Physical Science:

Sometimes celestial objects such as some planets appear to move backward in their orbit around the sun as shown in the animation below. Retrograde Motion

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astro.unl.edu/classaction/animations/renaissance/retrograde.html

Please click on start button in the animation. In the animation retrograde motion is shown for the planet "Mars". The projection of mars in the sky is shown by the background stars. We notice that when mars reaches certain positions it looks suddenly moving backward as seen from the earth and after some time in the year it again starts moving forward in its orbit around the sun. Such an apparent reverse motion of the planets is called retrograde motion. Ptolmy’s geocentric theory was not able to explain this event cleary. So in 1543, Polish astronomer, Nicholas Copernicus publish revised but completly radical theory called Heliocentric Theory. In this theory the unmovable sun at the center of the universe nd all the planets are moving around it in the specific oribit. This theory was able to solve almost all the puzzles of the time. In heliocentric model of the solar system Copernicus proposed that the Earth has three distinct motions: a daily axial rotation, an annual rotation about the Sun, and a precessional motion of its axis.