Green, argument for orbital eccentricity

Experiment: build a model in our solar system. Let Blue = Earth, move Venus out a bit, rename it "Ishtar," and let's see. I want Ishtar to orbit within Martyn Fogg's "Moist Greenhouse" range, which would be .85 AU to .95 AU for our system. ("Moist Greenhouse" is one step away from "Runaway Greenhouse.") So place Ishtar at .9 AU, with a year of 311 days. Earth is at 1 AU. If we give both planets perfectly circular orbits, then they will be at their closest point about once every Ishtar year (plus a little), a fraction of an Earth year. We want an orbit more "cometary," yet we want it to remain within a boundary (Moist Greenhouse) more constrained than comets. If we give Ishtar an orbital eccentricity of .05 (less than Mars's; about what Jupiter has), then Ishtar will swing from .855 AU to .945 AU. It will do this in the course of its year: speeding up as it nears the Sun, slowing down as it reaches the farthest point. I don't know the formula, but my guess is that Ishtar at apehelion while Earth is near (leaving aside Earth's perihelion) would not happen once a year. Starting both at the closest separation: Day 0 Ishtar arrives back at point on Day 311 Earth arrives back at point on Day 365 They are 54 days out of synch (Earth "falling behind"), suggesting that it will take, what, 6.75 years for them to get back into synch? ISHTAR EARTH 0 0 311 365 622 730 933 1095 1244 1460 1555 1825 1866 2190 = 6 years 2177 2488 2464 = 6.75 years Something like that. =mantis= Sirius Fiction booklets on Gene Wolfe, John Crowley Now with UPDATES! http://www.siriusfiction.com/