When the great Greek astronomer Claudius Ptolemy gazed at the heavens, he felt the particular curiosity many of us have come to know. He was exceptionally curious as to what caused the stars, the sun, the moon, and the planets to seem to rotate around Earth every night and day. This caused him to formulate the Ptolemaic Model of the Universe, otherwise known as the geocentric model of the universe. The geocentric model comprises our planet, Earth, in the center surrounded by the rotating heavens known as the celestial sphere. Everything, including the distant stars and planets, the sun, the other planets in the solar system, and everything else, revolved around planet Earth. The rotation of the heavenly bodies around Earth were said to be perfect circles in the Ptolemaic Model, but the problem with that was that the observational evidence didn’t align with the hypothesis. Ptolemy began watching how the sun, the moon, and the planets, at least from Earth, didn’t seem to be traveling in perfect circles. Observations made on planets perplexed him; some planets were seen to be reversing their course on some nights, and then resuming their course later on. In revising his model, he described what are known as epicycles and deferents. The epicycle is a small intermediate orbit whose center moves around the circumference of a larger orbit. This, of course, was wrong. With the heliocentric model, we now know that the apparent reverse motion of some of the sun, the moon, and the planets is a result of what is known as retrograde motion. In this episode, we will learn about what retrograde motion is and how it works.

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References

Ptolemaic Model of the Universe

https://socratic.org/questions/how-did-ptolemy-s-model-account-for-retrograde-motion

Ptolemaic Model of the Universe

https://www.britannica.com/science/Ptolemaic-system

Apparent Retrograde Motion + Great Animation

https://en.wikipedia.org/wiki/Apparent_retrograde_motion

Retrograde Motion

https://people.highline.edu/iglozman/classes/astronotes/retrograde.htm

Retrograde Motion

https://earthsky.org/space/what-is-retrograde-motion