Monday, November 21, 2011

The Physics of Retrograde Mercury

Planets generally move west to east in the sky. However, sometimes a planet will appear to slow down, stop its motion, and start moving in the opposite direction.  Eventually the planet starts moving west to east again, but this period of east to west motion is known as retrograde planetary motion.  This movement is related to Kepler’s Laws, which are:

1.The orbit of every planet is an ellipse with the Sun at one of the two foci

2.A line joining a planet and the Sun sweeps out equal areas during equal intervals of time.

3.The square of the orbital period of a planet is directly proportional to the cube of the semi-major axis of its orbit.

Kepler’s third law is the most relevant to retrograde planetary motion, since it basically states that the farther a planet is from the sun, the longer its orbital period will be. The equation below shows this relationship demonstrated in Kepler’s third law:

Even though Mercury has a much smaller gravitational force because it has a smaller mass than the Earth, which is related to the period as shown in the denominator of this equation, Mercury also has a much smaller semi-major axis (a), so it has a much smaller period. For Mercury, the orbital period is about 88 days, so for each revolution of the Earth about the Sun, Mercury makes a few revolutions.  Because of this, at some points in the Earth’s revolution, Mercury will appear to be moving backwards since it will move around the same side of the Sun as us, which appears as retrograde motion, as shown in the diagram below:

To see an animation of this, go to

Post written by Jillian Smith

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