Motion of the Planets

The word "planet" derives from Greek meaning "wanderer" - unlike the fixed stars, planets move against the background stars.

Apparent Motion

Planets usually move eastward against the stars (direct motion). However, periodically they reverse direction and move westward (retrograde motion) for a few weeks before resuming direct motion.

Retrograde Motion Explained

Retrograde motion is an optical effect caused by Earth overtaking slower-moving outer planets in its orbit. Imagine passing a car on the highway - the other car appears to move backward relative to distant scenery, even though both are moving forward.

Inner vs. Outer Planets

• Inner Planets (Mercury, Venus) - Orbit inside Earth's orbit. They show phases like the Moon and are never far from the Sun in the sky. They can appear at greatest eastern elongation (evening sky) or greatest western elongation (morning sky).
• Outer Planets (Mars through Neptune) - Orbit outside Earth's orbit. They appear opposite the Sun at opposition (highest in sky at midnight, brightest, and closest to Earth).

Inferior and Superior Conjunction

• Inferior Conjunction - Inner planet between Earth and Sun.
• Superior Conjunction - Planet on the far side of the Sun from Earth.