Well, see, there you go ... using a word that means different things
to different people, and may even mean different things to the same
people at different times.
What does "nearly" mean ? ? And how do you measure how far
or near to a circle it is ? ?
Every closed gravitational orbit is an ellipse. An ellipse looks like a
circle that either has or hasn't been squashed. If it's perfectly round
and hasn't been squashed at all, then we call it a circle. If it's been
squashed at all, then we call it an ellipse.
To come up with a number that tells how squashed it is, we divide
(the distance from the center to one focus of the ellipse)
by
(the distance from the center to one vertex of the ellipse).
The eccentricity of a circle is zero. When you squeeze the circle,
the more you squash it, the higher the eccentricity gets, until ... if
you totally squash it down to a straight line ... the eccentricity is 1.
Perfect circle . . . . . . zero
Totally squashed . . . 1.00
Orbit Eccentricity Compared to Earth
Mercury 0.21 x 12.3
Venus 0.007 x 0.4
Earth 0.017 x 1.0
Moon 0.055 x 3.3
Mars 0.094 x 5.6
Pluto 0.244 x 14.6
Halley's Comet 0.97 x 57.1
Conclusions:
-- All of the planets (and their moons too) have nearly circular orbits.
-- While Pluto was considered a planet, it had the most eccentric orbit
of all. (That's one of the reasons it lost its standing as a planet. There
were other reasons.)
-- Now the planet with most eccentric orbit is Mercury. The orbits didn't
change. Pluto just got bumped from the list.
-- Most comets have very eccentric, far-from-circlular, elliptical orbits.
They go wy out, and come waaay in close to the Sun.
