Most of the time, the planets in our solar system move eastwards across the sky. However, they sometimes move westwards. This is known as retrograde or “backwards” motion. Take for example, Mars. This is illustrated in the diagram below.
As the orbit of Mars around the Sun is much larger than the orbit of the Earth, the relative movement of Mars appears to be largely tangential. The relative movement of the Earth can vary from being tangential to being normal to the orbit of Mars. The tangential vs. normal movement results in retrograde motion.
a. At point a, movement of the Earth is largely normal to Mars. Hence, it moves “towards” Mars. In turn, Mars seems to be moving “forward” (eastwards) across the sky.
b. At point b, movement of Earth becomes more tangential, with respect to Mars. It hence “catches up” with Mars. The “forward” motion of Mars is retarded at this stage.
c. At point c, Earth moves in a tangential path parallel to Mars. Since the Earth has a higher orbital speed, it “out-races” Mars. Hence, from the Earth, Mars appears to move “backwards” (westwards) across the sky at this stage.
d. At point d, Earth starts to resume normal movement, and moves “away” from Mars. Retrograde movement hence begins to diminish.
e. At point e, Earth resumes normal movement, while Mars continues its tangential motion. This creates the illusion from a geocentric view that Mars resumes its “forward movement” across the sky. This entire process, for Mars, takes about 4 months. In the same way, other planets in the solar system exhibit retrograde behaviour, when seen from Earth. The entire period for retrograde motion to occur also differs from planet to planet. The seemingly arbitrary movements of the planets in the sky led the ancients to believe that they were directly influenced by the gods. Hence the role of the planets as determiners of fate was not an incredible notion in ancient times.
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