Overview
This lecture explains the apparent movement of planets in the night sky, focusing on the concept of retrograde motion for superior planets and the history and cause of this phenomenon.
Star and Planet Movements
- Stars maintain fixed positions relative to each other over thousands of years.
- Planets change positions night-to-night relative to the background stars, appearing as "wanderers."
- All celestial objects rise in the east and set in the west due to Earth's rotation.
- A planet’s eastward motion is due to its revolution around the Sun.
Retrograde Motion Explained
- Retrograde motion: a planet appears to move westward, temporarily reversing its typical eastward motion.
- For superior planets (those further from the Sun than Earth), retrograde creates a looping path in the sky.
- Ancient model (Ptolemy): explained retrograde with complex epicycles and deferents in a geocentric system.
- Modern model (Copernicus): retrograde is an apparent effect from Earth's faster motion overtaking an outer planet.
- Retrograde occurs around opposition, when the planet is opposite the Sun in the sky.
Demonstrating Retrograde Motion
- A teaching model uses gears and a connecting rod to simulate Earth and a superior planet’s motions around the Sun.
- Perspective lines from Earth through the planet show its apparent position among background stars.
- Retrograde motion begins when eastward motion stalls; midpoint is at opposition; resumes eastward after retrograde ends.
- The phenomenon is solely due to perspective, not changes in actual planetary motion.
Characteristics of Retrograde Motion
- Synodic period: time between oppositions (and successive retrograde cycles) gets closer to one year for more distant planets.
- Retrograde interval: duration of westward (retrograde) motion increases with planet distance from Earth.
- Retrograde loop size: largest for Mars and shrinks to zero for very distant planets, due to perspective effects.
Key Terms & Definitions
- Superior Planet — a planet whose orbit is further from the Sun than Earth's.
- Retrograde Motion — apparent temporary reversal of a planet’s movement relative to stars.
- Opposition — when a planet is directly opposite the Sun in the sky from Earth.
- Synodic Period — the interval between alignments of Earth and a planet (e.g., opposition).
Action Items / Next Steps
- Draw or simulate a line of sight from Earth through a planet to identify the apparent location in the sky.
- Record your answer and explain your method.
- Visit http://astro.unl.edu for more teaching materials.