Why does the Sun appear to reverse direction in Mercury’s sky?

The Sun appears to reverse direction in Mercury’s sky due to the planet's unique orbital and rotational characteristics. Specifically, Mercury has a highly eccentric (elongated) orbit, which causes it to travel around the Sun with varying speeds.1

According to Kepler’s laws of planetary motion, Mercury moves so swiftly near its closest approach to the Sun—a point known as perihelion—that its orbital velocity temporarily exceeds its rotational velocity. This discrepancy causes the Sun to appear to stop its normal westward drift, briefly move eastward across the sky, and then resume its westerly advance.1

This phenomenon is distinct from the concept of "retrograde motion" often discussed in astrology, which describes an apparent backward movement of a planet as viewed from Earth due to the relative orbital speeds of the two planets. On Mercury, the Sun's reversal is a physical consequence of the planet's specific spin-orbit resonance, where it rotates three times for every two orbits it completes around the Sun.21