will surely be eclipsed soon, if it has not been already. Data compiled by Angus Maddison, an economist who died earlier this year, suggest that China and India were the biggest economies in the world for almost all of the past 2000 years. Why they fell so far behind may be more of a mystery than why they are currently flourishing.