Retrograde Motion: What is it & How Does it Work?

[Sobi]

What is retrograde motion? Can some one explain this movement while Earth is constant and the outer planet is moving with relative rotational speed.

 

[rootone]

The orbits of all the planets around the Sun is in the same direction ( counterclockwise when viewed from above the Sun's north pole).
However sometimes we observe a planet APPARENTLY is moving backwards against the background of stars.
In fact the planet's orbit has not changed at all, the 'retrograde' movement is because of the relative positions of the planet and Earth.

http://starchild.gsfc.nasa.gov/docs/StarChild/questions/question46.html
This may help, it's old but I think it make a good job of explaining.

 

[Ironmanlet]

good post rootone.

So Mercury and Venus have a shorter 'year' than Earth does, so they go through retrograde motion (the appearance of moving backwards) more than once a year. But the outer planets, with longer orbital periods go through it once a year as the Earth comes around.

If I remember correctly, the word 'planet' comes from a greek word which means 'wanderer'. Because of the path of the planets with relation to the path of the stars in the background (parallax) thought the year.