What are sun and planet gears?
Gears that orbit around another gear. In a planetary set, small "planet" gears travel around a central "sun" gear, like the Earth around the Sun. They pack a big gear change into a small space — they are inside automatic gearboxes and electric screwdrivers.
Try Sun-Earth-Moon Orrery for freeMechanical Wonders booklet pack: 30 PDF booklets for $5.99. Instructions only; individual booklets remain free.
Explore the printable packTry it yourself
Hold the sun gear still and turn the arm. Watch the planet spin as it orbits!
Build it: 2 builds that teach sun & planet gears
- 🌍 Sun-Earth-Moon Orrery: Push the arm round: the Earth circles the Sun, spinning as it goes, with the Moon in tow. (level 4 of 5, about 35 minutes, 12 steps)
- 🚂 Sun-and-Planet Beam Engine: James Watt's steam-engine trick: a gear that never spins makes the flywheel turn four times per pump! (level 4 of 5, about 50 minutes, 17 steps)
In the classroom
- M2 Energy Transfer (Engineers, Advanced)
- M6 Space Machines (Engineers, Advanced)
Common engineering questions
How can I try sun & planet gears at home?
Hold the sun gear still and turn the arm. Watch the planet spin as it orbits!
Which builds teach sun & planet gears?
2 MightyCog builds teach it: Sun-Earth-Moon Orrery and Sun-and-Planet Beam Engine. Start with Sun-Earth-Moon Orrery: 12 steps, about 35 minutes.