Guided learning / 5 lessons
Understand worlds in motion
Move from gravity and elliptical orbits to the changing viewpoint of a planet watcher.
Where this takes you
Connect orbital motion with the patterns we see from Earth.
Follow the lessons ↓Lesson 1
Gravity →
Begin with the interaction behind orbital motion.
Pause and consider
How can an orbiting world be falling without simply heading straight into the Sun?
Lesson 2 / Interactive figure
Kepler's laws →
Explore the patterns in planetary orbits.
Pause and consider
Try the equal-area figure. Where does the orbiting body move fastest?
Lesson 3
Eccentricity →
Describe how far an orbit departs from a circle.
Pause and consider
How would you distinguish eccentricity from the size of an orbit?
Lesson 4
Synodic period →
Compare orbital timing with repeat alignments seen from Earth.
Pause and consider
Why does a planet's own orbital period differ from the interval between repeated alignments with Earth?
Lesson 5
Retrograde motion →
Bring the observer's moving viewpoint into the picture.
Pause and consider
Explain apparent backward motion without making a planet reverse its orbit.
Put it into practice
Follow motion in the solar-system view
Explore the solar system and its time controls. Compare the motions of an inner and an outer planet. Keep the simulation's scale and illustration labels in mind while connecting the view to the lessons.
Explore orbital motion →