Retrograde Motion

A Matter of Perspective - Retrograde Motion

Sometimes Mars seems to move backward - every wonder why! Perhaps its a matter of Perspective

From earth most celestial bodies exhibit prograde motion (eastward). The stars do move a bit, due to earth’s rotation but the change is not that noticeable; therefore the stars appear fixed for a longer period of time.  

The prograde motion of the planets (wanderers) is far more easily observable because they too are busy orbiting the sun. Apparent Retrograde Motion is an interesting phenomena exhibited by planets as seen by the observer on earth. During retrograde motion, the east moving planet seems to slow down, stop, switch direction westward, continue west for a while, slow down, stop and then resume its eastward course. The planet appears to loop in the sky.

Ancient astronomers worked with a geocentric model and so Ptolemy attempted to, but could not quite explain, this phenomena using epicycles and deferents which also incorporated a loop de loop inside it to explain retrograde motion.

In the heliocentric model planets do not retrograde. The retrograde phenomena simply then becomes a matter of perspective, it’s from the point of view of the observer on earth.  That’s why it’s called Apparent Retrograde Motion. It’s all about our line of sight from earth to that planet against a fixed frame of reference (the stars).

Here’s Why - All the planets orbit the sun. Per Kepler’s third law of motion, the square of a planet’s orbital period is proportional to the cube of its orbital radius. That is,  the further the planet is from the sun, the slower its orbit.

Apparent Retrograde Motion of a Superior Planet eg: Mars/Jupiter, Saturn,...

Following Kepler’s third law, the faster orbiting Earth will overtake the slower Mars, during Mars’ orbit around the sun. Superior Planets retrograde when earth overtakes them in their orbit.

Mars and Earth are considered in ‘opposition’ when they line up ie: SUN-EARTH-MARS. Retrograde motion takes just before opposition and just after opposition.  ie: Opposition lies in the middle of the retrograde motion.  Our line of sight against the fixed background of stars and thus our perception seems to make the superior planet retrograde.

Retrograde Motion of Inferior Planets eg: Mercury, Venus

Following Kepler’s third law, the faster orbiting Venus will overtake the slower Earth on its orbit around the sun. Inferior Planets retrograde when they overtake Earth in their orbits.

Venus and Earth are considered in “conjunction” if they line up

• Inferior Conjunction: SUN - VENUS - EARTH
• Superior Conjunction: VENUS - SUN - EARTH

Venus shows retrograde motion in Inferior Conjunction. ( Venus moves super fast when in Superior Conjunction, as Earth and Venus are moving in opposite directions in their orbits. That’s why Venus always appears to be on earth’s side in the orbit.)

To add more interest, I read that from a geocentric perspective, there are specific angles to the pendulum like motion of the inferior planets. eg:. Venus always appears within 47 degrees of sun.  It swings back west 47 degrees, stops, chases the sun eastward, overtakes the sun further down, keeps its motion west  for another 47 degrees before retrograding again. Mercury’s pendulum swing is within 27 degrees of the sun. For retrograde of inferior planets