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The solar system - motions of celestial bodies

1) 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 massive object.

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.
The Sun 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.

- Planets: 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 their 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 planets.

- 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 solar system.

- 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 water vapor.

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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