FSS - Motion of the Planets Lesson

 

Motion of the Planets

The eight planets that compose our solar system today - Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune - are spread out through a large, spherical area that describes our solar system.   However, the planets all revolve around the sun (more or less) in the same plane.   If you were to look at the solar system "on edge", the planets would look like race cars going around circular tracks of different diameters, but all on the same table.   As material was pulled inward towards the sun, it started spinning, the same way that water spins around a drain as it is sucked down.   If you were to look down on the north pole of the sun, all of the planets would be moving in a counterclockwise direction.   Since the path travelled by each planet gets longer the further it is from the sun, Mercury has the shortest year- only 88 Earth days- while Neptune takes 165 Earth years to make a single orbit around the sun!

 

 

In addition to revolving around the sun, each planet rotates on an imaginary axis.   The rotational period of a planet determines the length of its day (Earth takes 23.9 hours to complete one spin on its axis), and its period of revolution determines the length of a year on the planet.   Earth makes 365.25 rotations during a complete revolution around the sun; every four years, those quarter days are accounted for by adding an extra day to the month of February, and we celebrate a "leap year".   The gas giants spin on their axes faster than the inner, rocky planets, and therefore have shorter days.

While all planets revolve around the sun in the same direction, the same generalization cannot be made for their rotations.   While there are definite trends, there are several exceptions among the planets that serve to remind us of the very individual histories of each of these bodies.   For example, all of the planets except for two rotate counterclockwise (looking down on its north pole), and all of the planets except for one are oriented with their rotational axis roughly perpendicular to their orbit around the sun.

 

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