# 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.

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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