Date: Wed, 13 Mar 2002 18:30:27 -0800
From: Michael Andre-Driussi 
Subject: (urth) 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=


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