Retrogradations

Retrogradations, also known as retrograde motion, refer to the apparent backward movement of a planet in the night sky, as observed from Earth. This phenomenon occurs due to the combined effects of Earth's and the planet's orbital motion. As Earth orbits the Sun, it overtakes the slower-moving outer planets, causing them to appear to move backward in the sky. This illusion is an example of parallax, where the observer's perspective changes, altering the apparent position of the observed object.

Retrogradations are most noticeable for the outer planets, such as Mars, Jupiter, and Saturn, which have longer orbital periods than Earth. The duration of a planet's retrogradation depends on its orbital period and the relative positions of Earth and the planet. For instance, Mars' retrogradation can last for about 72 days, while Jupiter's can last for approximately 130 days.

The concept of retrogradations has been known since ancient times, with early astronomers like Ptolemy and Hipparchus making observations and attempts to explain this phenomenon. Modern astronomy has provided a more accurate understanding of retrogradations, attributing them to the relative motion of Earth and the planets in their orbits around the Sun.