eli5: why are the orbits of celestial bodies elliptical?
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The math about how gravity works that in theory orbits all follow a certain shape.
The force of gravity declines with the square of the distance. aka the “inverse square law”.
If you plug that into some equations and ignore real world factors and things like general relativity, you get one standard equation about how two object should move in regards to one another on account of gravity.
You can group that into categories: circle, elliptical orbit, parabolic trajectory and hyperbolic trajectory. Circle and parabolic arch are just mathematical possibilities that appear when the numbers work out just right, which they don’t in real life.
A circle is just a special type of an ellipsis, like square is a special type of rectangle.
A circle is when mathematically the ellipsis has zero “eccentricity”, (you can think of that as how stretched the ellipsis is.). The closes the eccentricity gets to one the more stretched the ellipsis gets.
Once the eccentricity hits one you don’t have an ellipsis at all any more but a parable, if you go beyond on you get a hyperbole.
Parable and hyperbole means that it isn’t a proper orbit because the thing won’t come back around again.
So with circle only a mathematical possibility and the other stuff only working once and never coming round again, you are left with only ellipsis in real life.
A guy named Kepler worked that out in the early 1600s.
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