Kids – they’re such a treasure. One minute you’re having a nice chat, the next minutes they’re testing your knowledge of the natural world with a question like, “Why can we see the Moon during the day?” And before you know it, you’re building a CNC Earth-Moon orbital model.
We’ve got to applaud [sniderj]’s commitment to answering his grandson’s innocent question. What could perhaps have been demonstrated adequately with a couple of balls and a flashlight instead became an intricate tellurion that can be easily driven to show the relative position of the Earth and Moon at any date; kudos for anticipating the inevitable, “Where was the moon when I was born, Grampa?” question. The mechanism is based on the guts of a defunct 3D-printer, with the X-, Y-, and Z-axis steppers now controlling the Earth’s rotation and tilt and the Moon’s orbit respectively, with the former extruder drive controlling the tilt of the Moon’s orbital plane. A complex planetary gear train with herringbone gears, as well as a crossed-shaft helical gear set, were 3D-printed from PLA. The Earth model is a simple globe and the Moon is a ping-pong ball; [sniderj] is thinking about replacing the Moon with a 3D-printed bump-map model, a move which we strongly endorse. The video below shows the tellurion going through a couple of hundred years of the saros at warp speed.
There’s just something about machines that show the music of the spheres, whether they be ancient or more modern. And this one would be a great entry into our 3D-Printed Gears, Pulleys, and Cams contest too.
> The video below shows the tellurion going through a couple of hundred years of the saros at warp speed.
One. It goes through the cycle of exactly one year.
Not sure about the up/down motion of the moon in the Model, it looks a bit too constant in speed of motion (a triangular rather than sinusoidal wave). Other than that, looks like a cool project.
Yeah, that also irritated me. The moon should go up and down in a sinusoidal way with smooth turn pints. Not like a ping-pong ball.
+1 – Yes, didn’t slow to a stop and reverse, more of a triangle wave…
Oh – and i wouldn’t really expect it to be accurately portrayed here, but on the ‘kids’ note – recently I looked into proportion of earth size to distance to moon to make sure I had some decent facts for a kid’s question – quite possibly due to seeing many models/illustrations along the way as I was younger, I didn’t realize in scale how far away the moon really is, in case this is of benefit to any others having as bad of a mental model as I did: It looks like the earth-to-moon scale is roughly correct (4:1), but earth diameter to distance to moon is roughly 30:1 – so for this to be ‘to scale’, the moon would be ~30 feet away from the earth (given 12″ globe).
– Not nit-picking design, but clarifying for anyone else that has based their perceptions on models/illustrations like this or similar along the way. – And the other interesting fact – at that scale, the space station would be less than 1/2″ away from earth, with the moon 30′. Made ‘going to the moon’ that much more impressive to me (and interesting how close the space station actually is).
Yeah, I’ve had the same scale problem. I wish there was a design that could show correct scale, but even for earth-moon system it is already difficult, and sun would be ridiculously far away in any reasonably sized model.
Do a google search for: if the moon was a pixel and hit the first result. A true to scale model of the solar system. It’s quite mind boggling.
“planetary gear train”… perhaps it could be called a moonary gear train?
Huh, I always knew this mechanism as an orrery. It appears that’s the general term, and a tellurion is a specific type of orrery for the sun-earth-moon system?
An Orrery shows the motion of the planets and moons around the sun. A tellurion is indeed specific for the sun earth moon system (and the lunar and seasonal cycles on earth).
Anyone else try to skip over the philosophy wisdom-complex portions of post-text on HaD lately? cringe