The red 42-tooth old-style gear represents the Sun. The blue 15-tooth gear is the Earth, and the white 14-tooth the Moon.
The Earth rotates on its axis and also orbits the Sun. The Moon is stationary on its axis, and orbits the Earth.
The orrery is powered by one technic electric motor driving the Earth orbit gear train.
There are three gear trains. I tried to approach the actual ratios of the Earth and Moon orbits, and was able to come fairly close.
The gear train rotates the Earth 364.59 times for each rotation around the sun. In addition, because the three motions are coaxial, the earth picks up one extra rotation a year from the earth orbit drive for a total of 365.59 days per year. This is 0.33 days too long, or about 8 "hours" off over the course of a year.
The Moon drive produces one lunar orbit per 27.777 Earth rotations. The lunar drive picks up about 0.076 of a rotation per orbit around the Earth from the Earth orbit drive, for an actual period of one lunar orbit per 27.85 days. This is about half a day - 12 "hours" - longer than the actual lunar orbit of 27.32 days.
The arms holding the Moon and the Earth/Moon assembly are counterweighted to make their motion smoother
Build your Own!
With a remarkable program called Ldraw you can view and manipulate this model on your own computer. You can take it apart virtually, see how it works, and build your own! First, go to ldraw.org and download this free program and the associated files. Then return here and download the following two data files:
orrery_arm.dat
orrery_base.dat
More Lego Orreries!
Other Lego fans have tackled this challenge. Check them out!
Simon Bennett has constructed a simple sun-earth-moon orrery.
Thomas Johnson has a very nice looking orrery that demonstrates the effect of the tilt of the earth's axis on the seasons. Be sure to check out the movie of his orrery in action!
The Keppler Mission at NASA has pictures and instructions for building a 3-planet and 4-planet orrery that demonstrate the transit method of detecting extra-solar planets.
Notes:
- I had two general goals in designing and building this orrery. First was to approach as closely as possible the relative orbital periods of the earth and moon. Second was to build it entirely from Lego.
- The geartrain has many redundancies, mainly owing to the limitations of spacing the 16-tooth technic gears. With so many extra gears, proper alignment is crucial for smooth movement.
- I have rebuilt the orrery incorporating the battery box into the base. This has improved the overall stability of the model.
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