Based on the definition of systemic risk given by Jean-Claude Trichet at Clare College in Cambridge (Dec. 2009), we propose a simple methodology for ranking systemically important institutions. We incorporate both the cross sectional aspects of risks through firms interrelations and the time series aspects of the evolution of this interconnectedness over time. We view firm's risks as a network with vertices equal to the volatility shocks and edges equal to their correlations. Dynamic centrality measures allow us to rank the firms in terms of risk connectedness and firm characteristics. The resulting global systemic risk (GS) measure from applying this approach to all firms in the S&P500 for 2003-2011 reveals that the systemic risk in the financial sector stocks peaked in September 2008, but was greatly reduced by the introduction of TARP. Anxiety about European debt markets saw the systemic risk begin to rise again from April 2010. We further decompose these results to find that the systemic risk of insurance and deposit taking institutions differs importantly, the latter experienced generally declining systemic risk from late 2007, in line with burst of the housing price bubble, while risk for insurance companies continued to climb up to the rescue of AIG. Our systemic risk index emphasises interconnectedness: a comparison of this with the capital shortfall approach of Brownlees and Engle (2011) shows that while risk due to interconnectedness declined post September 2008, capital shortfall risk remained at sustained levels. The two approaches offer complementary information. Further, we show the importance of including the interconnectedness of the financial sector with firms in the real economy, in producing measures of systemic risk.