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The solar system - motions of celestial bodies
What is the solar system ?
We generally consider that the solar system consists of the Sun and all
objects revolving around this one.
2) Which celestial bodies belong
to the solar system ?
- The sun: set in the center of the solar system, it is its most
The Sun is a
medium sized star that was formed five billions years ago in the same
time as the other objects of the solar system.
seems to be really bigger than the other stars because they are farther
from the Earth and they don't belong to the solar system.
there are eight planets revolving around the Sun. In
order of distance from the Sun there are Mercury,
Venus, Earth, Mars that are called " telluric planets " because of
solid surface, then there are Jupiter, Saturn, Uranus and Neptune which
are some giant gaseous planets.
- Dwarf planets: they also revolve around the Sun but they are smaller
and farther than the other planets. Pluto, Charon, Eris and Sedna
belong to this category.
- Natural satellites: planets can be accompanied with small celestial bodies that
revolve around them and
that we call natural
satellites. For instance The Moon is
Earth's natural satellite, Mars has two tiny satellites ( Deimos and
phobos ), and there are dozens of them around each giant gaseous
- Asteroïds: These are some small sized objects ( the biggest
ones can reach hundred kilometers) revloving around the Sun.
They are mainly concentrated in two areas: the asteroid belt located
between Jupiter and Mars and the Kuipper belt at the limits of the
- Comets: like asteroids they orbit the Sun but they have a flatter
trajectory and they come periodically close to it which makes
them visible from Earth because of their long tail made of dust and
3) Motions in the solar system
The common feature of planets, dwarf planets, comets and asteroids is
in their almost circular motions around the Sun.
These motions result from the action of the Sun: this phenomenon
is called gravity
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