Answer to Question #14248 in Astronomy | Astrophysics for Donna Corbin
direction - from west to east - is not truly circular, but more
The orbit of a planet around the Sun is an ellipse, with the
one of the focal points of the ellipse. This focal point is
the barycenter of the Sun-planet system; for simplicity
explanation assumes the Sun's mass is infinitely larger than
planet's. The orbit lies in a plane, called the orbital plane.
point on the orbit closest to the attracting body is the
The point farthest from the attracting body is called the
There are also specific terms for orbits around particular
things orbiting the Sun have a perihelion and aphelion,
orbiting the Earth have a perigee and apogee, and things orbiting
Moon have a perilune and apolune (or periselene and
respectively). An orbit around any star, not just the Sun, has
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
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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