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Answer to Question #14248 in Astronomy | Astrophysics for Donna Corbin

Question #14248
The Earth and other planets go around the sun in what kind of orbits?
Expert's answer
The paths that the planets take as they travel around the sun in the
same
direction - from west to east - is not truly circular, but more
of an
egg-shape path.
The orbit of a planet around the Sun is an ellipse, with the
Sun in
one of the focal points of the ellipse. This focal point is
actually
the barycenter of the Sun-planet system; for simplicity
this
explanation assumes the Sun's mass is infinitely larger than
that
planet's. The orbit lies in a plane, called the orbital plane.
The
point on the orbit closest to the attracting body is the
periapsis.
The point farthest from the attracting body is called the
apoapsis.
There are also specific terms for orbits around particular
bodies;
things orbiting the Sun have a perihelion and aphelion,
things
orbiting the Earth have a perigee and apogee, and things orbiting
the
Moon have a perilune and apolune (or periselene and
aposelene
respectively). An orbit around any star, not just the Sun, has
a
periastron and an apastron.
Owing to mutual gravitational perturbations,
the eccentricities of the
planetary orbits vary over time. Mercury, the
smallest planet in the
Solar System, has the most eccentric orbit. At the
present epoch, Mars
has the next largest eccentricity while the smallest
orbital
eccentricities are seen in Venus and Neptune.
In the elliptical
orbit, the center of mass of the orbiting-orbited
system is at one focus of
both orbits, with nothing present at the
other focus. As a planet approaches
periapsis, the planet will
increase in speed, or velocity. As a planet
approaches apoapsis, its
velocity will decrease.

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