Elliptical orbit of the earth

A post of John Cook can surprise. The orbit of the earth around the sun is often displayed as a rather elongated ellipse. In truth, it is much closer to a circle than one could believe. Here is the plot in Mathematica using data from this Wikipedia page: p=perihelion=147100000; a=aphelion =152100000; r=(a+p)/2 S1 = ParametricPlot[{a*Cos[t],p*Sin[t]},{t,0,2Pi},PlotStyle->Red]; S2 = ParametricPlot[{r*Cos[t],r*Sin[t]},{t,0,2Pi},PlotStyle->Blue]; Show[{S1, S2}]

Here is a typical textbook picture John Cook notices: "It was only by obsessing over a discrepancy of 0.037 percent of a circle that Kepler was able to discover that Mars has an elliptical orbit."