Blast into 2026: Conquer the Astronomy Challenge – Your Cosmic Adventure Awaits!

Retrograde motion refers to the apparent motion of a planet as observed from Earth, where it appears to slow down, stop, and then reverse direction before resuming its normal forward motion. This phenomenon occurs due to the relative positions and motions of Earth and the other planet in their orbits around the Sun.

As Earth moves faster in its orbit, it can overtake and pass by outer planets like Mars, causing these planets to appear to move backward (or "retrograde") against the backdrop of the stars for a period of time. This effect is purely an optical illusion caused by the relative motion of the two planets, rather than an actual change in the direction of the planet's orbit.

The other options describe different celestial motions that are not related to retrograde motion. The circular motion of a moon around a planet is a stable orbit, which does not exhibit retrograde characteristics. The sustained orbit of a comet refers to its elliptical path around the Sun, and the constant westward drift of stars involves the motion of stars across the sky due to Earth's rotation, neither of which represent the retrograde phenomenon experienced by planets.